sciemnce

Mandatory Experiment 6.2

Studying the effects on reaction rate of (i) concentration and (ii) temperature
Theory
The reaction used is that between a sodium thiosulfate solution and hydrochloric acid:
2HCl(aq) + Na2S2O3(aq)

→ 2NaCl(aq) + SO2(aq) + S(s)↓ + H2O(l)

The precipitate of sulfur formed gradually obscures a cross marked on paper and placed beneath the reaction flask. The rate of reaction, and consequently the time taken to obscure the cross, depends on a number of variables such as temperature, concentration and volume. By varying one of these and keeping the others constant, the effect on rate can be studied.
The inverse of the time taken to obscure the cross is the measure of reaction rate used in this experiment.
(i) Effect of concentration
Chemicals and Apparatus
0.l M sodium thiosulfate solution
Dilute hydrochloric acid (3 M)
Graduated cylinders

Stop clock
Conical flasks
White paper

Procedure
NB: Wear your safety glasses.
1. Place 100 cm3 of the sodium thiosulfate solution into a conical flask.
2. Add 10 cm3 of 3 M hydrochloric acid to the flask, while starting the stop clock at the same time.
3. Swirl the flask and place it on a piece of white paper marked with a cross.
4. Record the time taken for the cross to disappear.
5. Repeat the experiment using 80, 60, 40 and 20 cm3 of. sodium thiosulfate solution respectively. In each case, add water to make the volume up to 100 cm3 and mix before adding HCl.
6. If the initial sodium thiosulfate concentration is 0.1 M, subsequent concentrations will be 0.08 M,
0.06 M, 0.04 M and 0.02 M respectively.
7. Record the results in results table.
8. Draw a graph of 1/time against concentration. This is effectively a graph of reaction rate against concentration.

Results:
Concentration of thiosulfate
Reaction time in seconds
1/time

0.l M
43
0.0233

0.08 M
52
0.0192

0.06 M
71
0.0141

0.04 M
114
0.0088

0.02 M
270
0.0037

(ii) Effect of temperature
Chemicals and Apparatus
0.05 M sodium thiosulfate solution
Dilute HCl (3 M)
Conical flasks
Graduated cylinder
Stop clock

White paper
Thermometer
Bunsen burner
Tripop
Wire gauze

Procedure
NB: Wear your safety glasses.
1. Place 100 cm3 of 0.05 M sodium thiosulfate solution into a conical flask.
2. Warm the flask gently until the temperature is about 20 0C.
3. Add 5 cm3 of 3 M HCl, starting a stop clock at the same time, before proceeding.
4. Without delay, swirl the flask, place it on a piece of white paper marked with a cross, and record the exact temperature of the contents of the flask.
5. Record the time taken for the cross to disappear
6. Repeat the experiment, heating the thiosulfate to temperatures of approximately 30 0C, 40 0C, 50
0
C and 60 0C respectively (before adding the HCl).
7. Record the results in results table.
8. Draw a graph of 1/time against temperature. This is effectively a graph of reaction rate against temperature. Results:
Temperature (0C)

19oC

Graph:

38 oC

51 oC

60 oC

105
0.0095

Reaction time in seconds
1/time

32 oC
46
0.0217

36
0.0278

18
0.0556

12
0.0833

Effect of Temperature on Rate of Reaction
0.1

0.09

0.08

0.07

1/Time

0.06

0.05

0.04

0.03

0.02

0.01

0
0

10

20

30

40

50

60

70

Temperature

Questions relating to the experiments
1. What is the effect of increasing the concentration on the reaction time?
2. What is the effect of increasing the concentration on the reaction rate?
3. What is meant by saying that two quantities are directly proportional?
4. What is the effect of raising the temperature on the reaction time?
5. What is the effect of raising the temperature on the reaction rate? Suggest two factors responsible for the result observed.
6. Suggest a reason why it is not recommended to carry out the experiment at temperatures higher than about 60 0C.
7. Which is the limiting reactant in the temperature experiment?

Suggested Answers to Student Questions
1. What is the effect of increasing the concentration on the reaction time?
The reaction time is decreased.
2. What is the effect of increasing the concentration on the reaction rate?
The rate is increased.
3. What is meant by saying that two quantities are directly proportional?
If one of the quantities is increased/decreased by a certain factor, the other changes in exactly the same way.
4. What is the effect of raising the temperature on the reaction time?
The reaction time is decreased.
5. What is the effect of raising the temperature on the reaction rate? Suggest two factors responsible for the result observed.
The rate is increased. The higher temperature results in greater kinetic energy of the particles present. This causes:
(i)
more collisions per unit time, and
(ii)
a greater proportion of the collisions to have the activation energy needed for products to form.
Both (i) and (ii) result in a rate increase.
6. Suggest a reason why it is not recommended to carry out the experiment at temperatures higher than about 60 0C.
The reaction occurs so quickly that it is not possible to measure the time accurately.
7. Which is the limiting reactant in the temperature experiment?
100 cm3 of 0.05 M Na2S2O3 contains:
100/1000 x 0.05 = 0.005 moles Na2S2O3
5 cm3 of 3 M HCl contains:
5/1000 x 3 = 0.015 moles HCl
According to the balanced equation, the reacting ratio is Na2S2O3 : HCl = 1:2
The amounts used are in the ratio
Na2S2O3 : HCl = 0.005: 0.015 = 1 : 3
Clearly Na2S2O3 is the limiting reactant.