C7 Energy changes

Question 1

Exothermic reaction

Answer 1

Energy is transferred from the reacting substances to their surroundings

Question 2

Endothermic reaction

Answer 2

Energy is transferred to the reacting substances from their surroundings

Question 3

Example of exothermic reactions

Answer 3

• Burning of fuels
• Explosives
• Reaction of acids with metals

Question 4

Example of endothermic reactions

Answer 4

• Endothermic chemical reactions are relatively rare
• A few reactions that give off gases are highly endothermic
• Dissolving salts in water is another process that is often endothermic
• Photosynthesis

Question 5

Energy level diagram

Answer 5

A graph that shows how the energy in a reaction changes as the reaction progresses

Question 6

Activation energy

Answer 6

The minimum amount of energy needed for a reaction to take place

Question 7

How to determine whether a reaction is endothermic or exothermic?

Answer 7

Step 1: energy must be supplied to break bonds
Step 2: energy is released when new bonds are made
- A reaction is exothermic if more energy is released during bond making than supplied for bond breaking
- A reaction is endothermic if more energy is supplied for bond breaking than released during bond making

Question 8

Reaction profiles - exothermic vs endothermic

Answer 8

Exothermic
- reactants at a higher energy level than the products
- energy is released

Endothermic
- reactants are at a lower energy level than the products
- energy is taken in

Question 9

Bond energy

Answer 9

The energy needed to break the bond between two atoms

Question 10

Voltage for metal combinations

Answer 10

The greater the difference in reactivity between the two metals, the higher the voltage produced by the cell