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Key notes on Module 3 of Unit 2 for OCR AS Chemistry

Enthalpy is the heat content that is stored in a chemical system
Although we cannot directly measure enthalpy, we are able to measure the change in enthalpy,
whether that be directly or indirectly (this depends on circumstances later…

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Standard enthalpy change of formation of a compound is the enthalpy change that takes place
when one mole of a compound is formed from its constituent elements in their standard states
under standard conditions
o The enthalpy change of formation of an element is defined as 0 kJ mol-1

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o More than one reaction taking place
Enthalpy changes can be measured indirectly using enthalpy change of combustion ­ A = B ­ C
Enthalpy changes can be measured indirectly using enthalpy change of formation ­ A = C ­ B
An enthalpy cycle is a diagram that shows alternative…

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o Enzymes allow pure products to be created without any side reactions
The Boltzmann distribution is the distribution of energies of molecules at a particular
temperature, often shown as a graph
o Molecules have different energies within a reaction
o Important features of the distribution include:
The area under the…

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Catalyst presence ­ this has no affect on the position of the equilibrium because it speeds up the
rate of the forward and reverse reactions and actually increases the rate that the equilibrium is
The Haber Process is the process that produces ammonia by reacting together nitrogen from


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