Doc Brown's A Level Chemistry Advanced Level Theoretical Physical Chemistry - AS A2 Level Revision Notes - Basic Thermodynamics GCE Thermodynamics-thermochemistry sub-index links below Part 1 - ΔH Enthalpy Changes - The thermochemistry of enthalpies of reaction, formation, combustion and neutralisation Parts 1.2a-b(i)-(iii) Thermochemistry - Hess's Law and Enthalpy Calculations This page describes Hess's Law of 'constant heat summation' and the use of thermochemical cycles to determine unknown enthalpy changes from known data e.g. involving the calculation of enthalpy of reaction, enthalpy of formation, enthalpy of combustion. As well as Hess's Law calculations other methods of solving numerical enthalpy problems without using thermochemical cycles are also described. Energetics index: GCSE Notes on the basics of chemical energy changes - important to study and know before tackling any of the three Advanced Level Chemistry pages Parts 1-3 here * Part 1a-b ΔH Enthalpy Changes 1.1 Advanced Introduction to enthalpy changes - reaction, formation, combustion : 1.2a & 1.2b(i)-(iii) Thermochemistry - Hess's Law and Enthalpy Calculations - reaction, combustion, formation etc. : 1.2b(iv) Bond Enthalpy Calculations : 1.3a-b Experimental methods for determining enthalpy changes and treatment of results : 1.4 Some enthalpy data patterns : 1.4a The combustion of linear alkanes and linear aliphatic alcohols : 1.4b Some patterns in Bond Enthalpies and Bond Length : 1.4c Enthalpies of Neutralisation : 1.4d Enthalpies of Hydrogenation of unsaturated hydrocarbons and evidence of aromatic ring structure in benzene : Extra Q page A set of practice enthalpy calculations with worked out answers ** Part 2 ΔH Enthalpies of ion hydration, solution, atomisation, lattice energy, electron affinity and the Born-Haber cycle : 2.1a-c What happens when a salt dissolves in water and why? : 2.1d-e Enthalpy cycles involving a salt dissolving : 2.2a-c The Born-Haber Cycle *** Part 3 ΔS Entropy and ΔG Free Energy Changes : 3.1a-g Introduction to Entropy : 3.2 Examples of entropy values and comments * 3.3a ΔS, Entropy and change of state : 3.3b ΔS, Entropy changes and the feasibility of a chemical change : 3.4a-d More on ΔG, Free energy changes, feasibility and applications : 3.5 Calculating Equilibrium Constants : 3.6 Kinetic stability versus thermodynamic feasibility * PLEASE note that delta H/S/G values vary slightly from source to source, so I apologise in advance for any inconsistencies that may arise as I've researched and developed each section. |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1.2 Thermochemistry - Hess's Law and Enthalpy Calculations 1.2a Hess's Law and its importance Hess's Law is a version of the general law of conservation of energy i.e. the first law of thermodynamics which can be stated as energy cannot be destroyed or created but merely changed in form or distributed in different ways. Hess's Law states that the energy change from reactants A to products B is independent of the pathway taken no matter how many stages it involves. This is shown in the diagram of enthalpy cycles below.
ΔH_{1} = ΔH(A to B) Pathway 1 the most direct route with no intermediate stages, or ΔH_{1} = ΔH_{2} + ΔH_{3} pathway 2 involving one set of intermediates C, or or ΔH_{1} = ΔH_{4} + ΔH_{5} + ΔH_{6} pathway 3 involving two sets of intermediates D and E etc. etc. - there is no limit to the complexity of the Hess's Law Cycle as long as A and B are constant. 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 From the following thermochemical data to do
1.2b(ii) Solving enthalpy problems using an 'algebraic-equation style' method 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 enthalpy summation method
ΔH_{reaction} = ∑H_{products} - ∑H_{reactants} ΔH^{θ}_{reaction,298} = ∑ΔH^{θ}_{f,298}(products) - ∑ΔH^{θ}_{f,298}(reactants) ΔH^{θ}_{esterification} = {ΔH^{θ}_{f}(ethyl ethanoate) + ΔH^{θ}_{f}(water)} - {ΔH^{θ}_{f}(ethanoic acid) + ΔH^{θ}_{f}(ethanol)} ΔH^{θ}_{esterification} = {-481 + -286} - {-487 + -278} ΔH^{θ}(esterification reaction) = (-767) - (-765) = -2 kJmol^{-1} What would be ΔH^{θ}(hydrolysis)? Answer! just reverse the sign! i.e. +2 kJmol^{-1} 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, ΔH^{θ}_{f}(oxygen) = 0, therefore after rearranging we get
A set of enthalpy calculation problems with worked out answers - based on enthalpies of reaction, formation, combustion and bond enthalpies A level Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) revision aids for revising A level chemistry courses revision guides Website content copyright © Dr W P Brown 2000-2012 All rights reserved on revision notes, puzzles, quizzes, worksheets, x-words etc. * Copying of website material is not permitted chemhelp@tiscali.co.uk Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |