Chapter 9

Measure of heat energy stroked in a chemical system

Difference in enthalpy between reactants and products in a reaction

Exothermic reactions give out heat energy Has a negative enthalpy change because energy needed to make bonds is larger than energy released to break bonds Energy transferred from system to surroundings

Reaction that absorbs heat Has a positive enthalpy change Surroundings to system

Minimum amount of energy needed to break reactant bonds and start a chemical reaction

Standard set conditions for experimental measures that allow comparisons of different sets of data to be made

100 kPa

298 K

1 mol dm-3

Physical state of substance

Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation, under sc

Enthalpy change when 1 mole of a compound is formed from its elements

Enthalpy change when 1 mole of a substance completely reacts with oxygen, under sc

Enthalpy change when an acid and an alkaline react together to form 1 mole of water

Pour 150cm3 water into beaker Record initial temp Add methanol to spirit burner Weigh spirit burner Burn methanol whilst stirring water with thermometer After 3 mins - record max temperature of water Re weigh spirit burner

• heat lost to surroundings other than water • incomplete combustion • evaporation of methanol from wick- must reweigh quick • non standard conditions Values will be less exothermic

Combine known substances In insulated container Lid on Measure temp change Calculate heat given out using Q=mct

The energy required to break one mole of a specified type of bond in a gaseous molecule Calculated from actual bond enthalpies in different chemical environments

• actual bond enthalpies are different • species need to be gaseous molecules • non standard enthalpy

If a reaction can take place by 2 routes and the start and finish are the same the total enthalpy change is the same for each