Rate and Extent of Chemical Change
What's covered
Key facts
A catalyst provides an alternative reaction pathway with a lower activation energy; reactants find a route that requires less energy to reach the transition state.
Collision theory is the theory that explains how reactant collisions lead to a reaction.
Colorimetry measures how much light is absorbed by a coloured solution.
Increasing the concentration of a reactant shifts the equilibrium toward the products (and vice versa).
A catalyst is a substance that speeds up a reaction without being used up.
Increasing concentration in solution increases rate because more particles per unit volume means more frequent collisions.
Gas production rate can be measured using a gas syringe or an inverted measuring cylinder.
At dynamic equilibrium, the forward and reverse reactions are both still occurring, but at equal rates, so net concentrations stay constant.
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.
Increasing concentration speeds up a reaction because particles collide more frequently.
Sample questions
A taste of the 45 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 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
- •It reacts with the reactants to form an intermediate product
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
- •Temperature rise of the mixture, or colour change with a colorimeter
- •Titrate remaining acid at intervals, or measure electrical conductance
- •Watch for reactants dissolving, or time until the solution turns clear
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
How does increasing temperature affect reaction rate?
- ✓Particles collide faster and more often
- •Rate falls because heat damages reactants
- •Rate falls because particles spread out
- •Rate stays exactly the same
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