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In today's T-Level Chemistry lesson, we are going to study A twelve point five, Reversible Reactions.

By the end of this lesson, you should be able to define dynamic equilibrium, state the conditions it requires, and use Le Chatelier’s principle for concentration, temperature and pressure.

The symbol with arrows both ways means the reaction proceeds in both directions. The forward reaction forms products. The reverse reaction reforms reactants.

Reversible is not the same as equilibrium. Equilibrium needs specified conditions.

Dynamic equilibrium occurs when a reversible reaction in a closed system has equal forward and reverse rates, so macroscopic concentrations or amounts remain constant.

The reactions do not stop. Both continue. Constant composition does not mean equal concentrations.

Check the conditions. The reaction must be reversible. The system must be closed. Forward and reverse rates must be equal. Forward and reverse reactions continue. Equal reactant and product concentrations are not required.

A sealed flask with equal forward and reverse rates can be at dynamic equilibrium. An open vessel that lets a gaseous product escape is not closed, so it is not at equilibrium. A sealed flask where the forward rate is greater than the reverse rate is not at equilibrium yet.

Le Chatelier’s principle: when a system at equilibrium is subjected to a change in conditions, the equilibrium shifts in the direction that opposes the effect of that change.

Use a method. Identify the change. Decide how the system opposes it. Predict a left or right shift. State which side is favoured. Justify using concentration, heat direction, or gas moles.

Concentration, for A plus B in equilibrium with C plus D. Add A, shift right to use some added A. Remove A, shift left to form more A. Add C, shift left. Remove D, shift right to form more D.

Temperature: treat heat as a product or a reactant. You must identify the exothermic and endothermic directions first.

If the forward reaction is exothermic, write heat on the product side. Increasing temperature shifts left, the endothermic reverse. Decreasing temperature shifts right.

If the forward reaction is endothermic, write heat on the reactant side. Increasing temperature shifts right. Decreasing temperature shifts left.

Increasing temperature does not always shift left. It depends on the enthalpy direction. For an exothermic forward reaction, increasing temperature decreases product yield.

Pressure applies to gaseous equilibria. Increasing pressure favours the side with fewer moles of gas. Decreasing pressure favours the side with more moles of gas. If the gas moles are equal, the equilibrium position does not shift. Solids and liquids are not counted.

Nitrogen plus three hydrogen in equilibrium with two ammonia. Four moles of gas on the left, two on the right. Increasing pressure shifts right.

Hydrogen plus iodine in equilibrium with two hydrogen iodide. Two moles of gas on each side. Increasing pressure causes no shift.

Two sulfur dioxide plus oxygen in equilibrium with two sulfur trioxide. Three moles of gas to two. Increasing pressure shifts right. If that forward reaction is exothermic, increasing temperature shifts left.

Calcium carbonate solid in equilibrium with calcium oxide solid and carbon dioxide gas. Zero gas moles on the left, one on the right. Increasing pressure shifts left.

Correct the misconceptions. Reactions do not stop at equilibrium. Constant does not mean equal concentrations. Not every reversible system is at equilibrium. Increasing pressure does not always shift right. Increasing temperature does not always shift left.

Examination-style question.

Part a. Define dynamic equilibrium and state one required condition. Three marks.

Forward and reverse reactions continue at equal rates in a closed system.

Part b. For nitrogen, hydrogen and ammonia, state the effect of increasing pressure. Two marks.

The equilibrium shifts right because there are two moles of gas against four.

Part c. The forward reaction is exothermic. What happens to product yield if temperature increases? Two marks.

The equilibrium shifts in the endothermic reverse direction, so product yield decreases.

Part d. What happens if you add a reactant? Two marks.

The equilibrium shifts to use some of the added reactant, so products are favoured.

Before we finish, complete the model. Dynamic equilibrium is a closed reversible system with equal continuing rates and constant composition. Le Chatelier’s principle predicts the opposing shift for concentration, temperature and pressure.

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