EJC 2023 Chemical Energetics Lecture Notes

2023 JC1 H2 CHEMISTRY (9729) CORE IDEA 3 - TRANSFORMATION Topic 5: CHEMICAL ENERGETICS - THERMOCHEMISTRY & THERMODYNAMICS Name: ___________________________________________ Civics Group: _________ Students should be able to: (a) explain that most chemical reactions are accompanied by energy changes, principally in the form of heat usually associated with the breaking and forming of chemical bonds; the reaction can be exothermic (H negative) or endothermic (H positive) (b) construct and interpret an energy profile diagram, in terms of the enthalpy change of the reaction and of the activation energy (c) explain and use the terms: (i) enthalpy change of reaction and standard conditions, with particular reference to: formation; combustion; hydration; solution; neutralisation; atomisation (ii) bond energy (H positive, i.e. bond breaking) (iii) lattice energy (H negative, i.e. gaseous ions to solid lattice) (d) calculate enthalpy changes from appropriate experimental results, including the use of the relationship: heat change = −mcT (e) explain, in qualitative terms, the effect of ionic charge and of ionic radius on the numerical magnitude of a lattice energy (f) apply Hess’ Law to construct simple energy cycles, e.g. Born -Haber cycle, and carry out calculations involving such cycles and relevant energy terms (including ionisation energy and electron affinity), with particular reference to: (i) determining enthalpy changes that cannot be found by direct experiment, e.g. an enthalpy change of formation from enthalpy changes of combustion (ii) the formation of a simple ionic solid and of its aqueous solution (iii) average bond energies (g) explain and use the term entropy (h) discuss the effects on the entropy of a chemical system by the following: (i) change in temperature (ii) change in phase (iii) change in the number of particles (especially for gaseous systems) (iv) mixing of particles [quantitative treatment is not required] (i) predict whether the entropy change for a given process or reaction is positive or negative (j) state and use the equation involving standard Gibbs free energy change of reaction, G : G H= TS− [the calculation of standard entropy change, S , for a reaction using standard entropies, S , is not required] (k) state whether a reaction or process will be spontaneous by using the sign of G (l) understand the limitations in the use of G to predict the spontaneity of a reaction (m) predict the effect of temperature change on the spontaneity of a reaction, given standard enthalpy and entropy changes

C h e m i c a l E n e r g e t i c s : T h e r m o c h e m i s t r y & T h e r m o d y n a m i c s E u n o i a J u n i o r C o l l e g e 2 | P a g e LECTURE CONTENT 1 Introduction ................................ ................................ ................................ .............................. 3 1.1 Enthalpy and enthalpy changes ................................ ................................ ........................ 3 1.2 Thermochemical equation ................................ ................................ ................................ . 4 1.3 Standard conditions and standard states ................................ ................................ .......... 5 2 Definitions of Enthalpy Changes of Reactions (Part I) ................................ .......................... 7 2.1 Standard enthalpy change of formation, fH ................................ ................................ .. 7 2.2 Standard enthalpy change of combustion, cH ................................ .............................. 7 2.3 Standard enthalpy change of neutralisation, neutH ................................ .......................... 8 2.4 Bond energy, BE ................................ ................................ ................................ ............... 9 3 Experimental Determination of Enthalpy Change (Calorimetry) ................................ ......... 10 3.1 A Simple Calorimeter ................................ ................................ ................................ ...... 10 3.2 Calculations using experimental results ................................ ................................ .......... 11 3.3 Calorimetry to determine enthalpy change of combustion ................................ ............... 15 4 Theoretical Calculation of Enthalpy Change (Hess’ Law) ................................ ................... 18 5 Definitions of Enthalpy Changes of Reactions (Part II) ................................ ....................... 25 5.1 Standard enthalpy change of atomisation, atH ................................ ............................ 25 5.2 Lattice Energy, LE................................ ................................ ................................ ........... 27 5.2.1 Factors affecting lattice energy ................................ ................................ ............ 27 5.2.2 Experimental versus theoretical lattice energies ................................ .................. 28 5.3 Ionisation Energy, IE ................................ ................................ ................................ ....... 28 5.4 Electron Affinity, EA ................................ ................................ ................................ ........ 29 6 Born-Haber Cycle ................................ ................................ ................................ ................... 31 7 Standard Enthalpy Changes of Hydration and Solution ................................ ..................... 34 7.1 Standard enthalpy change of hydration, hydH ................................ ............................... 34 7.2 Standard enthalpy change of solution, solH ................................ ................................ .. 35 7.3 Relationship between lattice energy, enthalpy change of hydration and enthalpy change of solution ................................ ................................ ................................ ....................... 35 8 Entropy ................................ ................................ ................................ ................................ ... 38 8.1 Spontaneous processes ................................ ................................ ................................ . 38 8.2 Entropy and entropy changes ................................ ................................ ......................... 39 8.2.1 Factors affecting Entropy of a Chemical System................................ .................. 39 9 Gibbs Free Energy ................................ ................................ ................................ ................. 43 9.1 Limitations in the use of G to predict spontaneity of a reaction ................................ .. 44 9.1.1 Standard states ................................ ................................ ................................ ... 44 9.1.2 Temperature ................................ ................................ ................................ ........ 44 9.1.3 Kinetics considerations ................................ ................................ ........................ 45 9.2 Calculations involving standard Gibbs free energy change of reaction ............................ 45 9.3 Calculations of rG from fG ................................ ................................ ..................... 47 9.4 Temperature dependence of G ................................ ................................ ................. 48 9.5 Effect of Temperature on spontaneity of a reaction ................................ ......................... 50 10 Summary Of Enthalpy Terms ................................ ................................ ................................ 55 REFERENCES 1. Peter Cann & Peter Hughes, Cambridge