7. Rates of Reaction and Energy Changes

Question 1

What is the equation for rate of reaction?

Answer 1

Rate of reaction = amount of reactant used or product formed / time (s)

Question 2

What units could be used for rate of reaction?

Answer 2

g/s
cm³/s
mol/s

Question 3

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

Answer 3

• Measure time and collect gas in an upside down measuring cylinder in a trough of water or in a gas syringe to measure the volume of gas produced.
• Measure time and the change in mass.

Question 4

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

Answer 4

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 5

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

Answer 5

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

Question 6

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

Answer 6

When a gas is produced this will cause mass to decrease. The experiment can be carried out on a digital balance and the rate of reaction can be calculated by recording the mass at regular time intervals.

Question 7

What does the term ‘activation energy’ mean?

Answer 7

The minimum amount of energy required for a reaction to occur between two reacting particles.

Question 8

What must happen for a reaction to occur?

Answer 8

Particles must collide at the correct orientation with sufficient energy to react.

Question 9

In terms of particles, what 2 things could happen when the rate of reaction increases?

Answer 9

• More frequent collisions.
• Energy of collisions increases.

Question 10

How can the rate of a reaction be increased?

Answer 10

• Increase the temperature.
• Increase the concentration.
• Increase surface area to volume ratio.
• Add a catalyst.
• Increase the pressure (for gases).

Question 11

How does temperature affect the rate of reaction?

Answer 11

Increasing the temperature increases the rate of reaction. This is because the reactants have more energy so more particles have energy above the activation energy meaning more collisions will be successful. Collisions also occur more frequently because the particles have more kinetic energy.

Question 12

How does surface area affect the rate of reaction?

Answer 12

Increasing the surface area of reactants increases the rate of reaction. This is because a greater surface area means there are more exposed particles so more frequent successful collisions.

Question 13

How does a catalyst affect the rate of reaction?

Answer 13

A catalyst increases the rate of reaction. This is because it provides an alternative reaction pathway with a lower activation energy. More particles will have sufficient energy to overcome the activation energy and react so more successful collision occur in the same time.

Question 14

How does concentration affect the rate of reaction?

Answer 14

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

Question 15

How does pressure affect the rate of a gaseous reaction?

Answer 15

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

Question 16

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

Answer 16

Since gaseous carbon dioxide is released, the rate can be measured by using a digital balance to measure the change in mass over a period time.

Question 17

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

Answer 17

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

Question 18

After completing an experiment, how can you find the rate of reaction using a graph?

Answer 18

Plot:
* X axis: time
* Y axis: amount of reactant used or product formed

Draw a tangent to a point on the graph and find the gradient of this line to find the rate of reaction at that time.

Question 19

A graph showing time and the amount of gas given off during an experiment has a steep gradient. What does this tell you about the rate of the reaction?

Answer 19

Fast rate of reaction.

Question 20

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

Answer 20

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 21

What is a catalyst?

Answer 21

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

Question 22

How does a catalyst speed up the rate of a reaction?

Answer 22

It provides an alternate reaction pathway with a lower activation energy.

Question 23

What are enzymes?

Answer 23

Enzymes act as biological catalysts that increase the rate of reactions in living cells.

Question 24

Which enzyme is used to produce ethanol from glucose?

Answer 24

Yeast

Question 25

What is an exothermic reaction?

Answer 25

A reaction that gives out energy to the surroundings.

Question 26

What is an endothermic reaction?

Answer 26

A reaction that takes in heat energy from the surroundings.

Question 27

Are neutralisation reactions endothermic or exothermic?

Answer 27

Exothermic

Question 28

Are displacement reactions endothermic or exothermic?

Answer 28

Either exothermic or endothermic.

Question 29

Is a salt dissolving in water endothermic or exothermic?

Answer 29

Either exothermic or endothermic.

Question 30

Are precipitation reactions endothermic or exothermic?

Answer 30

Exothermic

Question 31

How could you measure the temperature change of a neutralisation reaction?

Answer 31

1) Measure initial temperature of the solutions.
2) Mix both reactions in a polystyrene cup.
3) Record the highest temperature reached.
4) Calculate the temperature change.

Question 32

When measuring the temperature change of a reaction, heat loss means that measurements may be inaccurate. How could heat loss me minimised during an experiment?

Answer 32

• Use polystyrene cup.
• Place reaction cup in a beaker full with cotton wool for extra insulation.
• Lid on the reaction cup.

Question 33

In terms of bond energies, what happens during a chemical reaction? Describe exothermic and endothermic reactions in terms of bonds breaking/forming.

Answer 33

Energy is needed to break bonds and is released when bonds are made.

Exothermic: energy released from breaking bonds is greater than the energy used to make bonds.

Endothermic: energy released in forming new bonds is greater than the energy used to break old bonds.

Question 34

What two types of reaction are exothermic?

Answer 34

Combustion and neutralisation

Question 35

Give an example of an endothermic reaction

Answer 35

• Thermal decomposition
• Photosynthesis

Question 36

How can the energy change of a reaction be calculated from bond energies?

Answer 36

Energy change (kJ mol⁻¹) =
total energy of bonds broken - total energy of bonds made

Question 37

If the energy change of a reaction is negative, is the reaction exothermic or endothermic?

Answer 37

Exothermic, energy has been lost to the surroundings.

Question 38

What is a reaction profile?

Answer 38

A graph showing the relative energies of reactants and products, as well as the activation energy of a reaction.

Question 39

Draw a reaction profile for an endothermic reaction.

Answer 39

(search up picture)

Question 40

Draw a reaction profile for an exothermic reaction.

Answer 40

(search up picture)