8 Reaction kinetics

Question 1

H2CO3 + H2O -> HCO3– + H3O+ Explain how this buffer system acts to control the blood pH. Include equations in your answer. Ref: 9701/41/O/N/16

Answer 1

addition of acid: H+ + HCO3– → H2CO3 OR H+ +HCO3– –> H2O +CO2 addition of base: OH–+ H2CO3 → HCO3–+H2O OR H+ + OH– → H2O and position of eqm moves to the right OR OH–+HCO3– →CO32– + H2O

Question 2

S2O82– + 2I– →2SO42–+I2 Suggest why, in the absence of a catalyst, the activation energy for this reaction is high. Ref: 9701/42/O/N/16

Answer 2

(two) negatively-charged species repel each other

Question 3

Write two equations to show how Fe3+(aq) ions can catalyse the reaction between S2O82–(aq) ions and I– (aq) ions. Ref: 9701/42/O/N/16

Answer 3

Equation 1: 2Fe3+ +2I– → 2Fe2+ +I2 Equation 2: S2O82– +2Fe2+ →2SO42– +2Fe3+

Question 4

The graph shown is a concentration-time graph for a typical reaction.

On the axes below, sketch the curve you would expect for the autocatalysed reaction in (i)

Ref: 9701/42/F/M/17

Answer 4

first section: flatter

second section: steeper, before flattening