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Enthalpy change




AQA Content

Define standard enthalpy of combustion
Define standard enthalpy of formation

Specification Notes

Reactions can be endothermic or exothermic.
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature


Enthalpy refers to all the heat energy that's stored in a system.
Enthalpy is very difficult to determine - the change in enthalpy is easier.
For example, neutralization reactions release heat energy - measure the temperature before and after then the change in temperature can be used to calculate the enthalpy change, ∆H.

When the enthalpy of the products is less than the reactants, ∆H is negative and the reaction is overall exothermic.

When the enthalpy of the reactants is less than the products, ∆H is positive and the reaction is overall endothermic.

Activation energy (Ea) is the minimum energy required for a reaction to take place. For a reaction to occur, the chemical bonds in the reactant molecules need to be broken – this requires energy, the Ea.

Energy profiles

Exothermic Reactions –∆H

Exothermic Reactions +∆H

Enthalpy changes are given under standard conditions

Standard Conditions:

Temperature 298K.
Pressure is 100 kilopascals.
Concentration in mole per cubic decimetres.
Chemical species in their standard states.

Common Enthalpy Changes

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions.

The enthalpy change when one mole of water is formed in a neutralization reaction under standard conditions.
(Note: This value is the same, –57kJ per mole, for any reaction of a strong acid with a strong alkali).

The enthalpy change when one mole of a compound is formed from its elements under standard conditions and species in their standard states.

(Remember this is for one mole of product. If you had 4 moles, for example, the value would need to be divided by 4).

The enthalpy change when a reaction takes place. (must be in molar quantities shown by the balanced chemical equation)


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