Equilibrium (AQA GCSE Chemistry): Revision Notes
Equilibrium
What is equilibrium?
Reversible reactions can reach a special state called equilibrium. This only happens when the reaction takes place in a closed system where the reactants and products cannot escape.
At equilibrium, something important occurs: the rate of the forwards reaction equals the rate of the backwards reaction. This means both reactions are still happening, but they balance each other out perfectly.
Think of equilibrium like a busy two-way street where the same number of cars travel in each direction every minute. Traffic flows in both directions continuously, but the overall traffic pattern stays constant.
The hydrogen and iodine example
Let's look at a clear example to understand equilibrium better.
Worked Example: Hydrogen and Iodine Equilibrium
When hydrogen gas and iodine gas are mixed in a sealed container, they react to form hydrogen iodide:
Forward reaction:
Backward reaction:
Step 1: Initial state Initially, you have lots of hydrogen and iodine molecules bumping around.
Step 2: Forwards reaction dominates As time passes, some of these combine to make hydrogen iodide molecules.
Step 3: Backwards reaction begins But as more hydrogen iodide forms, some of it starts breaking apart back into hydrogen and iodine.
Step 4: Equilibrium reached Eventually, the system reaches equilibrium. At this point, hydrogen iodide is forming at exactly the same rate it's breaking down. The concentrations of all substances stay constant - they don't change any further.
Key principles of equilibrium
Understanding equilibrium requires grasping several fundamental concepts that work together to create this balanced state.
Key Principles to Remember:
- Both forwards and backwards reactions continue - they never stop
- The rates of forwards and backwards reactions are equal
- Concentrations of all reactants and products remain constant
- The amounts of reactants and products do NOT have to be equal
That last point is crucial. Equilibrium doesn't mean you have equal amounts of everything - it just means the rates are balanced.
Three positions of equilibrium
Equilibrium can sit in different positions depending on the reaction. This position determines whether you'll see more reactants or more products in your final mixture.
Position 1: Equilibrium lies to the left
- More reactants than products
- The backwards reaction is favoured
- You see mainly hydrogen and iodine molecules
When equilibrium lies to the left, it means the original reactants are more stable under the given conditions, so they predominate in the mixture.
Position 2: Equilibrium is in the middle
- Roughly equal amounts of reactants and products
- Neither direction is strongly favoured
Position 3: Equilibrium lies to the right
- More products than reactants
- The forwards reaction is favoured
- You see mainly hydrogen iodide molecules
Spotting equilibrium in action
In laboratory settings, you can often tell when a system reaches equilibrium by observing when changes stop occurring.
Observable Signs of Equilibrium:
- Colour changes stop - if the reaction produces coloured products, the colour becomes constant
- Gas volumes become steady - if gases are involved, pressure stops changing
- Temperature becomes constant - energy changes level out
These observable changes are your practical indicators that the forwards and backwards reaction rates have become equal.
Essential Points About Equilibrium:
- Equilibrium only happens in closed systems where nothing can escape
- At equilibrium, forwards and backwards reaction rates are equal
- Concentrations stay constant, but reactions are still happening
- Amounts of reactants and products don't have to be equal
- Equilibrium can favour reactants, products, or be roughly balanced