C5: Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

More energy released from bonds made than energy required to break bonds
Increase in surrounding temperature
Net decrease in potential energy
Net increase in kinetic energy

Question 2

What is an endothermic reaction?

Answer 2

Less energy released from bonds being made than energy required to break bonds
Decrease in surrounding temperature
Net increase in potential energy
Net decrease in kinetic energy

Question 3

What happens when bonds are made?

Answer 3

Energy is released

Question 4

What is the practical for temperature change?

Answer 4

• React an acid and an alkali in polystyrene cup with lid and thermometer
• Measure max temperature reached
• Repeat with increasing volumes of alkali
• Max temp will decrease as alkali is in excess (same energy released but larger volume)
• Where the two LOBF meet: volume needed to neutralise

Question 5

What is the exothermic reaction profile?

Answer 5

Products lower than reactants

Question 6

What is the endothermic reaction profile?

Answer 6

Products higher than reactants

Question 7

What is the net energy released?

Answer 7

Starting bond energy - End bond energy

Question 8

How can cells/batteries power electrical devices?

Answer 8

They contain chemicals that when reacted, create a potential difference

Question 9

What do cells generally consist of?

Answer 9

Two different metals in contact with an electrolyte

Question 10

How can renewable batteries be used again?

Answer 10

They are able to perform the reverse reaction when a potential difference is supplied

Question 11

How do hydrogen fuel cells work?

Answer 11

• Contain hydrogen and oxygen that have been separated by electrolysis
• When they recombine this produces a potential difference
  2H2 -> 4H+ + 4e-
  2O2 + 4H+ + 4e- -> 2H2O