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28 Cards in this Set
- Front
- Back
In all chemical reactions, bonds are broken and formed all the time. What does it mean when bonds are broken?
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The breaking of bonds require energy which makes the reaction endothermic
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In all chemical reactions, bonds are broken and formed all the time. What does it mean when bonds are formed?
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The formation of new bonds releases energy making this process exothermic.
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Define Heat change
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It is the difference between the quantity of heat needed to break the bonds of the reactants and that evolved during the formation of new bonds in the products.
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What is meant by the enthalpy change of a reaction?
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It is the heat change at constant pressure
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what is the formula for the enthalpy change?
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(Total enthalpy change in the products) - (total enthalpy change in the reactant)
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What is the enthalpy change for exothermic reaction?
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Negative
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What is the enthalpy change for endothermic reactions?
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Positive
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What happens in an exothermic reaction?
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It gives out heat to the surrounding. The surrounding heat rises as the heat content of the system falls
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What happens in an endothermic reaction?
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It absorbs heat from the surrounding. The surrounding temperature falls as the heat content of the system rises
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Give an example of a exothermic reaction
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NaOH(aq) + HCl(aq) ----> NaCl(aq) + H2O(l)
ΔH=-56.7KJ/mol |
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Give an example of an endothermic reaction
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CaCO3(s) ----> CaO(s) + CO2(g)
ΔH=+572KJ/mol |
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Define what is meant by specific heat capacity of a substance
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It is the quantity of heat required to raise its temperature by 1Kelvin
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What is the formula for heat change?
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ΔQ=mcΔT
ΔQ=CΔT ΔQ= hear change/J ΔT= change in temp/k C= specific heat capacity |
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Define Standard enthalpy change of reaction
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It is the enthalpy change when amounts of reactions, as shown in the reaction equation, react together under standard conditions
2H2(g) + O2 ---> 2H2O(l) ΔH= -575KJ/mol N2(g) + 3H2(g) ---> 2NH3(l) ΔH= -100KJ/mol N2(g) + O2(g) ---> 2NO(g) ΔH= +180KJ/mol |
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Define Standard enthalpy change of Formation
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It is the Enthalpy Change when one mole of substance is formed from its constituent elements under standard conditions
Na(s) + O2(g) ---> NaCl(s) It is the enthalpy change when amounts of reactions, as shown in the reaction equation, react together under standard conditions 2H2(g) + O2 ---> 2H2O(l) ΔH= -575KJ/mol N2(g) + 3H2(g) ---> 2NH3(l) |
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Define Standard enthalpy change of Combustion
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It is the enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions
C2H5OH(l) + 3O2(g) ---> 2CO2(g) + 3H2O(l) ΔH= -1368KJ/mol |
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Define Standard enthalpy change of Hydration
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Standard enthalpy change of an ion is the energy released when one mole of gaseous ion is hydrated under standard conditions
X+(g) + aq --->X+(aq) Y-(g) + aq ---> Y-(aq) |
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Define Standard enthalpy change of Solution
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It is the enthalpy change when one mole of substance is completely dissolved in a sufficiently large volume of solvent under standard conditions such as the addition of more solvent does not cause any enthalpy change.
NaCl(s) + aq ---> Na+(aq) + Cl-(aq) |
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Define Standard enthalpy change of Atomisation
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It is the enthalpy change when a substance decompose to form one mole of gaseous atoms. This reaction is always exothermic
Na(s) ---> Na(g) 1/2Cl2(g) ---> Cl(g) atomisation energy is half of the bond energy. Bond energy can be obtained from the data booklet |
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Define Standard enthalpy change of Neutralisation
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It is the enthalpy change when an aqueous acid and aqueous base react together to form one mole of water under standard conditions.
NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(l) ΔH= -56.7KJ/mol |
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What is the relationship between standard enthalpy change of solution, hydration and lattice dissociation energy?
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ΔHsol = ΔHhyd + ΔHLDE
If ΔHhyd>ΔHLDE then ΔHsol = negative (soluble) If ΔHhyd<ΔHLDE then ΔHsol= positive (insoluble) |
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State Hess's Law
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Hess's law states that the enthalpy change in a reaction is the same regardless of the route taken by which a chemical change occurs provided that the initial and final conditions are the same.
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What is the Born-Haber cycle?
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It is a technique for applying Hess's law to the standard enthalpy changes which occur when ionic compounds are formed.
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Vaporisation of sodium
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Na(s) ---> Na(g) ΔHatm
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Ionisation of Sodium
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Na(g) ---> Na+(g) + e- ΔHIE
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Dissociation of chlorine molecule
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1/2Cl2(g) ---> Cl(g) ΔHatm
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Electron affinity of Cl atom
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Cl(g) + e- ---> Cl-(g) ΔHea
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Reaction between Na+ and Cl- ions
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Na+(g) + Cl-(g) ---> NaCl(s) ΔHLDE
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