Interpretation of graphs for chemical systems in dynamic equilibrium

1. Graphs Used to Represent Equilibrium

Graphs help visualise how equilibrium is established and how changes affect the system. Common types of graphs include:

• Reaction rate vs. time

• Concentration (or amount) of reactants/products vs. time For each graph:

• Time is the independent variable (horizontal axis).

• Reaction rate or concentration is the dependent variable (vertical axis).

2. Stages in a Reaction Graph

2.1 Initially (Before Equilibrium)

• The forward reaction rate is high because there are many reactant particles.
• The reverse reaction rate is zero as no products are available yet.

2.2 As the Reaction Proceeds

• Reactants decrease → forward reaction rate slows down.
• Products increase → reverse reaction rate speeds up.
• Eventually, both rates become equal, and equilibrium is established.

2.3 At Equilibrium

• The forward and reverse reactions occur at the same rate.
• The concentration of reactants and products remains constant (but not necessarily equal).
• On a reaction rate vs. time graph, the two curves flatten out at equal values.
• On a concentration vs. time graph, the lines become horizontal, showing constant concentrations.

3. How Equilibrium Responds to Changes (Le Châtelier's Principle in Graphs)

3.1 Adding More Reactant

• Sharp increase in reactant concentration.
• Equilibrium shifts right, increasing product concentration.
• The forward reaction rate spikes, then gradually returns to a new equilibrium.

3.2 Removing a Product

• Sharp decrease in product concentration.
• Equilibrium shifts right, producing more product.
• The forward reaction rate briefly increases, then stabilises.

3.3 Changing Temperature

• Increase in temperature favours the endothermic reaction (curve rises for that direction).
• Decrease in temperature favours the exothermic reaction (curve rises for that direction).
• The reaction rate graph shows temporary spikes, then levels out.

3.4 Changing Pressure (Gases Only)

• Increase in pressure shifts equilibrium to the side with fewer gas molecules (reduces pressure).
• Decrease in pressure shifts equilibrium to the side with more gas molecules (increases pressure).
• The concentration graph shows a sharp change, then stabilisation.

3.5 Adding a Catalyst

• Speeds up both forward and reverse reactions equally.
• Equilibrium is reached faster, but the position of equilibrium does not change.
• The reaction rate graph shows a sudden increase in both rates, then stabilises.