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Topic 12 ­ Enthalpy Changes
Revision Notes

1) Introduction

An enthalpy change is a change in heat energy measured at constant pressure.
Enthalpy changes refer to the chemicals not the surroundings.
The symbol for an enthalpy change is H ( = change, H = heat energy)
The units for enthalpy…

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Where n= number of moles of reactant (mass/molar mass or conc x

o Enthalpy changes measured from calorimetry are smaller than the expected
values because of heat loss to the apparatus and the environment
o Other reasons for differences from standard values are nonstandard conditions
and evaporation of water…

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7) Enthalpy Profile Diagrams

These diagrams show the difference in enthalpy between the reactants and products.

8) Average Bond Enthalpy

This is the energy needed to break one mole of gaseous bonds
Example equation:
H2(g) 2H(g)

For a bond such as CH that is found in many compounds, the value…

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The reaction is endothermic if more energy is needed to break bonds in the
reactants than is released when bonds are formed in the products

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Example ­ data is enthalpy of formation

Calculate the enthalpy change for the following reaction.

Li2CO3(s) Li2O(s) + CO2(g)

Enthalpies of formation (kJ mol1) Li2CO3(s) 1216, Li2O(s) 596, CO2(g) 394

H = Hf(products) Hf(reactants)
H = (596 + (394)) ­ (1216)
= 990 + 1216
= 226 kJ mol1



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