1. Thermochemistry

2. Chemical reactions
Bonds are broken and new ones are formed in a chemical reaction
Need energy for this to happen
Sometimes take energy from surroundings to break the bonds
Then either energy is released when the new ones are formed or more energy from the surroundings is required

3. Endothermic & exothermic
In an endothermic reaction heat is taken in from the surroundings, e.g., sodium hydrogencarbonate and citric acid
ΔH for an endothermic reaction is positive (heat is gained by the reaction)
(“ΔH “ represents the change in heat during a reaction)

4. In an exothermic reaction heat is given out to the surroundings, e. g
In an exothermic reaction heat is given out to the surroundings, e.g., magnesium metal and hydrochloric acid
ΔH for an exothermic reaction is negative (heat is lost by the reaction)

5. Bond energy
Bond energy is the energy required to break one mole of covalent bonds and to separate the neutral atoms from each other
Unit = kJ/mol
1 kJ = 1000 J

6. Heat of reaction
Heat of reaction is the heat change that occurs when the number of moles in a balanced equation react completely
Unit = kJ/mol

7. Heat of combustion
Heat of combustion is the heat change that occurs when one mole of a substance is completely burned in excess oxygen
Unit = kJ/mol

8. Measure it using a bomb calorimeter:
- compound placed in crucible
- oxygen is injected into calorimeter
- electric current passes through it igniting it
- as it combusts the heat generated passes into the container of water
- temperature increase is recorded by the thermometer
- water constantly stirred to ensure temperature is evenly distributed

9. Used in industry to measure the energy in food and fuels
The kilogram calorific value is the heat energy produced when 1 kg of a substance is completely burned in excess oxygen

10. Heat of formation
Heat of formation is the heat change that occurs when one mole of a compound is formed from its elements in their natural state
Unit = kJ/mol

11. Hess’s Law
Hess’s Law states that if a chemical reaction takes place in a number of steps the sum of the heat changes is equal to the heat change that would occur if it took place in one step
Law of Conservation of energy states that energy cannot be created or destroyed but transferred from one form to another

12. Heat of neutralisation
Heat of neutralisation is the heat change that occurs when one mole of an acid reacts fully with one mole of a base
Unit = kJ/mol

13. Calculation:
ΔH = m c ΔT
where m = total mass of solution in kg
c = specific heat capacity
ΔT = change in temperature

14. Q: When 50cm3 1M HCl solution is neutralised by 50cm3 1M NaOH solution the temperature rises by 6.7oC. Calculate the heat of reaction, given that the specific heat capacity for the reaction is 4.2 kJ kg-1 K-1.

15. A: m = = 100 cm3 = 0.1 kg c = 4.2 kJ kg-1 K-1 ΔT = 6.7oC = 6.7 K ΔH = m c ΔT = (0.1)(4.2)(6.7) = kJ

16. moles acid = vol x M 1000 = (50)(1) = 0. 05 0
moles acid = vol x M 1000 = (50)(1) = moles of acid produces kJ 1 mole of acid will produce kJ ΔH = kJ