Half life 1 = 38 seconds
Half life 2 = 38 seconds
Half life 3 = 37 seconds
Equal half lives so 1st order reaction with
respect to H2O2
Rate = k [H2O2]
k = Rate/ [H2O2] = 1.61 x 10-5s-1 from
results at 12 seconds.
(a) H2O2 + 2I- + 2H+ = I2 + 2H2O
(b) Rate = k [H2O2] [I- ]
(c) Overall order = 2
(d) First step is the rate determining step
– because it involves one molecule of
hydrogen peroxide and one ion of
iodine reacting and no hydrogen ions to
confirm the rate equation.
(a) Zero order with respect to X – see
experiments 1,4 and 5 – an increase in
the concentration of X does not affect
the reaction rate.
(b) Second order with respect to Y – see
experiments 1 and 2 – doubling the
concentration of X results in a four times
Rate = k [Y]2
Rate = k [Y]2
0.0001 = k (0.1)2
So k = 0.001 mol -1dm3s-1
Y + Y = Y2
‡ + X = XY2
Chemists study rate equations in order to
determine the mechanisms of reactions and
then predict factors that could make
reactions more economically viable.
The sodium thiosulphate reacts with the
iodine which is initially produced and
remove it from the reaction mixture. This
allow a fixed end point of the reaction to
be determined once all the sodium
thiosulphate is used up.
The starch solution detected the end of
the initial reaction and the point at
which iodine is no longer reacting with
the sodium thiosulpate.