AQA GCSE Single Science Chemistry: The rate and extent of chemical change

How would you calculate the mean rate of reaction?

Quantity of product formed or reactant used up/time

Which factors affect the rate of chemical change? And how?

• Concentration (more reactant particles means more collisions)

• Pressure (more particles within given volume, increased number of collisions)

• Temperature (increased energy particles collide with and increased speed means more collisions)

• Surface area (more surface particles can collide over)

• Presence of catalyst (lowers activation energy, provides alternative reaction pathway)

Required practical for rate of reaction: Turbidity/colour change

• Add 10cm³ of sodium thiosulfate solution, ensure concentration is known, to a conical flask atop a black cross

• Add 10cm³ of hydrochloric acid

• Swirl flask gently and start stopwatch. Stop the watch after cross can no longer be seen.

• Repeat process with different concentrations of sodium thiosulfate solution

Define collision and activation theory

Collision theory - Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.

Activation energy - The minimum amount of energy required for reaction to take place

Define equilibrium and Le Chatelier’s Principle

Equilibrium - When the forward and backward reaction occur at the exact same rate

Le Chatelier’s Principle - When a change is made to a system at equilibrium, the system moves to counteract that change

What happens if you change the concentration (providing the system is at equilibrium)?

• If the concentration of reactants is increased, more products will be formed

• If the concentration of products is decreased, more reactants will react

What happens if you change the temperature (providing the system is at equilibrium)?

• An increase in temperature means the endothermic reaction will be favoured.

• A decrease in temperature means the exothermic reaction will be favoured.

What happens if you change pressure (providing the system is at equilibrium)?

• An increase in pressure favours the side with the least amount of moles

• A decrease in pressure favours the side with the most amount of moles