kinetics PMT Flashcards
Q4a The student uses a stopwatch to measure the time. The stopwatch shows
each time to the nearest 0.01 s
Suggest why the student records the times to the nearest second and not
to the nearest 0.01 s
it is hard to judge to the nearest 0.01 second
Q9a Explain why the use of a large excess of H2O2 and I– means that the rate of
reaction at a fixed temperature depends only on the concentration of
H+
(aq).
H2O2 and/or I– concentration (effectively) constant
so have a constant/no effect on the rate
Q9b Samples of the reaction mixture are removed at timed intervals and titrated
with alkali to determine the concentration of H+
(aq).
State and explain what must be done to each sample before it is titrated
with alkali.
Stop the reaction / quench
By dilution / cooling / adding a reagent to react with H2O2/I
Q9g A general equation for a reaction is shown.
A(aq) + B(aq) + C(aq) → D(aq) + E(aq)
In aqueous solution, A, B, C and D are all colourless but E is dark blue.
A reagent (X) is available that reacts rapidly with E. This means that, if a
small amount of X is included in the initial reaction mixture, it will react with
any E produced until all of the X has been used up.
Explain, giving brief experimental details, how you could use a series of
experiments to determine the order of this reaction with respect to A. In
each experiment you should obtain a measure of the initial rate of reaction
Stage 1 Preparation
1a Measure (suitable/known volumes of) some reagents
(ignore quoted values for volume)
1b Measure (known amount of) X / use a colorimeter
1c into separate container(s) – (allow up to two reagents and X
measured together into one container); reference to A, B or C added
last. NOT if X added last.
Stage 2 Procedure
2a Start clock/timer at the point of mixing (don’t allow if only 2 reagents
mixed)
(allow even if X not added or added last)
2b Time recorded for appearance of blue colour/specific reading on
colorimeter/disappearing cross
2c Use of same concentration of B and C / same total volume / same
volume/amount of X
2d Same temperature/use water bath
2e Repeat with different concentrations of A (can be implied through
different volumes of A and same total volume)
Stage 3 Use of Results
3a 1/time taken is a measure of the rate
3b plot of 1/time against volumes/concentrations of A or plot log(1/time)
vs log(volume or concentration of A)
3c description of interpreting order from shape of 1/time vs volume or
concentration graph / gradient of log plot gives order / allow
interpretation of time vs concentration graph / ratio between change
in concentration and change in rate (e.g, 2x[A] = 2 × rate so 1st orde
Q10b Explain the effect that lowering the temperature would have on the rate of
reaction.
Fewer particles will have energy greater than or equal to the activation
energy
1
Fewer successful collisions in a given time
Q10c Explain the effect that this change has on the rate of reaction (amount of gas present has been reduced)
Rate of reaction decreases (no mark)
Particles are spread further apart
Fewer collisions between gas particles so fewer successful collision
Q17d The rate equation for a reaction is
rate = k[E]
Explain qualitatively why doubling the temperature has a much greater
effect on the rate of the reaction than doubling the concentration of E.
Reaction occurs when molecules have E>Ea
Doubling T by 10 °C causes many more molecules to have this E Whereas doubling [E] only doubles the number with this E
