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216 Cards in this Set
- Front
- Back
why is the sea so salty |
Cracks in the Earth‟s crust allow water to contact molten rock. Vast amounts of minerals aredissolved in the super heated water and these are released into the oceans via hydrothermalvents. |
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why is the sea so salty |
Other salts come from the weathering of rock and eventually get washed into the sea. |
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what happens as the sea water evaporates |
salts become more concentrated. |
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where is the element bromine extracted from |
bromide ions found in sea water |
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why does the Dead Sea in Israel contain the largest number of bromide ions |
salts are constantlywashed into it but cannot escape. |
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how are ionic solids held together |
by the electrostatic attraction between the anions and cations. Each positive ion (cation) exerts an attraction on several surroundingnegative ions (anions).The ions build up to form an ionic lattice. |
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eg sodium chloride |
Each Na+ion has coordination number = 6 aseach Na+is surrounded by 6 Clions.Coordination number of Cl-= 6 as each Clisalso surrounded by 6 Na+ions.The attractive forces are stronger overall in thisstructure than repulsions. |
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what happens when an ionic solid dissolves in water |
theions are surrounded by water molecules. |
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why |
The -oxygens on water molecules areattracted to the positive ions and the + Hatoms in water are attracted to the negativeions |
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what are ions surrounded by water molecules said to be? |
hydrated |
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why do anions and cations behave independently when dissolved in water |
separated widely from each other |
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what is sea water |
a mixture of positive and negative ions |
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What are hydrated salts |
Some salts contain molecules of water packed into their lattices. This isshown in the formulae e.g CuSO4. 5H2O(s). The water in these salts is calledwater of crystallisation and the salts are said to be hydrated. |
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what does strong heating of hydrated salts cause |
drive out this water to leave anhydrous salts.CuSO4. 5H2O(s) CuSO4 (s) + 5H2O(g) |
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what is a precipitate |
2 types of ions coming together to form a solid (precipitate) that watermolecules cannot dissolve. |
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What happens to the appearance of a solution with a precipitate |
The solid will sink to the bottom of the solution. Theappearance of the solid in solution will cause the liquid mixture to go from clear to cloudy untilall the solid has settled out. |
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Are nitrates soluble |
yes, all are |
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are Chlorides soluble |
All chlorides are soluble except AgCl, PbCl2 |
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Sulphates? |
Sulphates are soluble except BaSO4, PbSO4 , SrSO4 |
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Carbonates? |
Carbonates are insoluble except for NH4+and group 1 carbonates. |
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sodium, potassium and ammonium salts? |
all are soluble |
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What is a reducing agent? |
substance that provides electrons (= species oxidised inthe equation). |
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What is an oxidising agent? |
Oxidising agent = substance that removes electrons (= species reduced inthe equation) |
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What type of oxidation states do elements on its own have? |
0 eg O2 and Fe |
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what are the oxidation states of simple ions |
charge on the ions |
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what is the oxidation of Fluorine |
-1 |
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what is the oxidation state of O |
-2 (exceptionsare peroxides where it is –1, and F2O where it is +2) |
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Oxidation number of hydrogen |
+1 (exceptions arecompounds with H and reactive metals e.g MgH2 or NaH where it is –1) |
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Cl in compounds is |
–1 except in compounds with O or F |
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what must the oxidation states in a neutral compound add up to |
0 |
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what must the oxidation states of molecular ions add up to |
overall charge of ion |
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why are oxidation states in transition metals shown in the name |
Transition metals can show more than one oxidation state in compounds. |
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what are oxyanions |
molecular ions with negative charge that contain oxygen andanother element. The oxidation state of the element joined to oxygen isshown in brackets after the name of the ion in Roman numerals. The ionname starts with part of the name of the element that is joined to oxygen andends in -ate. eg ClO3-chlorate (V |
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when is an element oxidised |
oxidation state increases (becomes morepositive). (number of electrons decreases) |
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when is an element reduced |
its oxidation state decreases (becomes morenegative).(number of electrons decreases) |
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what is an oxidising agent |
element within a compound which causes oxidation (the element itself gets reduced) |
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what is a reducing agent |
element within a compound which causes reduction (the element itself gets oxidised) |
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how is the reducing agent or oxidising agent named |
as it's compound |
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what happens if a question asks for the systematic name of an inorganic compound |
include oxidation states where appropriate |
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how are halogen elements found |
diatomically |
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what are the physical trends down the group? |
melting point and boiling points increase down the group colour of compounds darkens down the group |
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why do boiling and melting points increase down the group |
as the number ofelectrons (and thus size of atom and relative mass) in the non-polar molecules rises, leading to strongerinstantaneous dipole-induced dipole intermolecular bonds. |
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what is fluorine like (F2) |
volatility= gas reacts with water soluble in organic solvents (hexane pale yellow gas at rtp |
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Cl2 |
green gas at rtp volatility=gas slightly soluble with water to give pale green solution with hexane is soluble to give pale green solution |
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Br2 |
dark red liquid at rtp volatility=liquid, forms brown gas on warming slightly soluble with water to grive red-brown solution soluble with hexane to give red solution |
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I2 |
shiny blood-red solid at rtp volatility=sublimes on warming to give a purple vapour barely soluble with water to gives brown solution soluble with hexane to give violet solution |
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what is the solubility of halogen (non polar) molecules like |
dont dissolve easily in water (low solubility) but are much more soluble in organic solvents have low solubility in water as of covalent bonds |
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what is the colour of chlorine in hexane like |
pale green |
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bromine in hexane |
red |
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iodine in hexane and water |
violet in hexane brown in water |
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how do halogens usually react |
by gaining an electron to form a noble gas configuration. This makes them oxidising agents |
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what type of ions are halogens |
halide ions (eg Cl-) |
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What happens to halogen's reactivity and oxidising power down the group |
decreases down the group |
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what type of compunds do halogens form with metals |
ionic compounds |
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what type of compounds do halogens form with other non metal compounds |
covalent bonds |
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what are the oxidation states like in interhalogen compunds |
the more reactive halogen will be the -1oxidation state (the other will be in a positive oxidation state) |
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what happens when halogens react in solutions |
a displacement reaction occurs A more reactive halogen will displace the halide ions of a less reactivehalogen in solution. Halide ions are oxidised, the halogen is reduced. colour produced is of the diatomic halogen molecule produced |
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what is electronegativity |
tendency of atom to attract a bonding pair of electrons |
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why happens to electro-negativity down the group |
decreases |
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why |
larger atoms attract less than smaller ones; as positive nucleus is further away and more shielded as number of shells increases down group atoms become larger (more shells, blocking attraction of positive nucleus) so electronegativity decreases |
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Chemical properties of halogens |
|
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what happens to reactivity down the group and strength as oxidising agents |
decreases |
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why? |
The atoms get larger. The outer shell gets further from the nucleus. The attraction between the nucleus and electrons gets weaker, so an electron is less easily gained. |
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how can you test for halide ions |
using silver nitrate solution (AgNO3) |
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what happens when chloride ions react with silver ions |
white precipitate of silver chloride forms Ag+(aq) + Cl-(aq) give AgCl(s) |
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bromide ions and silver ions |
cream precipitate of silver bromide Ag+(aq) + Br-(aq) AgBr(s |
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iodide ions and silver ions |
Ag+(aq) + I-(aq) AgI(s) A yellow precipitate of silver iodide |
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what happens when silver bromide is decomposed by light |
forms metallic silver This is the basis for most filmbased photographic processes.Silver chloride is decomposed the fastest by light, AgI does not decompose easily onexposure to light. |
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what is the first ionisation enthalpy |
theenergy needed to remove 1electron from every atom in 1mole of isolated gaseous atoms ofthe element.X(g) to X+(g) + e - NB element must be in gas state |
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how can the existance of electron shells be proved |
by measuring the successiveionisation enthalpies of an element = energy needed to remove eachelectron in turn from an atom until there are none left. |
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what is the second ionisation of sodium |
Na+(g) to Na2+(g) + e |
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why is there a large jump in energy for the second ionisation enthalpy |
This isbecause the second electron is in the second shell (the first electron camefrom the 3rd shell). |
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why does the second electron need more energy to be removed? |
It is in a shell (2nd) closer to the nucleus. It is being removed from an atom that already has a positive charge. Fewer filled electron shells between this e- and the nucleus so lessshielding from the attraction of the nucleus. (The first electron isshielded by 2 full inner shells) |
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what are the large energy jumps evidence for |
existance of shells |
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where do electrons exist |
in shells, designated n=1, n=2, the further away a shell from the nucleus, the larger the n number |
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what are these shells divided into |
sub shells s, p, d, f |
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how are sub shells further divided |
into atomic orbitals each atomic orbital can hold a maximum of two electrons. These two electrons must have opposite (or paired) spins |
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description of s sub shell |
1 orbital hold max number of 2 elesctron |
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description of p sub shell |
3 orbitals hold max number of 6 electrons dumbell shape |
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description of d sub shell |
5 orbitals hold max number of 10 electrons |
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what is electronic configuration |
arrangement of electrons in orbitals |
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how are orbitals filled? |
in orbitals of increasing energy |
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how are orbitals filled up (when there is more than 1 orbital at the same energy) |
orbitals are first occupied singly by electrons then when each orbital is occupied singly by electrons, electrons then pair up |
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what tpe of spin do singly occupied orbitals have |
parallel spins |
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what type of spin do electrons in doubly paired orbitals have |
opposite spins |
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why does the 4s orbitall fill up before the 3d orbital? |
has a lower energy |
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what do arrows in boxes represent |
arrows represent electrons box represents orbital arrows pointing in opposite directions have paired (opposite) spins |
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what type of electronic configuration does chromium have |
1s2, 2s2, 2p6, 3s2, 3p6,3d5,4s1 |
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what type of electronic configuration does copper have |
1s2, 2s2, 2p6, 3s2, 3p6, 3d10,4s1 |
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Why are chromiu and copper exceptions? |
These exceptions are due to the high stability of a full or half filled d sub-shell and thereduction of repulsion between electrons that would have otherwise been paired in the 4s. |
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Why are group 1 and 2 elements known as s block elements |
as have two electrons in their outermost sub shell, which is an s orbital |
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why are groups 3,4,5,6,7 and 0 elements known as p block elements |
as contain 3,4,5,6,7, or 8 electrons in outer most sub shells which are in the p orbitals |
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why are the first transition block elements know as d block elements |
as have electrons which fill d sub shell |
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what do first ionisation enthalpies of group 1,2 and 3 elements provide evidence for? |
sub shells |
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why? |
E.g small dips between Beand B, also Mg and Al as go from s to psubshells. Also dips N to O, P to S are evidence of p3going to p4and electrons having to pair inorbitals leading to repulsion and a slightdrop in ionisation enthalpy as a result. |
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what is the first step for manufacturing bromine from sea water |
acidified seawater withhigh Br ionconcentrationhas chlorineand steamadded to it. |
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2 |
The Cl2oxidises Br ionsto Br2. A displacement reaction Cl2(aq) + 2Br-(aq) to Br2(aq) + 2Cl-(aq) Steam evaporates the Br2 from the water with a little unreacted Cl2 |
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3 |
A condenser cools the mixture and bromine and steam liquefy. Liquidbromine and water mix very poorly so most of the water is separated and returned tothe steaming process |
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4 |
Damp bromine (with a little dissolved Cl2) is distilled. The chlorine boilsout and is reused to react with fresh sea water |
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5 |
Last traces of water are removed from the bromine using ConcentratedSulfuric acid as a drying agent. Br2(l) + H2O(l) + 98% H2SO4(l) to Br2(l) + dilute H2SO4 (aq) |
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what is feedstock |
industrial term for chemical reactants |
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what are the typical sequences in a chemical process |
|
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what are the two main ways chemical processes can be carried out |
batch continuous |
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What is the batch process |
Reactants mixed in reaction vessel, reaction is carried out and products removed when reaction is complete. Process then repeated withnext “batch” of reactants. |
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What is the continuous process |
reactants constantly fed in at one end of the plantand products constantly removed at other end. The processing does not stop. |
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pros of batch process |
Better %conversion in agiven time thancontinuous Versatile: severaldifferent reactions canbe carried out using thesame reaction vessel. Plant cheaper:Equipment = largelygeneral purposevessels –can be boughtoff the shelf. Most cost effectivewhen smallamounts of productneeded or for slowreactions |
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cons of batch process |
More difficult tocontrol the releaseof heat (exothermicreactions) Larger workforceneeded More time wasted.If a new process isbeing done in the samevessel, time will be lostcleaning out the vesselcarefully to avoidcontamination. Contamination more likely |
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pros of continuous process |
Best for making ahigh tonnage ofjust one product. Minimal labourrequired. (may notshut down for severalyears) Safer. Avoids theneed to have largeamounts of reactants inone place so easier toremove heat More easilyautomated. (Manyparts of the processcan be run by machine–reliable, less labourcost) |
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cons of comtinous process |
Specially designedplant needed. Oftenvery difficult to adapt itfor other purposes. Initial capital costmuch higher. Settingup specialised plant isvery costly. Only cost effectiveif running at fullcapacity. (productmust be in constantdemand). High contaminationrisk if used to makemore than 1product. Extensive,specialised machinerymuch harder to clean thana single open-shutreaction vessel. |
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what must materials used to construct plants be like |
Materials used to construct the plant must not react with reactants, productsor catalysts. (Often glass ,special alloy or glass reinforced linings used instead of steel) |
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how may feedstocks be prepared before reactions take place |
Purification Getting the feedstocks into the correct ratio of quantities for the reaction. Converting feedstocks into a state that allows them to be easily handled. (e.ggases or liquids best as can be pumped or piped around easily. Solids are costly to move about so theymay be either melted or mixed with a liquid and pumped as a “slurry”). |
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where are inorganic feedstocks usually obtained from |
ores, minerals, sea water or the air. |
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where do organic feedstocks come from |
Cracking of natural gas produces ethene and propene. Distillation of crude oil produces a range of fractions. Cracking producesalkenes. Reforming “naptha” fraction gives aromatic hydrocarbons, branchedalkanes and cycloalkanes( the majority of these are used in unleaded petrol |
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what is co product |
when a process produces more than 1 useful product. The processmay become uneconomical if demand for one of these products falls. |
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what is by product |
non-useful additional products, often made from side reactions.A lot of effort is put into minimising side reactions as far as possible.It is important to make the best use of both co- and by- products. |
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how long does it take to pay off initial setting up costs |
several yearsbefore the sale of products actually balances all these costs and gives anoverall profit |
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what are the two types of costs |
fixed costs variable cost |
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what are fixed costs |
these are independent of the amount of product made.e.g : land purchase, wages, and the capital costs of setting up the plantand surrounding infra structure |
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variable costs |
relate directly to the amount of product made.e.g raw materials, transport of product, waste treatment. |
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what is efficiency |
the best combination of pressure, temperature and rate of mixingavailable must be used |
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pros and cons of high temps and pressure |
need more expensive plant , usemore energy can make reactions more difficult to control but dogive a very high rate |
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efficiency in equilibrium processes |
conditions for the bestyield of products must be considered. A compromise temperature and pressure is often necessary which givesthe best balance of yield and rate.An effective catalyst (if available) greatly reduces costs. |
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how are processes made as effcient as possible to maximise profit? |
-finding / using the optimum conditions for the process. -using suitable catalysts. recycling unreacted feedstock - separate it from products and send it back tothe reactor. effective insulation of piping etc. to reduce energy losses. heat exchangers - use heat released in exothermic parts of theprocess to provide energy for endothermic processes (this may includeproviding hot water and central heating for the factory). The heat is often transferredas heated steam/water around the plant. |
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where are most plants located |
good communications network (i.e easily reached by road, rail or water. Many bulk materials transported bysea). supply of skilled labour in the area. plenty of water available. waste disposal and treatment areas being available. flat, firm land with room for expansion. |
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what do strict safety laws require |
Strict health and safety reviews at all planning stages. Rigorous risk assessments carried out. Special health and safety responsibilities for some employees. Frequent emergency practice drills. |
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what are the principles of green chemistry |
Minimise waste (recycling or finding uses for waste products) Reduce energy consumption (catalysts to reduce temperature,increasing atom economy). Reduce feedstock consumption (e.g reduce number of steps in aprocess) |
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what do laws require for waste disposal |
must be environmental friendly e.g waste added to rivers must be first adjusted for temperature, pHand heavy metal content and its effect on the dissolved oxygen contentof water must be negligible. SO2 from the roasting of ores is converted into H2SO4 in neighbouring factories. The acid can be sold atprofit.Gases containing contaminants can be passed through solutions that remove contaminants –“scrubbers”. E.g NaOH(aq) can scrub air clean of bromine vapour. |
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how do you calculate precentage yield |
actual moles of product divided by x100 Expected moles of product |
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how do you calculate atom economy |
relative formula mass of useful product divide by x100 relative formula mass of all reactants used To get a better idea of waste we use atom economy |
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how can chlorine be manufactured |
Chlorine can be produced by the electrolysis of concentrated solutions ofsodium chloride (“brine”) NaCl. |
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what are the two methods of manufacturing chlorine |
mercury cell membrane cell |
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what are cathodes and annodes |
cathodes are negatively charged annodes are positively charge |
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what is happening to the mercury cell process |
Uses a mercury cathode and is being phased out because of the amount oftoxic mercury lost into the environment. |
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how is chlorine manufactured in the membrane cell 1 |
an electric current is passed through a concentrated solution of sodium chloride |
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2 |
A membrane separates the solutionaround the anode and cathode. Membrane made of PTFE (Teflon) so heatresistant and inert. Negative side chains addedto PTFE allow positive ions to pass through but repel negative Clions. |
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3 |
Na+ ions hydrated with water can passthrough the membrane but ClandOHcannotpass through.Also H2 and Cl2 don‟t get the chance tomix (explosive if they do). |
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4 |
The result is sodium hydroxide and hydrogen gas are formed at thecathode (= negative electrode) and Cl2(g) is formed at the anode (positiveelectrode). These are valuable co-products |
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what are the equations |
Anode: 2Cl-(aq) to Cl2 (g) +2e- Cathode: 2H2O (l) + 2e- to H2 (g) + 2OH-(aq)Oxidation Cl –1to 0 Reduction H +1 to 0 Overall equation:2Cl-(aq) + 2H2O(l) Cl2(g) + H2(g) + 2OH-(aq) |
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what are advantages of membrane cells over mercury cells |
Lower running costs Replacing mercury cells with membrane cells of same size producesmore chlorine. No expensive treatment to remove toxic waste products needed. Less environmental pollution |
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what is the bond polarity like in covalent bonds such as O=O |
covalent bonds are a shared pair of electrons in eg O=O and H-H bonding electrons shared equally |
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What happens If the atoms sharing the bonding electrons are different |
one nucleusmay attract the bonding electrons more strongly than another. |
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what is electro negativity |
Electronegativity is a measure of an atom‟s attraction for bonding electrons.The higher the electronegativity value the more strongly that atom will attractbonding electrons. |
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what are The most electronegative elements in decreasing value |
F,O,Cl,N |
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what are the least electronegative elements |
elements are the metals in the bottom of group 1. |
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what is the electronegativity like in H-F |
there is a significant difference in electronegativity. This means the more electronegative F has a greater share of the bondingelectrons. The greater electron density around F causes it to become slightly negativelycharged (“delta negative” . The H has lost some its share of the bondingelectrons so becomes slightly positive in charge (“delta positive” |
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what are polar bonds |
Bonds where there is a significant electronegativity difference |
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what happens the difference in electronegativity becomes large enough |
forms an ionic bond (where the bonding electrons stay on the negative ion so non metals and metals... |
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what are C-H bonds like? |
effectively non polar |
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what are C-C bonds like? |
completely non polar |
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what must substances have if they are in solid or liquid state? |
forces between particles holding them together |
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what are intermolecular bonds |
bonds between molecules |
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what are intermolecular bonds like if substances have high melting or boiling points |
strong asmore energy will have to be put in to overcomeintermolecular bonds to separate molecules boiling involves breaking all the intermolecular bonds |
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are instantaneous dipoles induced dipoles present in all substances |
yes |
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how do idid bonds form 1 |
Electrons inside a molecule (or noblegas atoms) are constantly moving randomly |
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2 |
Electron movements createuneven distribution of charge (instantaneous dipole). E.g sometimesthere is more electron density on one side of a molecule making that molecule (briefly)slightly negatively charged (δ-). |
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3 |
A dipole is induced in a neighbouringmolecule creating an attraction. |
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4 |
this is an id id attraction id id attractions are the weakest type of intermolecular bond |
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why are id id interactions continusously forming and breaking |
as electron cloud is continuously moving |
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when does an idid bond increase |
atomic mass gets larger (more electrons & outer electrons moreloosely held) Molecules get larger (as molecular mass rises, more electronsagain). The area of contact between molecules rises e.g a straight chainC5H12 pentane will have astronger intermolecular bondthan its branched chain isomer2,2-dimethylpropane. Thepentane molecules‟ straightchains can pack closertogether than chains in thebranch chained isomer. |
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why do idid forces increase as carbon chains get longer |
more molecular surface area so more electrons to interact so as molecules get longer it takes more energy to overcome idid forces and seperate (why polyethene) is solid at room temp |
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why are idid forces weak in branched chain (alkanes) |
branched chain (alkanes) cant pack closely together and have smaller molecular surface areas, so idid forces reduced |
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when are pd pd dipoles created |
significant electronegativity difference betweenatoms bonded together causing bonding electronsto be shared unevenly. slightly positively charged end of molecule attracts slightly negatively charged end of next molecule, and an intermolecular bond occurs |
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strength of bonding |
hydrogen bonding then pdpd the idid (pdpd bonds responsible for holding polyester together) |
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what also affects pd pd bonding |
shape of the molecule In some molecules (e.g CCl4) symmetrical dipoles cancel out each other‟seffects and the molecule is non-polar overall |
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how is fluorin made as needed (in situ) |
it is very reactive, too reactive to store made by electrolysing liquid hydrgoen fluoride |
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how is chlorine transported |
by rail or road tanker as liquid it is a higly toxic gas |
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how is bromine transported |
in lead lined steal tanks supported in strong metal frames |
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how are transport routes planned |
to minimise accident risk |
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uses of fluorine |
Used to make the plastic polytetrafluoroethene (PTFE or “teflon”), usedin non-stick coatings on cookware. HCFC‟s used in air conditioning and refrigeration. sodium fluoride used in toothpaste. |
|
risks of using flourine |
element itself is highly reactive and handling must be kept to a minimum |
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uses of chlorien |
is an importnat intermediate in the manufacture of HCl and chlorinated solvents used in plastic industry (PVC and polurethanes) used in water treatmennt to kill pesticides medicine bleach 2NaOH + Cl2 NaCl + NaClO + H2O |
|
risks of chlorine |
pesticides can accumulate in the environment CFC destroy stratospheric ozone |
|
uses of bromine |
used in flame retardent manufacturing agricultural fumigants and in photography |
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risks of bromine |
organic bromine compounds can destroy stratospheric ozone |
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uses of iodine |
used in antiseptics, germicides and dyes iodine 131 is used to diagnose thyroid disease |
|
further risks of bromine |
volatile liquid that is toxic by inhalation and burns on skin contact. precautions included as mentioned above and Sensors detect any leakage immediately. Air contaminated with Br2 is passed through an alkaline “scrubbing” solution” toprevent Br2 escaping into the atmosphere. 2NaOH(aq) + Br2(g) to NaBr(aq) + NaBrO(aq) +H2O(l) all fairly non-hazardous products. Transported in lead lined steel tanks (lead does not react with Br2, steel does. Outersteel is needed for strength |
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what happens when you electrolyse a solution containing iodide or bromide ions? |
halogen element is released at annode (halide ions lose electrons to annode, and are oxidised at cathode, hydrogen ions from water forms hydrogen gas |
|
what are halogenalkanes |
= hydrocarbon chains in which one or more H‟s are replaced with halogenatoms. homologous series : R-Hal |
|
naming haloalkanes |
alkane chain name is prefixed with halogen name halogens listed in alphabetical order (bromo, chloro, fluoro, iodo) with numbers indicating ppositin and (di, tri, tetra, penta) numbering to give lowest number total overall |
|
what type of bond is a carbon halogen bond? |
polar, but polarity is not high enough to allow them to dissolve in water |
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what happens to boiling point of haloalkanes as you go down the group? |
inreases. atomic number increases, number of electrons increases stronger id id forces more energy needed to overcome forces |
|
why do boiling points increase as length of carbon chain increases? |
stronger idid forces, more electrons |
|
why does boiling point increase when number of halogens on chain rises |
more electrons, so stronger idid forces |
|
what happens to bond enthaply as size of halogen increases? |
bond strength decreases |
|
what happens to reactivity down the group? |
increases |
|
what happens to bond polarity down the group |
decrease (ie C-F, is more polar than C-I) |
|
what do these trends show |
that it is bond strength rather than bond polarity which has the greatest effect on the reactivity of haloalkanes (bromo and iodo compounds are reactive and so are useful as chemical intermediates in chemical synthesis) |
|
what type of reaction occurs when haloalkanes reach the stratosphere (eg CFCs) |
undergo homolytic fission these involve the breaking of the C-halogen bond. In the atmosphere, UV radiation of the correct energy breaks these bonds byhomolytic fission |
|
what happens in heterolytic fission of R-Hal bond |
CH3Cl to CH3+ + Cl-= heterolytic fission of chloromethane.The halogen leaves with both bonding electrons to become a halide ion.The carbon is left with a positive charge. An ion with a positively charged carbon is called acarbocation. CH3+is an example of a carbocation- covered in Polymer revolution PR laterHeterolytic reactions are most likely to happen in polar solvents where radiation is not involved. ) |
|
why can nucleophilic substitution occur in haloalkanes |
carbon to which the halogen is joined has a slight positive charge due tothe polarity of the C-halogen bond. This means that the carbon joined to the halogen can be attacked bynucleophiles. |
|
what is a nucleophile? |
atom/ ion/ molecule that can donate a lone pair ofelectrons to a positively charged carbon atom to form a new covalentbond. |
|
what happens when haloalkanes undergo nucleophilic substitution? |
nucleophile takes the placeof the halogen (substitution) whichleaves as a halide ion. |
|
what do curly arrows show |
show the movement of a pair ofelectrons |
|
Explain what happens |
nucleophile attacks partially positive (δ+) carbonattached to halogen. causes C-Halogen bond to break. Both electrons from C-Hal bond goto the halogen atom which becomes the halide ion (Hal-). R-Hal + Nu- to R-Nu + Hal- |
|
where do you draw the arrows? |
curly arrow showing bond breaking starts in the middle of the bond andfinishes on the halogen atom. curly arrow from the nucleophile starts from the lone pair and finishes at theδ+ carbon. |
|
what are reagents |
chemical(s) added to cause a desired reaction. |
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hydrolysis, reagents, conditions and reaction |
Reagent: Aqueous sodium hydroxideConditions: Heat under reflux (ethanol may be added as a co-solvent). Reaction: R-Hal + OH-(aq) R-OH + Hal- |
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hydrolysis using water |
Here the reaction occurs in two stages as the water needs tolose a proton (H+) after attackto restore the normal numberof two bonds for oxygen. reaction for water: R-Hal + H2O R-OH + H+ + Hal |
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why is this reaction with water slower for making alcohols than the other |
this reaction with water is a much slower way to make an alcohol. The negativelycharged hydroxide ion used in the alkaline hydrolysis will react much faster than the neutralwater molecule, plus only one step is involved in the hydroxide reaction. |
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reaction with ammonia |
Reagent: Concentrated aqueous ammoniaConditions: Heat in a sealed tube! R-Hal + NH3 R-NH2 + H+ + Hal reaction takes place in two stages as aproton (H+) must be lost to restore nitrogen‟s usual number of 3 bonds |
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how can you make a haloalkane? |
reaction of an alcohol with suitable halide ions in the presenceof a strong acid: e.g Chloroalkanes made from suitable alcohol and conc. HCl Bromoalkanes made from suitable alcohol and a mix of sodium bromide andconc.sulphuric acid. Conditions: Heat under Reflux |
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what happens? |
Reaction between the alcohol andthe conc.acid adds a proton onto theO of the OH group. halide ion acts as nucleophile andpushes off a molecule of water. The halogenoalkane and water areimmiscible so a separating funnelcan be used to separate them. Sodium hydrogencarbonateNaHCO3 is used to remove acidicimpurities. (neutralises them to form a sodium salt, water and carbon dioxide) |
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what are drying agents? |
used to remove the last traces of water.Good dryingagents = anhydrous sodium sulphate (or Anhydrous calcium chloride) |
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what are titrations? |
a method of quantitatively finding concetration of a solution by reacting a known volume of soultion with another solution of known concentration |
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how is the end point of reaction detected |
by adding an indicator eg phenolphthalein red to colourless (when adding acid to alkaline, acid base reaction or methyl orange(yellow in akali/ red in acid |
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why dont you use universal indicator |
colour change is too slow; want indicators that change colours quickly in small pH range |
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1st step |
a fixed volume os solution of unkown concentrstion is placed into a conicle flask using a pipete |
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2 |
a few drops of suitable indicator are addedand the mixture placed on a white tile, to see end point more clearly |
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3 |
soltion of known concentration is added slowly from burette with constant swirling |
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4 |
as you approach end point add solution from buretter drop wise |
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5 |
after a rough titration, accurate titrations followed until concordant results obtained |
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how do you calculate concentration |
c=n/v c=moldm to the -3 n=amount of substance in moles v=volume of solution in dm cubed |
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how do you use a seperating funnel and why? |
Used to separate 2 liquids which are immiscible (don‟t mix with each other). Open tap to let the bottom liquid flow out . Close the tap just as the top liquid reaches the tap.(Stopper is only put on when shaking contents) |
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what is refluxing |
Lets you keep a volatile liquid at boiling point for a longtime without evaporation losses. Condenser is mounted vertically. As liquid in flask boils, the vapour rises up into thecondenser Condenser cools vapour back into a liquid. Liquid drips back into the reaction flask and isheated again. |
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what is the good use of the co products of the electrolysis method for making chlorine? |
hydrogen and chlorine to make hydrogen chloride.H2 + Cl2 2HClHCl(g) is dissolved in water to create concentrated hydrochloric acid. Transporting inconcentrated form is most economical (can be diluted later).Making PVC also produces HCl(g), so hydrochloric acid can be a useful co-product of thistoo. |
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how are Many organic halogen compounds found |
naturally occurring in the oceans. These undergo hydrolysis reactions with water and sometimes radical substitution (see Aand PR for details of how these happen) such as 2CH3Cl + I2 + hv 2CH3I + Cl2 - note thisis not an easily predictable displacement – UV/ visible radiation usually needed for thesereactions |