Practical 7 - study of an 'iodine clock' reaction

Question 1

what’s the aim of this practical?

Answer 1

to study the kinetics of the oxidation of iodide ions by hydrogen peroxide in acid solution

Question 2

What’s the issue with H2SO4?

Answer 2

irritant

Question 3

Which solution used in this practical is the irritant?

Answer 3

H2SO4

Question 4

What would we use to measure volumes of 10cm3?

Answer 4

measuring cylinders

Question 5

What would we use to measure volumes of 25cm3?

Answer 5

volumetric pipette and filler

Question 6

Why do we add water to the hydrogen peroxide?

Answer 6

so that the volume added is 5cm3 every time

Question 7

The volumes of which solutions do we keep constant throughout this practical?

Answer 7

H2SO4
Na2S2O3
KI

Question 8

How many different concentrations of peroxide should we test and why?

Answer 8

At least 3 for a good spread of results

Question 9

What must the total volume of not exceed 5cm3?

Answer 9

H2O2 solution and deionised water

Question 10

When do we add the hydrogen peroxide in this practical?

Answer 10

after adding everything else, including the starch

Question 11

What’s the method for this practical?

Answer 11

1. decide what volumes of H2O2 solution and deionised water you will mix together to get 5 different concentrations of H2O2
2. in separate conical flasks (250cm^3), make up solutions according to the table (same volume of H2SO4, Na2S3O3 and KI and chosen volumes of H2O and H2O2 that must not exceed 5cm^3 together). DO NOT add hydrogen peroxide yet
3. add 1cm^3 starch solution to each flask and mix thoroughly
4. rapidly add the hydrogen peroxide to flask 1, starting the stopwatch immediately after this addition
5. stop timing when the solution turns from transparent to blue-black and record the time
6. repeat the experiment for flasks 2-5

Question 12

Equation for calculating a new concentration

Answer 12

C1V1 = C2V2

Question 13

What do the symbols mean in C1V1 = C2V2?

Answer 13

C1 - staring concentration
V1 - starting volume
C2 - final concentration
V2 - final volume

Question 14

What type of graph do we plot and what will it show?

Answer 14

Rate v.s concentration of peroxide
straight line graph

Question 15

why does a graph of rate v.s concentration of peroxide provide a straight line?

Answer 15

directly proportional

Question 16

What must also be kept constant in this practical except temperature? why?

Answer 16

concentrations and volumes of other reagents involved in the reaction
results collected are only affected by a change in the concentration of the hydrogen peroxide

Question 17

how can we investigate the effect of the concentration of another reagent using the iodine clock reaction?

Answer 17

keep the concentration and volume of other reagents, including hydrogen peroxide, constant

Question 18

Why is it important to keep the temperature constant during an iodine clock reaction and why does this occur?

Answer 18

small changes in temperature can cause significant changes in the results
higher temperatures = higher kinetic energy of particles = greater number of particles with an energy greater than the activation energy = greater chance of successful collisions

Question 19

how can we investigate the effect of temperature using an iodine clock reaction?

Answer 19

keep the concentrations and volumes for all reagents constant

Question 20

Why is the concentration of hydrogen peroxide directly proportional to the rate of reaction?

Answer 20

increased concentration of H2O2
increases the number of H2O2 particles in a given solution
increases the number of collisions per second
increases the number of SUCCESSFUL collisions per second

Question 21

Why is the concentration of hydrogen peroxide directly proportional to the rate of reaction?

Answer 21

increased concentration of H2O2
increases the number of H2O2 particles in a given solution
increases the number of collisions per second
increases the number of SUCCESSFUL collisions per second