Topic 2: Calculating Enthalpy Change using molar enthalpies of formation (a version of Hess’s Law)
‐ A formation reaction is a reaction in which a compound is formed from its constituent elements. ‐ The enthalpy change calculated from measurements of a formation reaction under standard conditions is called the standard enthalpy of formation, ∆fH°
‐ The potential energy decreases from 0 kJ for the reactants to ‐285.8 kJ for the product.
‐ In other words, the reactants are at a higher chemical potential energy than the product.
Draw the potential energy diagram for this reaction
Formation of H2O(l)
H2(g) + 1/2O2(g)
Ep
(kJ)
=­285.8kJ
H2O(l)
Reaction Progress
Thermal stability is the tendency of a compound to resist decomposition when heated. ‐ The standard molar enthalpies of formation give an indication of thermal stability. ‐ The lower (more negative) the value, the greater the thermal stability
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Using Molar Enthalpies of Formation to calculate Enthalpy change of a reaction
Enthalpies of reaction and molar enthalpies of reaction for many chemical reactions can be predicted from the molar enthalpies of formation of elements and compounds
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Practice Sheet 5
1. Laboratory quantities of ethene can be prepared by an elimination reaction of ethanol using an acid catalyst. Calculate the enthalpy change for the conversion of ethanol into ethene and water using standard molar enthalpies of formation.
2. Calculate the molar enthalpy of combustion for acetic acid using standard molar enthalpies of formation.
3. An initial step in the production of iron in a blast furnace involves the conversion of iron﴾III﴿ oxide and carbon monoxide into iron﴾II, III﴿ oxide, Fe3O4﴾s﴿, and carbon dioxide. Calculate the enthalpy of this reaction using standard molar enthalpies of formation.
4. The fertilizer urea is produced along with liquid water by the reaction of ammonia and carbon dioxide. Calculate the enthalpy of this reaction using standard molar enthalpies of formation.
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