10.3

Question 1

10.3 Rates – Activation energy

Answer 1

Potassium manganate(VII), KMnO4(aq), reacts with ethanedioc acid, H2C2O4(aq), to produce a brown colour. 
The rate equation for this reaction is shown below.

rate = k[KMnO4(aq)][H2C2O4(aq)]

The rate of this reaction can be followed by observing the formation of the brown colour.

From the start of each experiment, you will time how long it takes for the brown colour to appear. The initial rate of the reaction can be determined from this time.

In this experiment, you will measure this time at several different temperatures.
You will then use the Arrhenius equation to determine the activation energy, Ea, for the reaction.

The Arrhenius equation shows how the rate constant, k, of a chemical reaction varies with temperature T.

Arrhenius equation: k = Ae–Ea/RT

• k: rate constant
• A: pre-exponential factor (a factor that includes collision frequency and orientation of collision)
• Ea: activation energy
• R: ideal gas constant
• T: temperature (in K)

Question 2

Aqueous potassium manganate(VII), KMnO4(aq), 0.0200 mol dm–3

Answer 2

Currently no hazard classification at this concentration

Question 3

Aqueous ethanedioic acid, H2C2O4(aq), 0.500 mol dm–3

Answer 3

WARNING

Causes serious eye irritation

Question 4

Important Notes:

Answer 4

• The following procedure should be carried out at five different temperatures. The temperatures should be monitored carefully and if the temperature increases or decreases during the reaction then the average of the starting temperature and the final temperature should be taken as the reaction temperature. 

• In order to achieve thermal equilibrium, the conical flask and test tube should be placed in the water bath for at least five minutes before mixing. 


Question 5

Suggested Temperatures: 


•

Answer 5

Choose five temperatures from 0 °C to 50 °C and set up appropriate water baths. Run the first experiment at room temperature to get used to the procedure and colour change.
• To make water baths at temperatures below room temperature, you will have to mix ice and water, in plastic containers
• Above room temperature you can use a water bath, or if unavailable you will need to heat water and then transfer to the plastic container.
• Alternatively, select your own method for obtaining the correct temperatures.

Question 6

produruw

Answer 6

1. Add 20 cm3 of 0.500 mol dm–3 H2C2O4 to a conical flask and 10 cm3 of 0.0200 mol dm–3 KMnO4 to a test tube. 

2. Place both containers in your water bath for about five minutes. A test tube holder may be useful to prevent the test tube falling over. While you are waiting, prepare the solutions for the second run of this experiment at this temperature. 

3. If you are doing the reaction at 0 °C then a single experiment is enough as the reaction can take up to 30 min.
4. Mix the reactants and record the time it takes for the solution to go yellow/brown. Don’t forget to swirl the contents regularly. 

5. Repeat the procedure with another sample at this temperature. 

6. Repeat this procedure for the other suggested temperatures. 


Question 7

Analysis of your results

Answer 7

When complete, you can determine the activation energy, Ea by plotting ln k against 1/T. Remember to plot the temperature in kelvin (K).

You will need the following information

rate = [KMnO4(aq)]/t
rate = k[KMnO4(aq)][H2C2O4(aq)]

1. Use this information to calculate the rate constant k at each temperature T.
2. Refer to the background theory at the start of this activity for help with construction and interpretation of the graph.

Question 8

1. Use your results and the value for Ea that you have calculated to determine the value of the pre-exponential factor (collision frequency), A.

Answer 8

A = 4.5 × 105 (dm3 mol–1 s–1; A has the same units as k)

Question 9

Equipment

Answer 9

• Electronic water baths – with test tube / boiling tube holders (or learners make their own water baths)
• Plastic containers in which to set up
• –10–110 oC thermometer
• Burettes, or graduates pipettes (10 cm3 and 20 cm3)
• Clamp stand, with boss and clamp (for supporting the burette)
• Measuring cylinders, various sizes up to 50 cm3
• Beakers
• Conical flask, 100 cm3
• Test tubes
• Test-tube rack
• Glass rod
• Dropping pipettes
• Stop clock or stop watch

Question 10

note

Answer 10

The most suitable glassware to use for measuring out solutions is a burette or a graduated pipette but some learners may initially choose a measuring cylinder. After learners have had an opportunity to choose glassware it might be helpful for them to share the use of burettes or pipettes.