F321 Enthalpy

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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 change are kJ mol1
2) Exothermic Reactions
In exothermic reactions, the chemicals lose energy so H is negative. The
surroundings gain energy so feel hotter.
In terms of a reaction with oxygen, oxidation is an exothermic process.
Examples include:
Combustion ­ burning fuels for heating and in engines e.g.
CH4 + 2O2 CO2 + 2H2O
Respiration ­ oxidation of carbohydrates in living things
C6H12O6 + 6O2 6CO2 + 6H2O
3) Endothermic Reactions
In endothermic reactions, the chemicals gain energy so H is positive. The
surroundings lose energy so feel cooler.
Endothermic reactions require an input of heat energy or they will stop.
Examples of endothermic processes include:
Thermal decomposition of calcium carbonate
CaCO3 CaO + CO2
Photosynthesis
6CO2 + 6H2O C6H12O6 + 6O2
4) Measuring Enthalpy Changes
o Measuring enthalpy changes by experiment is called calorimetry
o Calorimetry works by using the energy released or absorbed in a reaction to
change the temperature of a known mass of water
o Calculating an enthalpy change from experimental results involves a two step
process
o Firstly: q = mc T/1000
Where m = mass of water in g
c= specific heat capacity of water (4.18 Jg1K1)
T = change in temperature
o Secondly: H = q/n

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Where n= number of moles of reactant (mass/molar mass or conc x
vol/1000)
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
o In the case of measuring enthalpies of combustion, differences from standard
values can occur through incomplete combustion and evaporation of the fuel (if
it is a liquid)
Example
The combustion of 0.…read more

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Enthalpy Profile Diagrams
These diagrams show the difference in enthalpy between the reactants and products.…read more

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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…read more

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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
Example ­ data is enthalpy of combustion
Calculate the enthalpy change for the following reaction.…read more

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