Heat of reaction (energy change)

​

Enthalpy change: the amount of energy absorbed or released in a chemical reaction carried out under constant pressure

​

ΔH = HEAT ABSORBED DURING BOND BREAKING - HEAT RELEASED DURING BOND FORMING

​

Catalyst: reduced the activation energy needed, not the enthalpy change

Concept

Characteristics:

• temp of surroundings increase
• products have less energy than reactants
• negative ΔH

(temp increases rapidly after the reaction begins as heat energy is given off to the env, temp falls gradually after reaction is complete and returns to room temp)

​

Why negative?

*the total amount of energy absorbed in breaking the bonds in the reactants is LESS THAN the total amount of energy released from bond forming in the products

Exothermic reactions

1. Nuclear fission; large atomic nucleus, eg uranium, split into 2/ smaller
2. Combustion; use oxygen to burn fuel
3. Neutralisation; free moving aq H+ and OH- come together and form bonds
4. Respiration; use oxygen to burn glucose

C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + energy

• Converting anhydrous salt to hydrated; CuSO4 + H2O -> CuSO4.5H2O
• Haber process; manufacture ammonia
• Contact process; 2 SO2 + O2 ->2 SO3
• Condensation, freezing, dissolving of anhydrous salt/acids (physical process)

Exothermic reactions (examples)

Characteristics:

• temp of surroundings decrease
• products have higher energy than reactants
• positive ΔH

(temp decreases rapidly after reaction begins as heat energy is taken in from the env, temp gradually increases after reaction is complete and returns to room temp)

​

Why positive?

*the total amount of energy absorbed in breaking the bonds in the reactants is MORE THAN the total amount of energy released from bond forming in the products

Endothermic reactions

1. Thermal decomposition of carbonate and nitrate salts
2. Dissolving crystals of potassium or ammonium salts in water
3. Photography; light falls on silver bromide → agitate e in Br- → move to Ag+ compound break up → form dark areas on the film

2AgBr (under light) -> Br2 + 2Ag (redox)

• Photosynthesis;

6 CO2 + 6 H2O -> C6H12O6 + 6 O2

• Evaporation, melting, dissolving some ionic compounds(physical process)

Endothermic reactions (examples)

Energy profile diagram

Energy Diagrams

Energy level diagram