2.2 Rates of Reaction

Question 1

What is collision theory?

Answer 1

For a chemical reaction to take place:
- Reactant particles must collide with the correct orientation.
- Reactant particles must have the activation energy.

Question 2

Define activation energy

Answer 2

The minimum amount of energy required for a reaction to take place between two particles.

Question 3

Describe and explain the gradient of the curve on a rate of reaction graph

Answer 3

Initially the gradient is very steep because the rate of reaction is fastest at the start (there are more reacting particles so more frequent successful collisions).
The gradient decreases over time as the reactants are used up.
The curve eventually levels off when the reaction is complete (one or all of the reactants have been completely used up).

Question 4

How can you calculate rate of reaction?

Answer 4

Rate of reaction =
(Amount of reactant used or product formed) / Time (s)

Question 5

What units could be used for rate of reaction?

Answer 5

g/s
cm3/s
mol/s

Question 6

What conditions can be changed to increase the rate of a reaction?

Answer 6

• Increase temperature
• Increase pressure
• Increase surface area of reactants
• Increase concentration of reactants

Question 7

How does temperature affect the rate of reaction?

Answer 7

Increasing temperature increases the rate of reaction because the reactants have more energy so more particles have energy greater than or equal to the activation energy so there will be more successful reactions. Collisions also occur more frequently because the particles have more kinetic energy.

Question 8

How does surface area affect the rate of reaction?

Answer 8

Increasing surface area increases the rate of reaction because more reacting particles are exposed so there are more frequent successful collisions.

Question 9

How does concentration affect the rate of reaction?

Answer 9

Increasing concentration increases the rate of reaction because there are more reacting particles in the same volume. This means more frequent successful collisions occur.

Question 10

How does pressure affect the rate of a gaseous reaction?

Answer 10

Increasing the pressure of a gaseous reaction increases the rate of reaction because there are more reacting particles in the same volume of gas (or the same number of reacting particles in a smaller volume) so there are more frequent successful collisions.

Question 11

Draw a reaction profile diagram for an exothermic reaction, labeling activation energy

Answer 11

Question 12

Draw a reaction profile diagram for an endothermic reaction, labeling activation energy

Answer 12

Question 13

What does the Maxwell-Boltzmann distribution look like for a temperature increase?

Answer 13

Question 14

What can be added to speed up the rate of a reaction?

Answer 14

A catalyst

Question 15

What is a catalyst?

Answer 15

A substance which speeds up the rate of reaction without being chemically changed at the end.

Question 16

How does a catalyst affect the rate of reaction?

Answer 16

A catalyst increases the rate of reaction because it provides an alternate reaction pathway with a lower activation energy. More particles have energy equal to or greater than the activation energy so more frequent successful collisions will occur.

Question 17

How can a catalyst be identified in a reaction?

Answer 17

They are chemically unchanged so can be distinguished from the product. They are not involved in the reaction equation as they are not used up.

Question 18

How can you measure the rate of reaction when a gas is given off?

Answer 18

• Collect the gas produced in an upside down measuring cylinder in a trough of water or in a gas syringe to measure the volume of gas produced.
• Measure the amount of gas collected over regular time intervals.

Question 19

How can you measure the rate of reaction when a precipitate is formed?

Answer 19

Put a black cross below a beaker containing one reactant. Time how long it takes for the cross to disappear after the second reactant is added.

Question 20

Why is the precipitation method not a very accurate way to investigate rate of a reaction?

Answer 20

It’s subjective so people are likely to disagree over the exact point at which the cross is no longer visible.

Question 21

How can you measure rate of reaction using a mass balance?

Answer 21

A mass balance can be used when a gas is produced, as this will cause the mass to decrease as the reaction proceeds. The experiment can be carried out on a mass balance and the rate of reaction can be calculated by recording the mass at regular time intervals.

Question 22

A graph is plotted to show time and the amount of gas given off during a reaction. Describe the shape of the curve

Answer 22

• Initially the curve is very steep as the rate of reaction is relatively fast at the start.
• The curve becomes less steep as the reactants get used up because there are fewer successful collisions occurring.
• At the end, the graph is a flat line because all the reactants have been turned into products.

Question 23

How do the concentrations of reactants and products change during a reaction?

Answer 23

The concentrations of the products increase rapidly at the start then slows down before staying constant when the reaction is complete.
The concentrations of the reactants decrease as the reaction progresses. They decrease rapidly at the start then slow down before staying constant when one or all of the reactants have been used up.

Question 24

Marble chips react with hydrochloric acid to produce calcium chloride, water and carbon dioxide. How could you calculate the rate of reaction?

Answer 24

Since gaseous carbon dioxide is released, the rate can be measured by using digital balance to measure the change in mass over a period of time or by using a gas syringe to collect the gas and measure its volume at regular intervals.

Question 25

Marble chips react with hydrochloric acid to produce calcium chloride, water and carbon dioxide. How could you increase the rate of this reaction?

Answer 25

• Increase the surface area of the marble chips by turning them into a powder.
• Increase concentration of acid.
• Increase temperature of the reaction.