C7 - Energy Changes

Question 1

define exothermic reaction

Answer 1

A reaction that transfers energy to the surroundings

Question 2

define endothermic reaction

Answer 2

A reaction that takes in energy from the surroundings

Question 3

give 2 examples of exothermic reactions

Answer 3

• respiration
• combustion
• neutralisation
• oxidation

Question 4

give 2 examples of endothermic reactions

Answer 4

• photosynthesis
• thermal decomposition
• reduction

Question 5

What is an energy change represented by?

Answer 5

ΔH

Question 6

Endothermic or Exothermic?
1. breaking bonds
2. making bonds

Answer 6

1. breaking bonds is endothermic (energy is needed to break bonds)
2. making bonds is exothermic

Question 7

Endothermic or Exothermic?
1. positive ΔH
2. negative ΔH

ΔH - energy change

Answer 7

1. endothermic because more energy is stored in the products
2. exothermic because less energy is stored in the products

Question 8

define activation energy

Answer 8

the minimum energy required for a reaction to start

Question 9

What diagram is used to show if a reaction is exothermic or endothermic?

Answer 9

Reaction profile - potential energy against time

Question 10

When do reactions occur in terms of particles?

Answer 10

Chemical reactions occur when reacting particles collide and have sufficient kinetic energy.

Question 11

steps for bond energy calculations

Answer 11

1. balance the equation
2. draw the displayed formula for all the molecules
3. calculate the energy required to break bonds in the reactants, E_R
4. calculate the energy required to make bonds in the products, E_P
5. ΔH = E_R - E_P

Question 12

What type of reaction occurs in an electrical cell?

Answer 12

A redox reaction (oxidation reduction) - displacement

Question 13

Write a half equation for the reaction occurring at the negatively charged electrode in a hydrogen-oxygen fuel cell

higher

Answer 13

2H₂ + 4OH^- → 4H₂O + 4e^-

Question 14

Write a half equation for the reaction occurring at the positively charged electrode in a hydrogen-oxygen fuel cell

higher

Answer 14

O₂ + 2H₂O + 4e^- → 4OH^-

opposite electrolysis of OH^-

Question 15

What does a fuel cell use?

Answer 15

A constant supply of fuels to produce a voltage

Question 16

What does a chemical cell use?

Answer 16

2 metal electrodes and a salt bridge to produce a voltage

Question 17

What are the advantages of hydrogen-oxygen fuel cells?

Answer 17

• No pollutants produced
• No need to be electrically recharged

Question 18

What are the disadvantages of hydrogen-oxygen fuel cells?

Answer 18

• Hydrogen is highly flammable
• Hydrogen is difficult to store

Question 19

What is the product of hydrogen-oxygen fuel cells?

Answer 19

Water