Rate and Extent of Chemical Change
What's covered
Key facts
Catalysts
A catalyst is a substance that speeds up a reaction without being used up.
Factors Affecting Rate
Collision theory is the theory that explains how reactant collisions lead to a reaction.
Measuring Rate
Colorimetry measures how much light is absorbed by a coloured solution.
Reversible Reactions and Equilibrium
Increasing the concentration of a reactant shifts the equilibrium toward the products (and vice versa).
Catalysts
A catalyst increases the rate of both the forward and reverse reactions in a reversible reaction by the same amount, so the equilibrium position is unchanged.
Factors Affecting Rate
Increasing concentration speeds up a reaction because particles collide more frequently.
Measuring Rate
Gas production rate can be measured using a gas syringe or an inverted measuring cylinder.
Reversible Reactions and Equilibrium
At dynamic equilibrium, the forward and reverse reactions are both still occurring, but at equal rates, so net concentrations stay constant.
Catalysts
A catalyst speeds up a reaction by providing an alternative reaction pathway with a lower activation energy.
Factors Affecting Rate
Increasing the concentration of a reactant in solution increases the reaction rate because more reactant particles per unit volume means more successful collisions per second.
Practice questions with answers
A taste of the 35 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
How does a catalyst speed up a reaction?
- •It gives the reactant particles extra kinetic energy to collide
- •It increases the concentration of the reactants
- ✓It provides an alternative reaction pathway with a lower activation energy
- •It raises the temperature of the reaction mixture
How does increasing the concentration of a reactant in solution increase the reaction rate?
- •Higher concentration raises the activation energy, slowing the reaction
- •Higher concentration slows particles and reduces collisions per unit time
- ✓More reactant particles per unit volume: more successful collisions per second
- •Particles become larger at higher concentration and collide with more force
Which two methods measure the rate of a gas-producing reaction?
- ✓Gas syringe volume over time, or mass loss on a balance
- •Measure the boiling point of the mixture, or test its pH with litmus
- •Measure the density of the acid, or the starting temperature
- •Temperature rise of the mixture, or colour change with a colorimeter
According to Le Chatelier's principle, if the pressure in a gaseous equilibrium is increased, what happens?
- •Equilibrium is unchanged: pressure does not affect equilibrium
- ✓Equilibrium shifts to the side with fewer moles of gas, lowering pressure
- •Equilibrium shifts to the side with more moles of gas
- •Rate of both forward and reverse reactions decreases
Why are catalysts important in many industrial chemical processes?
- •They increase the equilibrium yield by shifting the position in favour of products
- •They permanently convert reactants into more valuable industrial products
- •They prevent dangerous side reactions by being chemically used up first
- ✓They speed up reactions at lower temperatures, saving energy and cost
Why does increasing gas pressure speed up reactions?
- •Higher pressure heats the reactants
- ✓Particles are closer together, increasing collisions
- •Pressure attracts catalyst molecules
- •Pressure changes activation energy
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