Advanced Higher Chemistry Unit 3

Front 1

What is Gravimetric analysis

Back 1

Gravimetric analysis is a quantitative determination of an analyte based on the mass of a solid.

The mass of the analyte present in a substance can be determining by changing that substrate into another substrate of known chemical composition and farmula. The precipitate needs to be readily isolated, purified and weighed.

The final product has to be dried completely which is done by heating to constant mass. This involves heating the substance, allowing it to cool in a dessicator. This process of heating, drying and weighing is repeated until a constant mass is obtained. This shows that all the water has been driven off.

Front 2

What is meant by "heating to constant mass"

Back 2

Heating the compound repeatedly until the mass does not change

Front 3

Why is a dessicator used

Back 3

To prevent the compound from absorbing moisture, keeping it dry

Front 4

Definition of "weigh approximately.."

Back 4

The mass of the substance must be in the region of (mass) and the exact mass of the substance must be known.

Front 5

Why is gravimetric analysis more suitable

Back 5

An insoluble product is formed (precipitation)

Front 6

Advantage of gravimetric analysis

Back 6

Impurities can be clearly seen so a correction can be made

Front 7

Disadvantage of gravimetric analysis

Back 7

Time consuming

Front 8

What is volumetric analysis

Back 8

Volumetric analysis is the use of a standard solution to determine the concentration of another solution. The procedure is known as titration and can be a standard, back or complexometric titration.

Front 9

What is a standard solution

Back 9

A standard solution is a solution of an accurately known concentration. Can be prepared directly from a primary standard.

Front 10

What is a primary standard

Back 10

A primary standard has the following characteristics:

- a high state of purity >99.9 %

-very soluble

-non-volatile

-high molecular mass

- stability in water and air

Front 11

Examples of a primary standard

Back 11

- sodium carbonate

-Oxalic acid

Sodium hydroxide cannot be used as a primary standard as it absorbs moisture and has a low molecular mass

Front 12

How is a standard solution prepared

Back 12

-calculate the mass of the primary standard required to make the concentration of the solution required in the appropriate volume of solution.

-weigh the primary standard as accurately as possible

-Dissolve the primary standard in a small volume of deionised water in a beaker.

-Transfer the solution and all the rinsing to a standard flask.

-make the solution up to the graduation mark with some more deionised water

-invert the stoppered flask several times to ensure through mixing.

Front 13

Repeating titrations

Back 13

A titration must be repeated until concordant results are obtained (within 0.1 cm3 of each other).

Front 14

What is back titration

Back 14

Involves the addition if excess reagent and then titrating to determine the concentration of the excess reagent

Front 15

What is complexometric titration

Back 15

Based on reactions in which complexes are formed. EDTA is an important complexometric reagent and can be used to determine the concentration of metal ions

Front 16

Why is a control used

Back 16

A control validates technique and may consist of carrying out a determination on a solution of an accurately known concentration.

Front 17

What is distillation

Back 17

Distillation is a powerful tool for both the identification and purification of a compound. The boiling point of a compound-determined by distillation- is well defined and is thus one of the physical properties by which it can be identified. Distillation is used to purify a compound by separating it from a non-volatile or less volatile material

Front 18

What is refluxing

Back 18

Refluxing is a technique used to apply heat energy to a chemical reaction over an extended period of time. The liquid reaction mixture is paced in a round bottomed flask with anti-bumping granules with a condenser connected at the top. The flask is heated vigorously over the course of the chemical reaction and any vapour given off are immediately returned back to the reaction vessel as liquids when they reach the condenser.

Front 19

Why are anti-bumping granules uses

Back 19

To spread the heat evenly

Front 20

What is vacuum filtration

Back 20

Using a buchner, hirsch or sintered glass funnel, these methods are carried out under reduced pressure and provide a faster means of separating the precipitate from the filtrate. The choice of filtrate medium depends on the quantity and nature of the precipitate

Front 21

What is recrystallisation

Back 21

A laboratory technique used to purify solids based upon solubility. The solvent for recrystallisation must be carefully selected such that the impure compound is insoluble at low temperatures, yet completely soluble at high temperatures. The impure compound is dissolved gently in the minimum volume of hot solvent and then filtered to remove insoluble impurities. The filtrate is allowed to cool slowly to force crystallization. The more soluble impurities are left behind in the solvent.

Front 22

Use of a separating funnel

Back 22

Solvent extraction can be an application of the partition of a solute between two liquids. It is based on the relative solubility of a compound in two different immiscible liquids, usually water and an organic solvent. The partition coefficient is express as the concentration of a solute in the organic layer over that in the aqueous layer. The two solvents form two separate layers in the separating funnel and the lower layer is run off into one container and the upper lower is poured into another container. The quantity of of solute extracted depends on the partition coefficient and the number of times the process is repeated.

Front 23

Thin layer chromatography

Back 23

Instead of using chromatography paper, TLC uses a fine film of silica or aluminum oxide spread over glass or plastic. Rf values can be calculated and under similar conditions the compound will always have the same Rf value within experimental error.

Rf value = distance travelled be the sample/ distance travelled by the solvent

Since a pure substance will show up as only one spot on the developed chromatogram, TLC can be used to assess the purity of a product prepared in the lab

Front 24

Melting and mixed melting point

Back 24

The melting point if an organic compound is one of several physical properties by which it can be identified. A crystalline substance has a sharp melting point falling within a very small temperature range. Determination of the melting point can also give an indication of the purity of an organic compound, as the presence of impurities lowers the melting point and extends its melting temperature range. Since impurities lower the melting point, the technique of mixed melting point determination can be used as a means of identifying the product of a reaction.

Front 25

How to improve titrations

Back 25

-Repeat the titration until concordant results are obtained

-Titrate a larger sample

-Dilute the standard solution

Front 26

Why was the filtrate tested

Back 26

-to check the reaction is complete

- to check that no more precipitate is formed

(If there is a precipitate, the reaction is not complete)

Front 27

Why is volumetric analysis more suitable than gravimetric analysis

Back 27

-Gravimetric method would produce too little/ no precipitate

-Titrations can be used with lower (chloride) concentrations

Front 28

Refluxing: explain why only one layer was observed when the reaction was complete

Back 28

- the (named) products are soluble/ have dissolved

-There are no reactants left

-There is only product left

Front 29

Factors that should be considered when selecting an appropriate solvent for recrystallisation

Back 29

-doesn't react with solute

-being more soluble in the hot solvent than in the cold

-Boiling point

-Polarity