Driving force: what makes reactions go

Then without doing any calculations, explain what the driving force of the reaction is in each case – ie qualitatively rather than quantitatively.

Introduction

This problem could be used immediately before students meet Gibbs free energy to demonstrate the inadequacy of the possible explanations. Repeating the problem once students become familiar with the concept demonstrates its utility. Students should use a qualitative approach. However, worked numerical examples are given which could be used as homework for consolidation.

Teachers who have not used the problems before should read the section Using the problems before starting.

Prior knowledge

Discussion can be at one of two levels, depending on whether students are familiar with the Gibbs equation:

ΔG^θ = ΔH^θ – TΔS^θ

For the third experiment, knowledge of lattice enthalpies/entropies and hydration enthalpies/entropies could form the basis of extension material.

Resources

Data books and textbooks (physical and inorganic) should be available for reference.

The following is also required:

• for the first experiment, magnesium ribbon;
• for the second experiment (after 10–20 minutes), citric acid and sodium
• hydrogencarbonate along with small (100 cm³) beakers;
• for the third experiment, ammonium nitrate and more small beakers;
• for the second and third experiments, thermometers (–10 to 110 oC).

Special safety requirements

Students should not look directly at burning magnesium.

Possible answers

a. From students who are unfamiliar the Gibbs equation

Most reactions are exothermic and are seen by students as ‘running down energy hills’ analogous to an object falling under gravity to a position of minimum potential energy. The most useful discussion can take place when they try to interpret the endothermic reactions and to look for the unusual in them in order to try to devise alternative explanations.

Burning magnesium

This is identified as an exothermic reaction, one moving to a position of minimum energy. The heat supplied is the activation energy needed to start the reaction.

Reaction between citric acid and sodium hydrogencarbonate

What is unusual here? The evolution of copious quantities of gas. Could this be the driving force?

Dissolving ammonium nitrate in water

There is nothing unusual here. Dissolving involves breaking the lattice and hydration of the ions. Clearly, the lattice energy has a larger absolute value than the ion hydration energies, but this does not explain why the reaction takes place.

b. From students who are familiar with the Gibbs equation

ΔG^θ = ΔH^θ –TΔS^θ

Burning magnesium

ΔH^θ is large and negative (favourable)

ΔS^θ is negative because of the gas being used up (unfavourable)

ΔH^θ is the larger of the two factors at the temperature concerned and makes

ΔG^θ negative.

Reaction between citric acid and sodium hydrogencarbonate

ΔH° is positive (unfavourable)

ΔS° is positive because of the evolution of gas (favourable)

TΔS° is the larger of the two factors at room temperature and makes ΔG^o negative.

Dissolving ammonium nitrate in water

ΔH° is positive (unfavourable)

ΔS° must be positive for the reaction to go

TΔS° is the controlling factor, and makes ΔG° negative.

There is more order in the lattice than there is in the solution of the hydrated ions.

Some salts dissolve endothermically as in this case, some dissolve exothermically, and others are thermally neutral, such as sodium chloride.

Suggested approach

During trialling the following instructions were given to students and proved to be

extremely effective:

1. Working as a group, carry out the first experiment.
2. As a group, discuss the change. What is the driving force that makes it go? Such discussion can play a vital part in working out solutions to problems like these. Several minds working on a problem together can stimulate ideas that one on its own could not manage.
3. Write up in note form what your group decided. Say what the driving force is and explain it with reference to this change.
4. Repeat steps 1 to 3 for the second experiment.
5. Repeat steps 1 to 3 for the last experiment.
6. Working as a group, prepare a short (ca 5-minutes maximum) presentation to give to the rest of the class. If possible, all group members should take part: any method of presentation (such as a blackboard, overhead projector, etc) can be used.

Outline the problem and describe what you did. Details of the practical work are not needed, but explain what you decided was the driving force for each change. After the presentation, be prepared to accept and answer questions and to discuss what you did with the rest of the class.