Chemical energetics

Question 1

Describe the meaning of exothermic and endothermic reactions

Answer 1

• Exothermic gives out heat
• Endothermic takes in heat
  for graphs (pg87)

Question 2

Describe bond breaking as endothermic and bond forming as exothermic

Answer 2

In exothermic:
- energy released when new bonds form > energy needed to break bonds in reactants
In endothermic:
- energy needed to break bonds > energy released when new bonds formed

Question 3

Describe the production of electrical energy from simple cells

Answer 3

• An electrochemical cell contains 2 different reactivity dipping into an electrolyte.
• When connected to each other by a a wire, the electric current flows from - electrode to + electrode.
• The metal higher in reactivity series is the - electrode because it is better at releasing electrons
• The further apart the metals are in the reactivity series, the greater the voltage

Question 4

Describe the use of hydrogen as a potential fuel reacting with oxygen to generate electricity in a fuel cell

Answer 4

In a fuel cell:

• Hydrogen gas is bubbled through the cathode
• Oxygen gas is bubbled through the anode
• Electrode is platinum coated onto porous materials to allow gases to pass through

Question 5

Acidic electrolyte

Answer 5

• Negative electrode: hydrogen loses electrons and forms hydrogen ions in the electrolyte:
  + 2H2 -> 4H + 4e
• Positive electrode: electrons move round the external circuit to positive electrode. Here, oxygen gains electrons and reacts with hydrogen ions from the acid electrolyte:
  + O2 + 4H + 4e -> 2H20
• Overall equation: the hydrogen ions removed at the positive electrode are replaced by those produced at the negative electrode => concentration of electrolyte remains constant
  + 2H2 + O2 -> 2H20
• Water is then removed

Question 6

Alkaline electrolyte

Answer 6

• Negative electrode: hydrogen reacts with hydroxide ions in the electrolyte and forms water:
  + 2H2 + 4OH -> 4H2O + 4e
• Positive electrode: oxygen gains electrons and reacts with water to form hydroxide ions:
  + O2 +2H2O +4e -> 4OH
• Overall reaction: the hydroxide ions removed at negative electrode are replaced by those produced at positive electrode => concentration remains constant
  + 2H2 + O2 -> 2H20
• Water is then removed

Question 7

Advantages of fuel cells

Answer 7

• Water is the only product made - no pollutants formed
• Lightweight
• High efficiency

Question 8

Describe radioactive isotopes, such as 235U, as a

source of energy

Answer 8

• It is bombarded with high speed neutron => cause the nucleus to split => large amount of energy released
• More neutrons are produced as well as atoms of lower mass such as thorium and radium => cycle repeats