Revision notes · The rate and extent of chemical change
Reversible reactions and dynamic equilibrium
Reversible reactions4.6.2.1
Definition: A reversible reaction is a chemical reaction where the products can react together to reform the original reactants.
In some chemical reactions, the products can react to turn back into the starting materials.
- •Reversible reactions are represented in chemical equations using a double arrow symbol (⇌).
- •The reaction moving from left to right is called the forward reaction.
- •The reaction moving from right to left is called the reverse reaction.
- •An example is hydrated copper(II) sulfate changing into anhydrous copper(II) sulfate and water.
⚠️ Common mistake: Thinking that all chemical reactions are easily reversible by simple physical means.
🧠 Remember: The double arrow (⇌) shows a reaction can go both ways.
Energy changes and reversible reactions4.6.2.2
Energy is conserved during reversible chemical reactions, changing forms between the system and surroundings.
- •If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction.
- •The amount of energy transferred to the surroundings in one direction equals the energy taken in from the surroundings in the reverse direction.
- •Heating hydrated copper(II) sulfate requires energy input because the forward reaction is endothermic.
- •Adding water to anhydrous copper(II) sulfate releases heat because the reverse reaction is exothermic.
⚠️ Common mistake: Believing that both the forward and reverse reactions can release energy at the same time.
🧠 Remember: Energy is conserved: exothermic one way = endothermic the other.
Equilibrium4.6.2.3
Definition: Dynamic equilibrium is reached when the forward and reverse reactions occur at the exact same rate in a closed system.
When a reversible reaction takes place in a sealed container, it eventually reaches a balanced state.
- •Equilibrium can only be achieved in a closed system where no reactants or products can enter or escape.
- •At equilibrium, the rates of the forward and reverse reactions are equal.
- •The concentrations of reactants and products remain constant at equilibrium.
- •The reaction does not stop at equilibrium; both forward and reverse processes continue continuously.
⚠️ Common mistake: Thinking that the reaction has stopped or that concentrations of reactants and products must be equal at equilibrium.
🧠 Remember: Equilibrium means equal rates, not equal amounts.
The effect of changing conditions on equilibrium4.6.2.4
Definition: Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in conditions, the system shifts to counteract that change.
Changing the conditions of a system at dynamic equilibrium causes the equilibrium position to shift.
- •The position of equilibrium determines the relative amounts of reactants and products.
- •Le Chatelier's Principle allows us to predict the qualitative effects of changing temperature, concentration, or pressure.
- •If conditions favor the forward reaction, the equilibrium shifts to the right to form more products.
- •If conditions favor the reverse reaction, the equilibrium shifts to the left to form more reactants.
⚠️ Common mistake: Assuming that adding a catalyst changes the position of equilibrium rather than just speeding up the rate at which equilibrium is reached.
🧠 Remember: Equilibrium always acts to oppose the change you make.
The effect of changing concentration4.6.2.5
Changing the concentration of any reactant or product shifts the equilibrium position to restore balance.
| Change | Equilibrium shifts... |
|---|---|
| Reactant concentration increased | Right — makes more product |
| Product concentration decreased (removed) | Right — makes more product |
| Product concentration increased | Left — makes more reactant |
- •Industrial processes continuously remove products to force the equilibrium to shift right and increase yield.
⚠️ Common mistake: Confusing an increase in reactant concentration with a shift to the left.
🧠 Remember: Add reactant → shift right; Remove product → shift right.
The effect of temperature changes on equilibrium4.6.2.6
Temperature changes shift the equilibrium depending on whether the forward reaction absorbs or releases thermal energy.
| Change | Equilibrium shifts... |
|---|---|
| Temperature increased | Endothermic direction — absorbs the extra heat |
| Temperature decreased | Exothermic direction — releases heat |
- •If the forward reaction is exothermic, heating the system decreases the yield of products.
- •If the forward reaction is endothermic, heating the system increases the yield of products.
⚠️ Common mistake: Assuming heating a reaction always increases the product yield regardless of whether the reaction is endothermic or exothermic.
🧠 Remember: Heat up = Endothermic direction; Cool down = Exothermic direction.
The effect of pressure changes on equilibrium4.6.2.7
Pressure changes only affect equilibrium systems that involve gaseous reactants and products.
| Change | Equilibrium shifts... |
|---|---|
| Pressure increased | Towards the side with fewer gas molecules |
| Pressure decreased | Towards the side with more gas molecules |
- •Gas molecule numbers are determined by adding the balancing coefficients of gaseous species in the chemical equation.
- •If both sides of the equation have the same number of gas molecules, changing pressure has no effect on equilibrium position.
⚠️ Common mistake: Counting solid or liquid molecules when evaluating the effect of pressure changes.
🧠 Remember: Higher pressure favors the side with fewer gas molecules.
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