7 Equilibria Flashcards
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
starch blue OR black to colourless towards / close to the end-point of the titration /when the solution goes yellow
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
Cu +4HNO3 → Cu(NO3)2+2NO2+2H2O or ionic Cu +4H+ +2NO3– → Cu2+ +2NO2+2H2O correct species correct balancing
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
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–
Explain what is meant by the term partition coefficient.
Ref: 9701/42/O/N/16
ratio of the concentration of a solute in the (two immiscible) solvents /liquids
at equilibrium
Write an equation to show how compound H, C10H13NH2, behaves as a base.
Ref: 9701/42/F/M/17
C10H13NH2 + H3O+ ⇌ C10H13NH3+ + H2O
Explain the difference in the partition coefficients of butylamine, C4H9NH2 and ammonia between organic solvent, trichloromethane and water.
Kpartition will be larger for butylamine than for ammonia because butylamine contains a hydrophobic/ non polar (C4) group.
