5.2.1 Reversible reactions and dynamic equilibrium

Some reactions run in both directions

1. In many reactions the products can react together to re-form the reactants.
2. Heating blue hydrated copper sulfate drives off water and leaves white anhydrous copper sulfate.
3. Adding water to the white solid turns it blue again, which reverses the change.
4. Heating white ammonium chloride breaks it into ammonia and hydrogen chloride, which recombine on cooling.
5. The conditions decide which direction dominates, so changing them can reverse the change.

The double arrow marks a reversible reaction

1. A reversible reaction is written with the symbol $\rightleftharpoons$ in place of a single arrow.
2. The copper sulfate change is written this way: $${CuSO}_4\cdot 5H_{2}O\rightleftharpoons {CuSO}_4+5H_{2}O$$
3. The left to right change is called the forward reaction and the right to left change the backward reaction.
4. The two reactions are opposites, so one is exothermic by exactly as much as the other is endothermic.
5. Both reactions are taking place at the same time whenever reactants and products are mixed together.

A closed system reaches dynamic equilibrium

1. Nothing can enter or leave a closed system, so every product stays in contact with the mixture.
2. As the reactants are used up the forward reaction slows down.
3. As the products build up the backward reaction speeds up.
4. The two rates eventually become equal, and at that moment the amounts stop changing.
5. The mixture has then reached equilibrium, and it stays there unless something is changed.

Equilibrium is dynamic, not still

1. At equilibrium both reactions are still running, and running at the same rate.
2. Particles are still being converted in both directions, which is what dynamic means.
3. What stays constant is the amount of each substance present, not the activity in the mixture.
4. The reactants and products are not present in equal amounts, only in unchanging ones.
5. A mixture that has simply stopped reacting is not at equilibrium, because nothing is happening at all.

Changing the conditions changes what the mixture holds

1. Once a mixture is at equilibrium, its composition stays fixed until a condition is altered.
2. Changing the temperature, the pressure or a concentration disturbs the balance of the two rates.
3. The mixture then settles at a new equilibrium holding different amounts of reactants and products.
4. This is why the direction of a reversible reaction can be controlled by choosing the conditions.
5. Industry uses that control to push a reversible reaction towards the product that is wanted.

5.2.2 The Haber process

Ammonia is made from nitrogen and hydrogen

1. The Haber process combines nitrogen and hydrogen to make ammonia.
2. The equation for the reaction is: $$N_2(g)+3H_2(g)\rightleftharpoons 2{NH}_3(g)$$
3. The ratio is one nitrogen to three hydrogen, and the gases are supplied in that proportion.
4. The forward reaction is exothermic, so it releases energy as ammonia forms.
5. Ammonia is the raw material for fertilisers, which is why the process is run on such a scale.

Where the two raw materials come from

1. Nitrogen is extracted from the air, which is about $78\%$ nitrogen by volume.
2. The air is cooled until it liquefies and is then separated by fractional distillation.
3. Hydrogen is obtained from natural gas, which is mainly methane.
4. Both raw materials are purified before they enter the reactor, because impurities spoil the catalyst.
5. The nitrogen is effectively unlimited, while the hydrogen depends on a supply of natural gas.

The reaction is reversible and reaches a dynamic equilibrium

1. Ammonia breaks back down into nitrogen and hydrogen under the same conditions that form it.
2. The gases are held in the reactor long enough for the mixture to approach equilibrium.
3. Where the forward and backward rates become equal, the mixture has reached a dynamic equilibrium.
4. At that point the mixture holds ammonia together with unreacted nitrogen and hydrogen.
5. The reaction can therefore never convert all of the nitrogen and hydrogen in a single pass.

The conditions used in the Haber process

1. The temperature used is $450{}^{\circ }C$.
2. The pressure used is $200$ atmospheres.
3. An iron catalyst is used.
4. The catalyst brings the mixture to equilibrium sooner without being used up.
5. These three conditions are the ones recalled for this process.

The mixture is cooled and recycled

1. The gases leaving the reactor are cooled until the ammonia condenses to a liquid.
2. Nitrogen and hydrogen stay as gases at that temperature, so the ammonia separates out.
3. Removing the ammonia means the liquid product is run off on its own.
4. The unreacted nitrogen and hydrogen are pumped back into the reactor.
5. Recycling means that very little of either raw material is wasted, even though each pass converts only some.

5.2.3 Changing the position of a dynamic equilibrium

The position of equilibrium describes what the mixture holds

1. A mixture at equilibrium holds both reactants and products, in unchanging amounts.
2. An equilibrium lying to the right holds mostly products.
3. An equilibrium lying to the left holds mostly reactants.
4. Changing a condition upsets the balance of the two rates, and the mixture settles at a new position.
5. The general rule is that the system responds so as to oppose the change that was made.

Temperature favours the endothermic or the exothermic direction

1. One direction of a reversible reaction is exothermic and the other is endothermic.
2. Raising the temperature shifts the position towards the endothermic direction, which takes energy in.
3. Lowering the temperature shifts the position towards the exothermic direction, which gives energy out.
4. In the Haber process the forward reaction is exothermic, so a lower temperature gives more ammonia at equilibrium.
5. Temperature shifts the position of every equilibrium, while pressure does so only where the two sides differ in moles of gas.

Pressure matters when the two sides hold different amounts of gas

1. Only gases respond to a change in pressure, so solids and liquids can be ignored.
2. Count the moles of gas shown on each side of the equation.
3. Raising the pressure shifts the position towards the side with fewer moles of gas.
4. Lowering the pressure shifts it towards the side with more moles of gas.
5. Ammonia forms from four moles of gas and makes two, so high pressure favours ammonia.

Concentration shifts the equilibrium away from whatever is added

1. Adding more of a reactant shifts the position to the right, making more product.
2. Adding more of a product shifts the position to the left, making more reactant.
3. Removing a product shifts the position to the right, as the system replaces what was taken.
4. Removing a reactant shifts the position to the left for the same reason.
5. Removing the product as it forms keeps shifting the position right, so the reaction can run close to completion.

A catalyst changes the speed but not the position

1. A catalyst speeds up the forward and backward reactions by the same factor.
2. Because both rates rise together, they still become equal at the same composition.
3. Equilibrium is therefore reached sooner, holding exactly the amounts it would have held anyway.
4. The yield at equilibrium is unchanged, however much catalyst is present.
5. Industry pays for a catalyst because of the time it saves, not for extra product.