Topic 5 Energetics

Section 3 (a)

Exothermic Reaction

An exothermic reaction releases heat energy to the surroundings. Therefore, the temperature of the surroundings increases.

Examples: combustion(of hydrocarbons), many oxidisation reactions and neutralisation

Endothermic Reaction

An endothermic reaction takes in heat energy from the surroundings. Therefore, the temperature of the surroundings decreases.

Examples: thermal decomposition of metal carbonate, ammonium nitrate, …

Calculate the heat energy change from a measured temperature change

Q = mc∆T

Q = heat energy change (in Joules)

m = mass of solution/water in grams ( density of water = 1g/cm³) 

*If 2 solutions are mixed, remember to add the masses of both solutions together for m.

c = specific heat capacity (usually specific heat capacity of water = 4.2 J/g/^oC)

∆T = change in Temperature (in ^oC)

Calculate molar enthalpy change(∆H)

Find the number of moles of the limiting reagent(the reagent with the fewest no. of moles) using no. of moles = Mass / Mr

＊if the limiting reagent is the solution, no. of moles = concentration(mol/dm³) x Volume (in dm³)

Divide Q( convert to kJ）by no. of moles.

Add a negative sign to the result if it’s an exothermic reaction.

Practical 3.8: Investigate temperature changes accompanying displacement reactions, salts dissolving in water and neutralisation reactions

Key points:

• Polystyrene is a good heat insulator so the polystyrene cup reduces heat loss.

• The lid prevents heat loss to the air above and the surroundings.

• The cotton wool provides insulation and reduces heat loss.

Temperature change in a combustion reaction

Key points

● The equation for this reaction is: C2H5OH + 3O2 → 2CO2 + 3H2O

● A copper can is used in this experiment because copper has a low specific heat capacity so only absorbs a small amount of the heat energy.

● The biggest source of error in this experiment is heat loss to the environment. This could be reduced by placing a lid on the copper can. The lid must have hole for the thermometer.