16 — chemical energetics

Question 1

Exothermic

Answer 1

System (reactants and products) releases thermal energy to the surroundings (reaction mixture, reaction flask, surrounding air). Temperature of the reaction mixture increases. When the CR is complete, tempt of the reaction mixture gradually decreases until it reaches room tempt

Question 2

Examples of exothermic changes

Answer 2

Physical processes
1. Freezing
2. Condensation
3. Dissolving acids and alkalis in water

Chemical reactions
1. Combustion
2. Oxidation of metals
3. Neutralisation
4. Displacement of metals
5. Respiration

Question 3

Endothermic

Answer 3

When [system] (reactants and products) reacts, energy is absorbed from the surroundings. The temperature of the
surroundings decreases.
When the dissolution is complete, the temperature of the reaction mixture gradually increases until it returns to room temperature.

Question 4

Examples of endothermic changes

Answer 4

Physical:
Melting
Boiling
Sublimation
Dissolution of some ionic compounds in water

Chemical:
Photosynthesis
Thermal decomposition
Electrolysis of water
Photography

Question 5

Enthalpy change

Answer 5

Enthalpy change, [triangle] H, measures the difference in energy content of the reactants and products

Question 6

Activation energy

Answer 6

Activation energy Ea is the minimum amount of energy that colliding reactant particles must possess to react w each other

Question 7

Bond energy

Answer 7

Bond energy is the amt of energy absorbed to break one mole of a chemical bond. It is also the amount of energy released when 1 mole of that bond is formed.

Question 8

Overall Enthalpy change (after calculation)

Answer 8

The energy absorbed for bond breaking in [reactants] is less (exo)/more (endo) than the energy released when bond making in [product]. Hence, the reaction is exothermic/endothermic.