10 Marking Scheme: End-Of-Chapter Test

10 Marking scheme: End-of-chapter test

11 Marking scheme: End-of-chapter test

1acorrect axes, suitably labelled[1]
plotting[1]
curve of best fit[1]

bhalf-life method[1]
three half-lives taken and shown to be similar[1]

cgradients[1]
new graph of rate against concentration[1]
plotting[1]
line of best fit[1]

dIt is a straight line[1]
through (0,0).[1]

2acorrect axes, suitably labelled[1]
plotting[1]
line of best fit[1]

bThe gradient, and therefore the rate, is constant, even though the concentration of I2 is changing. [1]

cNo;[1]
there must be a slow step which doesn’t involve iodine.[1]

dThe balanced chemical equation tells us about the numbers of molecules of reactants
consumed and products produced[1]
and their formulae;[1]
the rate equation tells us how many of each reactant molecule[1]
are involved in the slowest step.[1]

3aParticles have more kinetic energyso they collide more frequently;[1]
collisions are more energetic;[1]
there are more collisions with energy exceeding Ea.[1]

bk increases.[1]

4It is the slowest step;[1]
its rate limits the rate of the overall reaction.[1]

5aiThe order of reaction with respect to a particular reactant is the power [1]
to which the reactant’s concentration is raised in the rate equation. [1]

iiTo find the order with respect to A, use experiments 1,2 and 3;[1]

if you double [A], there is no significant effect on the rate;[1]

therefore the reaction is zero order with respect to A.[1]

To find the order with respect to B, use experiments 4, 5 and 6;[1]

if you double [B], the rate increases by a factor of four;[1]

therefore the reaction is second order with respect to B.[1]

To find the order with respect to C, use experiments 7, 8 and 9;[1]

if you double [C], the rate doubles;[1]

therefore the reaction is first order with respect to C.[1]

birate = k[B]2[C][1]

ii3[1]

ce.g. first step is B + B + C  BBC (slow)[1]
then BBC + A  ABC + B (fast)[1]

dk = 0.0079[1]
mol–2 dm6 s–1[1]
three values of k worked out and averaged[1]