Thermochemistry

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

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)

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)

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

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

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

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

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

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

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

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

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

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.

A: m = 50 + 50 = 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) = 2.814 kJ

moles acid = vol x M 1000 = (50)(1) = 0. 05 0 moles acid = vol x M 1000 = (50)(1) = 0.05 0.05 moles of acid produces 2.814 kJ 1 mole of acid will produce 56.28 kJ ΔH = - 56.28 kJ