Separate Chemistry - C5 Energy Changes

Question 1

Name the following reactions that:

a) Take in heat from the surroundings
b) Give out heat to the surroundings

Answer 1

a) Endothermic
b) Exothermic

Question 2

Name the type of reaction used in self-heating cups and hand warmers.

Answer 2

Exothermic reaction

Question 3

Give an example of a chemical reaction that is exothermic.

Answer 3

Examples include: Combustion, Oxidation, Neutralisation

Question 4

How do you know the reaction profile below is for an exothermic reaction?

Answer 4

Energy of products is lower than the energy of reactants

Question 5

What can be said about the amount of energy before a chemical reaction and the amount of energy after a chemical reaction?

Answer 5

They are equal

Question 6

Give an example of a chemical reaction that is endothermic.

Answer 6

Examples include: Thermal Decomposition and Reacting Citric Acid with Sodium hydrogencarbonate

Question 7

Name the type of reaction is used in sports injury packs.

Answer 7

Endothermic reaction

Question 8

How do you know the reaction profile below is for an endothermic reaction?

Answer 8

Energy of products is higher than the energy of reactants

Question 9

Draw an energy level diagram (reaction profile) for an exothermic reaction.

Answer 9

Question 10

What is the name for the minimum amount of energy needed for a reaction to happen?

Answer 10

Activation energy

Question 11

Higher Q. Complete the sentences:

In a chemical reaction:

a) Energy is needed to ……………….
b) Energy is released when ……….

Answer 11

a) Energy is needed to break bonds
b) Energy is released when bonds are formed

Question 12

Calculate the overall energy change in this reaction.

Bond energies: NN = 941 kJ/mol ; N-H = 391 kJ/mol ; H-H = 436 kJ/mol

*this is just an example, you may have to do this for any reaction. You will be given the bond energies.

Answer 12

Reactants: (6 x 391) = 2346

Products: (1 x 941) + (3 x 436) = 2249

2346 – 2249 = + 97 kJ/mol

Question 13

Draw an energy level diagram (reaction profile) for an endothermic reaction.

Answer 13

Question 14

Look at the reaction profiles below. Which arrows show the activation energy and which arrows show the overall energy change?

Answer 14

Question 15

Higher Q. Calculate the overall energy change in this reaction.

Bond energies: C-H = 413 kJ/mol ; O=O = 495 kJ/mol ; C=O = 799 kJ/mol ; H-O = 463 kJ/mol

*this is just an example, you may have to do this for any reaction. You will be given the bond energies.

Answer 15

Reactants: (4 x 413) + (2 x 495) = 2642

Products: (2 x 799) + (4 x 463) = 3450

2642 – 3450 = -808 kJ/mol

Question 16

Higher Q. Explain, in terms of bonds and energy, what makes a reaction…

a) … exothermic.
b) … endothermic

Answer 16

a) Exothermic: The energy released from forming new bonds is greater than the energy needed to break existing bonds.
b) Endothermic: The energy needed to break existing bonds is greater than the energy released when new bonds are formed.

Question 17

Separate Q. Suggest a factor that can affect the voltage produced by a cell.

Answer 17

Factors include: Type of electrode , Type of electrolyte

Question 18

Separate Q. Describe how to make a simple cell.

Answer 18

Connecting two different metals in contact with an electrolyte.

Question 19

Separate Q. What is a battery?

Answer 19

Two or more cells connected in series

Question 20

Separate Q. Are alkaline batteries rechargeable or non-rechargeable?

Answer 20

Non-rechargeable

Question 21

Separate Q. Name the component that contains chemicals that react to produce electricity.

Answer 21

A cell

Question 22

Separate Q. Why do non-rechargeable batteries stop working?

Answer 22

Chemical reactions inside the battery stop when one of the reactants has been used up.

Question 23

Separate Q. How are rechargeable batteries recharged?

Answer 23

A electrical current is supplied to the battery that reverses the chemical reaction inside.

Question 24

Separate Q. Describe how a fuel cell works

Answer 24

• An external source of fuel (e.g. Hydrogen, Oxygen or Air) goes into the fuel cell.
• The fuel is oxidised electrochemically
• To produce a potential difference

Question 25

Separate Q. What reaction takes place inside a Hydrogen Fuel Cell?

Answer 25

• Oxidation reaction.
• Hydrogen is oxidised to produce water.

Question 26

Separate Higher Q. Write the half equations at the anode and cathode of a Hydrogen Fuel Cell.

Answer 26

• Anode (negative electrode *different to electrolysis):

H₂ → 2H⁺ + 2e^-

• Cathode (positive electrode *different to electrolysis):

O₂ + 4H⁺ + 4e^- → 2H₂O

Question 27

Separate Q. List some advantages of Hydrogen Fuel cells as an alternative to rechargeable batteries and cells.

Answer 27

• Fuel cells are less expensive than rechargeable batteries.
• Rechargeable batteries need to be replaced after so many charges.
• Fuel cells store more energy than rechargeable batteries.

Question 28

Separate Q. List some advantages of using Hydrogen Fuel Cells in cars as an alternative to conventional fuels.

Answer 28

• Using fuel cells does not use up finite resources (e.g. fossil fuels).
• Fuel cells don’t produce pollutants, they only produce clean water.