Chemistry Unit 2

Unit 2.1 – Thermochemistry

Key Words

Enthalpy H– is the heat content of a system at constant pressure

Enthalpy change – the heat added to a system at constant pressure (measured in J/kJ)

Principle of conservation of energy – states that energy cannot be created or destroyed

Standard State- The standard state gives the state (solid, liquid or gas) and colour of a substance at room temperature (298K) and at 1 atmosphere.

Standard conditions - 298K (25 degrees Celsius) and pressure of 1 atmosphere

Enthalpy change of combustion- The process when one mole of a compound is completely burnt in excess oxygen

Enthalpy change of reaction- The energy change at constant pressure and a stated temperature for a process in which a specified amount of reactants are converted into products

Enthalpy change of formation- When one mole of a compound is formed from its elements in their standard states

Hess’s Law- The enthalpy change of a reaction is the same, whether it occurs in one step or a series of steps, providing that the initial and final conditions are the same for each route.

Entropy- The measure of how disordered a system is.

Exothermic reactions

This is a reaction that releases energy to the surroundings and there is a rise in temperature. Enthalpy change is negative because H products is smaller than H reactants.

Endothermic reactions

Reactions that take in heat energy from the surroundings and there is a temperature drop. Enthalpy change is positive because H products are greater than H reactants

Measuring the enthalpy change of combustion of different fuels

To work out the enthalpy change:

1. Work out the heat given out by the fuel, this can be done by heating up water, as water absorbs a specific amount of energy (4.18J per g).

Mass of water heated x 4.18 x temperature rise

1. Work out the number of moles of fuel that has been burnt. This can be done by working out the difference between the mass of fuel before the water heated, and the mass of fuel afterwards.

Mass
Mr

1. Work out the energy given out by 1 mole of the substance. (Always use the smaller of the mole values as this is the one that is not in excess)

Heat given out by the fuel
Number of moles

1. This gives you the J/mol^-1, so to get the answer in KJ, divide by 1000.

(You could also use energy per gram but that is only really to compare values with other substances

Hess's Energy Cycle

Use arrows to calculate enthalpy change. Remeber that the arrows will go in different direction depending…