enthalpy changes

Enthalpy

heat content stored in a chemical system

Enthalpy change

heat exchange with surroundings at constant temp.

Standard conditions

Temp= 298K

Pressure= 100kPa

Conc= 1M

Exothermic

NEGATIVE

Releasing energy to surroundings (increase in temp).

Enthalpy of products is less than enthalpy of reactants due to energy being lost to sureoundings.

Endothermic

POSITIVE

Energy taken in from the surroundings (decrease in temp).

Enthalpy of products is greater than reactants as energy has been taken in from sureoundings.

Calculating heat capacity

• Heat energy transferred:

Energy transferred:

(q = M x C x temp change / 1000)

M= vol C= 4.18 x temp change

DIVIDE BY

(moles of fuel used)

standard enthalpy change of combustion

• enthalpy change when 1 mol of substance reacts completely with oxygen.

standard enthalpy change of neutralisation

• enthalpy change when 1 mole of H2O formed due to neutralisation.

• H+ + OH- —> H2O

• so always -57

standard enthalpy change of formation

• enthalpy change when 1 mol of a compound formed from its elements

standard enthalpy change of reaction

• energy change associated with a given reaction can

suggest two reasons why enthalpy change of combustion is less exothermic than calculated

• non standard conditions

• heat released to surroundings

benefits of using a catalyst

• reaction takes place at lower temp

• less energy due to lower Ea

• less carbon dioxide emitted to atmosphere