Calculations And Procedure

Question 1

Gravimetric analysis

Answer 1

The mass of an element or compound present in a substance is determined by changing that substance into another of known chemical composition that can be readily isolated , purified and weighed

Question 2

To use gravimetric analysis we need:

Answer 2

The equation

The reaction to proceed to completion

Question 3

The accuracy of gravimetric analysis depends on…

Answer 3

The dexterity of the person carrying out the procedure

Question 4

The two types of gravimetric analysis

Answer 4

Precipitation/ filtration

Heating (volatilisation)

Question 5

Precipitation/ filtration

Answer 5

The substance undergoes a precipitation reaction which is then separated, washed and dried to constant mass. The filtrate is tested to ensure the reaction has gone to completion
Must be carried out carefully to ensure all material is transferred and all apparatus dried and weighed with extreme care

Question 6

For precipitation/ filtration the product must have

Answer 6

Low solubility so that all the product is precipitated
Particle size large enough for filtration
Be stable at temperatures of 100-105°C so that it can be dried in an oven

Question 7

Heating (volatilisation)

Answer 7

Involves heating to change one substance into another

E.g dehydration of a hydrated salt to calculate the number of miles of water of crystallisation

Question 8

Phase equilibriums

Answer 8

A solute is added to two immiscible liquids and the solute is soluble in both, then some of the solute will dissolve in both liquids. It will distribute itself in a definite ratio which we call the partition co-efficient

Question 9

Partition co-efficient

Answer 9

A measure of the distribution of a solute between two immiscible solvents and is simply a specific case of equilibrium

Question 10

The size of a partition co-efficient will vary as it depends upon

Answer 10

The solute
The nature of the two immiscible liquids
The temperature

Question 11

The partition co-efficient is independent of…

Answer 11

The amount of solute

The volume of solvent

Question 12

Partition co-efficient can be calculated with

Answer 12

[solutex]solvent B(top layer)
K= —————————————
[solutex]solvent A(bottom layer)

Question 13

What are the solvent requirements for phase equilibria

Answer 13

The solvent used must be immiscible with the liquid mixture or soloution
The solute must be more soluble in one of the layers
The solvent must be volatile which allows the solute to be obtained by evaporation of the solvent
The solvent must be interactive with the solute

Question 14

Most organic solvents are less dense in water so…

Answer 14

Will be on the top layer (except chloroform/ dichloromethane)

Question 15

Vacuum filtration

Answer 15

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 filtering medium depends on the quantity and nature of the precipitate

Question 16

Recrystallisation

Answer 16

A laboratory technique to purify solids based upon solubility.
The impure compound is dissolved gently in the minimum volume of hot solvent then filtered to remove the insoluble impurities. The filtrate is allowed to cool slowly to force crystallisation. The more soluble impurities are left behind in the solvent

Question 17

For recrystallisation must be carefully selected so…

Answer 17

The impure compound is insoluble at lower temperatures

Question 18

Refluxing

Answer 18

Heat energy is applied to a chemical reaction mixture over a period of time. The liquid is placed in a round bottomed flask along with anti-bumping granules with a condenser at the top. The flask is heated vigorously over the course of the chemical reaction; any vapours given off are immediately returned to the reaction vessel as liquids when they reach the condenser

Question 19

Melting point of a crystalline substance

Answer 19

A sharp melting point falling within a very small temperature range

Question 20

The presence of impurities in organic compounds ….

Answer 20

Lowers the melting point and extends the temperature range

Question 21

Since impurities lower the melting point, the techniques of mixed melting point determination can be used…

Answer 21

As a means of identifying the product of a reaction

Question 22

Colorimetry

Answer 22

Uses the relationship between colour intensity of a solution and the concentration of the coloured species present.
Colorimeter Is used to measure the absorbent light of a series of solutions of known concentration. this data is then used to construct a calibration curve

Question 23

Distillation

Answer 23

A separation technique used to separate two liquids. In organic chemistry it is a powerful tool used to be identification and purification organic compounds. The boiling point as well defined and so we can use this physical property to identify an extract compounds
Used to purify a compound my nonvolatile or less volatile compound

Question 24

Chromatography

Answer 24

Separate substances by making use of the differences in polarity or molecular size
Depends upon the protection of substances between two phases one stationary and one mobile
The separation occurs because the substances in the mixture have different partition coefficient between the stationary and mobile phases

Question 25

Paper chromatography

Answer 25

Involves spotting a piece of paper with the sample mixture which is to be separated The bottom of the paper is then placed in a pool of solvent what is drawn up the paper This is the mobile phase and the water held in the paper as the stationary phase When the paper is removed from the solvent the various parts of the mixture it will have moved different distances along paper

Question 26

In chromatography what factors does the separation rely on

Answer 26

Solvent rate of flow of solvent over the paper solubility of the compounds in the mixture and in the solvent the partition of the components between the moving solvent and the water bound to the surface of paper

Question 27

And chromatography for each of the components in the mixture the Partition coefficient remains....

Answer 27

Constant | It is the difference between the partition coefficient of each component that separates them

Question 28

Rad value=

Answer 28

distance travelled by component ——————————————— Distance travelled by solvent

Question 29

Thin-layer chromatography

Answer 29

Similar to Paper chromatography but uses a fine film of silica or aluminium oxide spread over glass or plastic Small sample of the mixture being tested is spotted onto the baseline The solvent used dissolves the compounds in the spot and carries them up the chromatogram The distance travelled to depend upon how soluble compounds are in the chosen solvent and how well they adhere to the plate

Question 30

What is the order of the reaction

Answer 30

The no of moles involved in the rare determining step

Question 31

A chemical reaction is said to be in equilibrium when

Answer 31

the composition of the reactants and products remaining constant indefinitely

Question 32

Equilibrium can only occur in

Answer 32

A closed system

Question 33

Closed system

Answer 33

One which allows energy to be transferred to and from the surroundings but not from the reactants or the products

Question 34

Hertogeneous equilibrium

Answer 34

All species are in more than one phase

Question 35

Homogeneous equilibrium

Answer 35

All the species are in the same phase

Question 36

The value of an equilibrium constant gives

Answer 36

And indication of the extent of reaction once it is reached equilibrium

Question 37

When the value of Kc is <10^-3

Answer 37

Effectively no reaction

Question 38

When the value of Kc is 10^-3 to 10^3

Answer 38

Significant quantities of the reactants and products at equilibrium

Question 39

When the value of Kc is >10^3

Answer 39

The reaction is affectively complete

Question 40

Changing the concentration of either a product or reactant and equilibria mixture will affect the

Answer 40

Equilibrium position but not the equilibrium constant

Question 41

Changing the temperature of an equilibria mixture will affect

Answer 41

The equilibrium constant and the equilibrium position

Question 42

For endothermic reaction is a rise in temperature causes

Answer 42

An increase in equilibrium constant

Question 43

For an exothermic reaction arise and temperature causes

Answer 43

A decrease in the equilibrium constant

Question 44

What is the effect of a catalyst on the equilibrium mixture

Answer 44

A catalyst does not affect the equilibrium position that merely affects the rate at which equilibriums achieved It will speed up the forward and reverse reaction by the same extent until the equilibrium constant is not affected

Question 45

What affect what changing pressure have on an equilibriums mixture

Answer 45

If the pressure of an equilibriums has changed the system will adjust itself to minimise that change and so the equilibrium constant is not effected If they’re equal numbers of gaseous molecules products and the alteration of the pressure does not affect the equilibrium mixture