Equilibrium (AQA GCSE Chemistry Combined Science): Revision Notes
Equilibrium
What is equilibrium?
Equilibrium happens when a reversible reaction reaches a special balance point. For this to occur, the reaction must take place in a closed system where none of the reactants or products can escape.
In a reversible reaction, there are two reactions happening at the same time:
- The forwards reaction (reactants → products)
- The backwards reaction (products → reactants)
When equilibrium is reached, something important happens: the rate of the forwards reaction equals the rate of the backwards reaction. This means the reactions are happening at the same speed in both directions.
A closed system is essential for equilibrium because if any reactants or products could escape, the balance would be constantly disrupted and true equilibrium could never be achieved.
Key example - hydrogen and iodine gases
Worked Example: Hydrogen and Iodine Equilibrium
The reaction: H₂(g) + I₂(g) ⇌ 2HI(g)
What happens step by step:
- Hydrogen and iodine gases are mixed in a sealed container
- The forwards reaction: H₂ + I₂ → 2HI
- The backwards reaction: 2HI → H₂ + I₂
- Both reactions continue happening all the time
- Eventually, equilibrium is reached when both reactions happen at the same rate
Key observation: At equilibrium, the concentrations of all substances (H₂, I₂, and HI) stop changing, even though the reactions are still happening.
Three positions of equilibrium
Equilibrium doesn't always mean equal amounts of reactants and products. The position of equilibrium can be in different places:
- Position to the left - mainly reactants present
- Position in the middle - roughly equal amounts
- Position to the right - mainly products present
The important thing to remember is that the rates are equal, but the amounts don't have to be equal. This is a common misconception that many students have about equilibrium.
Important points about equilibrium
Understanding equilibrium requires grasping several key concepts that work together:
- Equilibrium only happens in closed systems (nothing can escape)
- The rates of forwards and backwards reactions are equal
- The concentrations of all substances stay constant
- The amounts of reactants and products don't need to be equal
- Both reactions are still happening - it's a dynamic equilibrium
The equilibrium symbol ⇌ shows that the reaction can go in both directions.
Key Points to Remember:
- Equilibrium occurs when forwards and backwards reaction rates are equal
- It only happens in closed systems where nothing can escape
- Concentrations stay constant at equilibrium, but reactions are still happening
- The amounts of reactants and products don't have to be equal
- The position of equilibrium shows whether there are more reactants or products present