NJC Energetics H1 Lecture Notes Student

National Junior College SH1 H1 Chemistry 1 Content ● Enthalpy changes (ΔH): ΔH of formation; combustion; neutralisation; bond energy; lattice energy ● Hess’ Law Learning Outcomes: Candidates 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 (ΔH negative) or endothermic (ΔH 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; neutralisation (ii) bond energy (ΔH positive, i.e. bond breaking) (iii) lattice energy (ΔH negative, i.e. gaseous ions to solid lattice) (d) calculate enthalpy changes from appropriate experimental results, including the use of the relationship: heat change = mcΔT (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 carry out calculations involving given simple energy cycles and relevant energy terms (restricted to enthalpy changes of formation, combustion and neutralisation), 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) average bond energies [construction of energy cycles is not required] Chemical Energetics All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any other information storage and retrieval system, without prior permission in writing from the copyright owner. Copyright © 2022 National Junior College

National Junior College SH1 H1 Chemistry 2 Note: It is not possible to measure the absolute enthalpy of the reactant or product in a system directly. We can only measure changes in enthalpy of the system when it undergoes chemical or physical changes. 1 Introduction The study of energy changes which t ake place during chemical reactions is called chemical energetics or thermochemistry. 1.1 Enthalpy Change, H Learning Objective: Able to explain that most chemical reactions and physical processes are accompanied by changes in heat energy which is a result of the breaking and forming of chemical bonds; ΔH = Hfinal − Hinitial ● When there is a net release in e