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Doc Brown's A Level Chemistry - Advanced Level Theoretical Physical Chemistry GCE AS A2 IB Level Revision Notes Basic Thermodynamics thermochemistry Part 1 ΔH Enthalpy Changes The thermochemistry of enthalpies of reaction, formation, combustion and neutralisation
Parts 1.2ab(i)(iii)
Thermochemistry Hess's Law and Enthalpy
Calculations
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1.2 Thermochemistry Hess's Law and Enthalpy Calculations 1.2a Hess's Law and its importance
Many problems can be set out as a 'Hess's Law' triangle e.g.
Note: Superscript θ means a standard enthalpy value (see further down the page) and each ΔHθ could be a composite of several ΔHθ values - see enthalpy of combustion calculations below. 1.2b Using Hess's Law to perform enthalpy calculations 1.2b(i) Using known enthalpies of reaction or combustion etc. Using a Hess's Law Cycle you can calculate enthalpies of formation which you could not determine by laboratory experiment. However, using these calculated enthalpies of formation and experimentally determined enthalpies of combustion a huge variety of other enthalpies for other reactions can then be calculated. The 1st example of calculating the enthalpy of formation of methane illustrates the principles of using Hess's Law, especially as this cannot be determined by direct laboratory experiment! Given the following data below from text/data book calculate the enthalpy of formation of methane.
All of the three enthalpies above can be very accurately determined by direct experiment in a calorimeter.
2nd example of Hess's Law Cycle calculation From the following thermochemical data
Calculate the enthalpy change for the reaction
3rd example of Hess's Law Cycle calculation Problem solving from the following thermochemical data: 1.2b(ii) Solving enthalpy problems using an 'algebraicequation style' method BUT, remember, the question might specify solving the problem via a Hess's Law cycle! Given the following data
Calculate the standard enthalpy of combustion of propane
What you do is rearrange, if necessary, the data equations and add up the results both equation components and delta H values, cancelling out the equation components should leave you with the correct equation whose enthalpy value you require.
Note that here you are using factors of 3 and 4 to make the cycle balance in molar terms 2nd example of 'algebraic' calculation style Given the following thermochemical data:
Calculate the enthalpy change for the reaction
Note that you double the enthalpy of formation hydrogen chloride to make the cycle balance in molar terms 1.2b(iii) A method using the summation of enthalpies of reactants and products A very simple example of an enthalpy summation method
ΔHreaction = ∑Hproducts ∑Hreactants ΔHθreaction,298 = ∑ΔHθf,298(products) ∑ΔHθf,298(reactants) ΔHθesterification = {ΔHθf(ethyl ethanoate) + ΔHθf(water)} {ΔHθf(ethanoic acid) + ΔHθf(ethanol)} ΔHθesterification = {481 + 286} {487 + 278} ΔHθ(esterification reaction) = (767) (765) = 2 kJmol1 What would be ΔHθ(hydrolysis)? Answer! just reverse the sign! i.e. +2 kJmol1
2nd example of enthalpy summation method A more complicated example where you need to think more about mole ratios in the equation. Given the following data from laboratory measurements
Calculate the standard enthalpy of formation of butane gas, which cannot be determined by experiment.
To solve this you can use equation (3) and the data from equations (1) and (2) to obtain a value for equation (4)
Since oxygen is an element, ΔHθf(oxygen) = 0, therefore after rearranging we get
Enthalpy calculation problems with worked out answers based on enthalpies of reaction, formation, combustion Energetics-Thermochemistry-Thermodynamics Notes INDEX |
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