occurs in a closed system when the forward and backward reactions of a reversible chemical reaction occur at the same rate; and the concentrations of reactants and products remain constant.

is the speed with which a chemical reaction takes place, measured by the amount of a reactant used or amount of product formed in a given time.

is one in which matter cannot enter or leave the observed environment, allowing only energy transfer between the system and its surrounding environment.

Chemical equilibrium

Combined science

In the Rates and energy changes unit, pupils learned about how energy changes are always involved in reaction, along with a range of methods for measuring and comparing rates of reaction using properties of reactants and/or products.

Industrial chemistry

Physical activity and equipment requiring safe use

Hot coffee cooling to room temperature is a reversible process. It is a {{}} rather than a chemical change.

amounts of reactions and products are constant

amounts of reactants and products are equal

changes in amounts of reactants and products are equal

reaction rates of forward and backward reaction are equal

Equilibrium is a state where opposing forces or processes are balanced. Which statements MUST be true at chemical equilibrium?

Dynamic equilibrium occurs in a {{}} system when forward and backward reactions occur at the same rate, and the concentrations of reactants and products remain constant.

N$$_2$$O$$_4$$ can leave the system, which disrupts the balance of equilibrium.

N$$_2$$O$$_4$$ is a light brown gas and NO$$_2$$ is a dark brown gas.

There is never a full conversion of reactant molecules to product molecules.

A closed system of a glass container is filled with NO$$_2$$ under conditions which allows the equilibrium with N$$_2$$O$$_4$$ to be established. Why will you never observe a colourless situation?

system where matter cannot enter/leave, but energy can be transferred

opposing processes occur with equal rates; no observable changes

reaction where reactants form products; reaction goes to completion

system where both matter and energy can freely enter and leave

reaction where reactants form products, which revert to reactants

Match the following key terms to their definition.

The models of water with food colouring, or of cars entering/leaving a road, are unhelpful for the ‘dynamic’ aspect of equilibrium, as you cannot see that individual molecules are continuously {{}}.

Physical changes are easily reversible by changing the {{}}, for example changing the state of water from ice to liquid by heating, and back to ice by cooling.

addition is more easily achieved than decomposition

conditions for forward and backward reactions can easily be met

dehydration is more easily achieved than hydration

reactants and products have comparable stability

the reaction does not involve any energy change

If the forward reaction of a reversible reaction is exothermic, the backward reaction will be {{}}.

Place 5 g of powdered blue copper(II) sulfate in a test tube.

Heat the copper sulfate until the colour no longer changes.

Record any observations during the heating process.

Allow the anhydrous copper sulfate to cool back to room temperature.

Pour the collected water very slowly onto the cooled powder.

Record any observations when the water is poured back onto the powder.

To carry out the experiment with hydrated and anhydrous copper sulfate, put the steps in order:

changing the surface area of a solid reactant

How can reaction conditions be changed so that reaction rates change?

Models have limitations. What does this image of the particle model **NOT** attempt to show?

Content guidance

Risk assessment required - equipment

Adult supervision required

Starter Quiz