Hess’s Law as Chem Tutorial 2 Kajana
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Transcript of Hess’s Law as Chem Tutorial 2 Kajana
AS Chemistry – Energetics part 2 Tutorial 2
Redington College Kajana SIvarasa
1
Hess’s Law The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. Or The energy change for a reaction is the same whether a reaction takes place in one stage or in a series of stages.
• Hess’s law tells us that the enthalpy change of reaction for the direct route is the same as for the indirect route. It does not matter how many steps there are in the indirect route. • So we deduce the following (using the diagram)
ΔH1 = ΔH2 + ΔH3 + ΔH4
A D
B C
ΔH1
ΔH4
ΔH3
ΔH2
AS Chemistry – Energetics part 2 Tutorial 2
Redington College Kajana SIvarasa
2
Ø Calculating the enthalpy change of formation, ΔHf using the enthalpy change of combustion.
• Enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.
Example C(s) + 2H2(g) àCH4(g)
AS Chemistry – Energetics part 2 Tutorial 2
Redington College Kajana SIvarasa
3
Ø Calculating enthalpy of reaction, ΔHr from enthalpy of formations
• Enthalpy change of reaction is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions in their standard states.
Example
AS Chemistry – Energetics part 2 Tutorial 2
Redington College Kajana SIvarasa
4
Bond Enthalpy
Ø Bond enthalpy(BE) is the energy needed to break 1 mole of a particular covalent bond.
Ø Mean bond enthalpy is the mean bond dissociation enthalpy for a particular bond in a range of compounds.
Ø Bond Breaking : Requires energy to overcome attractive forces joining the atoms together : Endothermic
Ø Bond Making : Energy is released : Exothermic
Ø In a chemical reaction :
• If the energy needed to break the bonds is less than the energy released when new bonds are formed the reaction will release energy and the reaction is exothermic.
• If the energy needed to break the bonds is more than the energy released when new bonds are formed the reaction will absorb energy and the reaction is endothermic.
• Exothermic
AS Chemistry – Energetics part 2 Tutorial 2
Redington College Kajana SIvarasa
5
• Endothermic
Ø Calculating Enthalpy change of a reaction from bond enthalpies :
AS Chemistry – Energetics part 2 Tutorial 2
Redington College Kajana SIvarasa
6
Ø Using bond enthalpies in Hess cycle calculations Example Q : The enthalpy change of formation of ethanol vapour from its elements can be estimated by applying the standard enthalpies of atomization and bond enthalpies to a Hess cycle. Given the data:
ΔHat [ C(graphite)] = +718 kJ/mol ΔHat[H2(g)] = +218kJ/mol ΔHat [O2(g)] = +249kJ/mol
And the mean bond enthalpies given above.