Focus 5

Question 1

Explain how the rate of a chemical reaction is affected.

Answer 1

Reactions take place when particles collide with a certain amount of energy. The minimum amount of energy needed for particles to react is called the activation energy, and is different for each reaction.
The rate (speed) of a reaction depends on:
The frequency of collisions between particles
The energy with which particles collide – if particles collide with less energy than the activation energy, there is no reaction

Question 2

Explain how an increased temperature increases the rate of a reaction.

Answer 2

An increased temperature increases the energy and speed of particles, which means that there are more successful collisions per second

Question 3

Explain how an increased concentration of dissolved reactants increases the rate of a reaction.

Answer 3

An increased concentration increases the number of reactant particles present, which increases the number of collisions per second.

Question 4

Explain how the increased pressure of gaseous reactants increases the rate of a reaction.

Answer 4

An increased pressure will increase the number of particles in a certain volume, which increases the number of collisions per second.

Question 5

Explain how the increased surface area of solid reactants increases the rate of a reaction.

Answer 5

By decreasing the particle size of a reactant, there is an increased surface area. This means that there is an increased area for collisions to take place, which increases the number of collisions.

Question 6

Explain how the use of a catalyst increases the rate of a reaction.

Answer 6

Catalysts are substances which provide an alternate reaction pathway with a lower activation energy, and therefore can increase the rate of reactions without being consumed like reactants. They do not increase the number of products, but they can increase the overall speed of the reaction.

Question 7

Explain how stirring increases the rate of a reaction.

Answer 7

Stirring
Stirring increases the energy and speed of particles, which means that there are more successful collisions per second.

Question 8

How do you measure the rate of reaction?

Answer 8

The rate of a reaction is measured by an increase in the number of products and a decrease in the number of reactants over time. Reactants do not occur at a steady rate:
As the reaction progresses, the concentration of reactants decreases
The frequency of collisions reduces, therefore the rate slows down over time
The rate plateaus then stops once the reactants have all transformed