Chapter 7 - Energy Changes

Question 1

Define an exothermic reaction.

Answer 1

Reactions transferring energy to the surroundings.

Question 2

What is an endothermic reaction?

Answer 2

Reactions taking in energy from the surroundings.

Question 3

Give an example of an endothermic and exothermic reaction used in day to day life.

Answer 3

Endothermic - Cold packs

Exothermic - Hand warmers

Question 4

Describe an experiment to measure the maximum temperature reached using displacement reactions. (6)

You will use magnesium and copper (II) sulfate solution in the experiment.

Answer 4

1. Measure out 20cm3 of copper sulfate into a polystyrene cup. [1]
2. Use a thermometer to record temperature of copper sulfate solution. [1]
3. Add a small amount of magnesium. [1]
4. Record maximum temperature reached. [1]
5. Empty out and measure out 20cm3 of fresh copper sulfate. [1]
6. Repeat twice and calculate a mean. [1]

Question 5

Explain an example of an exothermic reaction taking place in day to day life.

Answer 5

A self heating can is heated by a reaction that triggers when a button is pressed on the base of the can to start the reaction.

Question 6

Define the activation energy.

Answer 6

Energy needed to start a reaction.

Question 7

State the four main features of a reaction profile.

Answer 7

Reactants.
Products.
Energy Change.
Activation Energy.

(Check Figure 1 and 3 on page 116-117)

Question 8

State which type of reaction bond breaking and bond making is.

Answer 8

Bond breaking - Endothermic

Bond Making - Exothermic

Question 9

What is the bond energy of a reactant?

Answer 9

The energy needed to break the bond between 2 atoms.

Question 10

Using Table 1 on page 118, calculate the overall energy change of the equation below:

2H – H + 2O=O ——> 2 H – O – H

State whether the reaction is exothermic or endothermic.

Answer 10

Break

2 x H – H
(436)

2 x O=O
(498)

+1868

Make

4 x H – O
(464)

-1856

Change

1868 - 1856 = 12

The reaction is endothermic.

Question 11

How do you recognise whether a reaction is endothermic or exothermic after completing a bond energy calculation?

Answer 11

If overall energy is negative = Exothermic

If overall energy is positive = Endothermic

Question 12

Give 3 things that determines the voltage of a battery.

Answer 12

• The electrolyte.
• Metal in the battery.
• Difference in reactivity between 2 metals.

Question 13

What energy does a chemical cell and a fuel cell provide?

Answer 13

Electrical energy.

Question 14

Give an advantage and a disadvantage of a H2 fuel cell.

Answer 14

Advantage - No recharge time.

Disadvantage - Explosive.

Question 15

What type of reaction happens in a fuel cell?

Answer 15

An electrochemical (redox) reaction.