Investigating the Effect of Temperature on Reaction Rate:

Introduction

It is a simple matter to show qualitatively that most chemical reactions can be made to proceed faster if the reactants are heated. There are also applications of this in every day life, for example the faster growth of plants in hotter climates. The advantage of this experiment is that it allows students to measure the rate of reaction at different temperatures, because the current produced by the cell is the rate of reaction - the faster the reaction goes, the more electrons are produced. The student can then thus quantitatively study the effect of temperature on reaction rate.

The current produced by the fuel cell, which is the rate of reaction, is measured by connecting an ammeter directly to it. The current produced will be strongly dependent on the resistance of the meter used. However, in all cases the proportional change in current will be much the same whatever meter is used. An important feature of this fuel cell is that no harm will be done if the cell is "short circuited" by an ammeter, even if the "short circuit" is maintained for weeks on end.

This experiment works best if temperatures above and below room temperature can be used, so it is important to ensure a supply of cooled electrolyte before the experiment is done. This can be done by putting a bottle of 1M KOH in a fridge the day before. Each student needs 75ml, so a 1 litre bottle should be sufficient.

The cell should be heated by placing it in a large beaker or dish containing a little hot or warm water. The cell should be disconnected from the meter while this is being done, so that spillages are less likely while the cell is being moved about.

Apparatus

The apparatus needed by each student is:-

• Fuel cell (liquid fuel) anode, cathode
• ammeter, 0-1A, 2 connecting wires
• 0-100 ^oC thermometer
• dish or large beaker containing hot water

Instructions

Take care, this experiment involves the use of methanol (which is poisonous), and KOH solution (which is corrosive).

1) Draw a table for the results, with two headings, "Temperature" and "Current".

2) Fill the fuel cell anode up to the fill line with chilled KOH solution. Add about 10 ml of methanol. Place the cathode into the cell. Gently shake or swirl to mix the fuel and clear any air bubbles.

3) Connect the ammeter. Place the thermometer in the cell through the holes provided.

4) Allow about 15 seconds for the meter to reach a steady value, and then record the temperature and the current.

5) Disconnect the meter, and gently heat the cell by placing it in a dish or large beaker of warm water. Wait till the temperature has risen by about 5 ⁰C. Do not attempt to take temperature readings at round number temperatures - the heating method is too simple to allow this to be done accurately.

6) Reconnect the ammeter and wait about 15 seconds for the meter to reach a steady value. Record the values of the temperature and the current.

7) Repeat steps 4 and 5 until the temperature has reached about 45 ^oC. DO NOT HEAT ABOVE 50 ^oC. You may need to renew the warm water supply in the dish or large beaker to reach 45 ^oC.

8) Plot a graph of reaction rate (current ) against temperature.

Likely Results

The current produced depends very strongly on the type of ammeter used, but typically you should expect about 200 mA at 5 ^oC, rising to about 500 mA at 45 ^oC. The graph will probably be quite linear, rather than the exponential that the Arrthenius equation would predict, but only one of the reactants is being heated.