RP7- Rate of Reaction by Initial Rate Method

Question 1

Iodide ions are oxidised to iodine by hydrogen peroxide in acidic conditions.
H2O2(aq) + 2H+ (aq) + 2I– (aq) → I2(aq) + 2H2O(l)
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. (2)

Answer 1

• stop the reaction
• by dilution/ cooling/ adding a reagent to react w/ H2O2 or I2

Question 2

After cisplatin enters a cell, one of the chloride ligands is replaced by a water molecule to form a complex ion, B.
During the experiment the concentration of cisplatin is measured at one-minute intervals.
Explain how graphical methods can be used to process the measured
results, to confirm that the reaction is first order. (3)

Answer 2

1. plot conc against time + take tangents –> calculate rate form gradient
2. plot rate against conc of cisplatin
3. straight line thru origin = 1st order