7 Equilibria

Question 1

An indicator was used in the thiosulfate-iodine titration. (i) Name a suitable indicator for this titration. [1] (ii) State the expected colour change you would observe at the end-point in this titration. from …………………..to …………………… [1] (iii) State when in the procedure you would add the indicator. [1] Ref: 9701/41/O/N/16

Answer 1

starch blue OR black to colourless towards / close to the end-point of the titration /when the solution goes yellow

Question 2

Copper is used to make alloys such as brass. The percentage of copper in a sample of brass can be determined by dissolving the sample in concentrated nitric acid and reacting the mixture with potassium iodide. The resulting solution is then titrated. A 1.75 g sample of the brass was dissolved in excess concentrated nitric acid. The reaction of the copper metal in the brass with the concentrated nitric acid released a brown gas and formed a green-blue solution. Write an equation for this reaction Ref: 9701/42/O/N/16

Answer 2

Cu +4HNO3 → Cu(NO3)2+2NO2+2H2O or ionic Cu +4H+ +2NO3– → Cu2+ +2NO2+2H2O correct species correct balancing

Question 3

Write two equations to show how a solution containing a mixture of H2PO4 – and HPO4 2– acts as a buffer.

Ref: 9701/42/O/N/16

Answer 3

addition of acid:

H+ +HPO4 2– Æ H2PO4 –

OR

H+ + H2PO4 – Æ H3PO4

addition of base:

HO–+ H2PO4 – ÆHPO42– + H2O

OR

OH–+HPO42– Æ H2O +PO43–

Question 4

Explain what is meant by the term partition coefficient.

Ref: 9701/42/O/N/16

Answer 4

ratio of the concentration of a solute in the (two immiscible) solvents /liquids

at equilibrium

Question 5

Write an equation to show how compound H, C10H13NH2, behaves as a base.

Ref: 9701/42/F/M/17

Answer 5

C10H13NH2 + H3O+ ⇌ C10H13NH3+ + H2O

Question 6

Explain the difference in the partition coefficients of butylamine, C4H9NH2 and ammonia between organic solvent, trichloromethane and water.

Answer 6

Kpartition will be larger for butylamine than for ammonia because butylamine contains a hydrophobic/ non polar (C4) group.