Many chemical reactions are reversible: they can proceed in both the forward and reverse directions. In a closed system, these reactions can reach a state of dynamic equilibrium, where the rate of the forward reaction is exactly equal to the rate of the reverse reaction.
🔑 Key Principle
Dynamic equilibrium is dynamic because both the forward and reverse reactions continue to occur. It is in equilibrium because the rates of these two reactions are equal, resulting in no overall change in the concentrations of reactants or products over time.
Reversible Reactions
Some reactions, like the combustion of magnesium, go to completion and are considered irreversible. However, many reactions can go both ways. We represent reversible reactions using the equilibrium arrow symbol: \( \rightleftharpoons \)
For example, the thermal decomposition of ammonium chloride is reversible:
Open vs Closed Systems
To understand equilibrium, we must distinguish between the type of container or system in which the reaction is taking place:
Open Systems
A system where matter and energy can be exchanged with the surroundings. If a gas is produced, it can escape, preventing the reverse reaction from occurring. Equilibrium cannot be reached in an open system.
Closed Systems
A system where only energy, not matter, can be exchanged with the surroundings. Reactants and products are contained, allowing them to continuously react. Equilibrium can only be established in a closed system.
Characteristics of Dynamic Equilibrium
If you are asked to state the characteristics of a system in dynamic equilibrium, you must provide the following key features:
- It must occur in a closed system.
- The rate of the forward reaction equals the rate of the reverse reaction.
- The concentrations of reactants and products remain constant.
- The macroscopic properties (such as color, temperature, pressure, and density) remain constant.
A chemical reaction that can occur in both the forward and reverse directions, where reactants form products and products can react to reform the original reactants.
A state established in a closed system when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
Do not confuse constant concentrations with equal concentrations. At equilibrium, the concentrations of reactants and products are constant, but they are very rarely equal. There might be a high concentration of products and a low concentration of reactants, or vice versa, depending on the position of equilibrium.
Graphing Equilibrium
We can visualise the establishment of dynamic equilibrium using graphs. The graph below shows how the rate of the forward and reverse reactions change over time until they become equal at equilibrium.
Step 1: Identify the cause of the initial change.
Initially, only reactants are present. The forward reaction occurs, consuming purple iodine gas (\( \text{I}_2 \)) to form colourless hydrogen iodide (\( \text{HI} \)). As a result, the concentration of iodine decreases, causing the purple colour to fade.
Step 2: Explain the establishment of equilibrium.
As \( \text{HI} \) molecules are formed, they begin to react to reform \( \text{H}_2 \) and \( \text{I}_2 \). The rate of this reverse reaction increases as the concentration of \( \text{HI} \) increases, while the rate of the forward reaction decreases.
Step 3: State the final equilibrium condition.
Eventually, the rate of the forward reaction equals the rate of the reverse reaction. Dynamic equilibrium is established. Because the rate of iodine consumption equals the rate of iodine production, its concentration remains constant, and therefore the intensity of the purple colour remains constant.
Position of Equilibrium
The position of equilibrium refers to the relative amounts of reactants and products present once equilibrium is reached.
- If equilibrium lies to the right, the products predominate (concentration of products is greater than concentration of reactants).
- If equilibrium lies to the left, the reactants predominate (concentration of reactants is greater than concentration of products).
When writing about equilibrium in exams, always specify whether the reaction has reached equilibrium. Use the term dynamic to indicate that reactions are still taking place. Never write that the reaction has "stopped", as this contradicts the definition of dynamic equilibrium.
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