ASRJC 2024 H2 Chem Thermochemistry Notes

2024 JC1 H2 CHEMICAL ENERGETICS I (THERMOCHEMISTRY) 2024/ASRJC/Chemistry 1 Anderson Serangoon Junior College H2 Chemistry CHEMICAL ENERGETICS I (THERMOCHEMISTRY) Contents/Outline 1 Introducing enthalpy change, exothermic and endothermic reactions 2 Defining common enthalpy changes: ∆ H of reaction, formation, combustion, neutralisation, atomisation and bond energy 3 Determining enthalpy changes using experiments and q = mcT 4 Defining enthalpy changes involving ionic compounds: ∆ H of ionisation, electron affinity, lattice energy, hydration and solution 5 Determining enthalpy changes by applying Hess’ Law through energy cycle (including Born −Haber cycles), formulae and algebraic manipulation. Learning Outcomes Students should be able to: (a) explain that some chemical reactions are accompanied by energy changes, principally in the form of heat energy; the energy changes can be exothermic (∆H negative) or endothermic (∆H positive) (b) construct and interpret a reaction pathway 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. 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 References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Understanding Advanced Physical Inorganic Chemisry, Jeanne Tan and Chan Kim Seng, WS education 3. Chemistry, The Molecular Nature of Matter and Change (Fourth Edition), Silberberg, McGraw Hill 4. Chemistry & Chemical Reactivity (Sixth Edition), Kotz, Treichel and Weaver, Thomson 5. Chemistry The Central Science (Ninth Edition), Brown, LeMay, Bursten, Prentice Hall

2024 JC1 H2 CHEMICAL ENERGETICS I (THERMOCHEMISTRY) 2024/ASRJC/Chemistry 2 Contents: Part 1: Introduction 1.1 System and surroundings 1.2 Heat transfer 1.3 Enthalpy and enthalpy change 1.4 Exothermic and endothermic reactions 1.5 Standard conditions and standard states Part 2: Types of enthalpy changes 2.1 Standard enthalpy change of reaction 2.2 Standard enthalpy change of formation 2.3 Standard enthalpy change of combustion 2.4 Standard enthalpy change of neutralisation 2.5 Standard enthalpy change of atomisation 2.6 Bond energy Part 3: Experimental method to determine enthalpy change 3.1 Calorimeter 3.2 Temperature correction 3.3 Experimental method to determine enthalpy change of combustion Part 4: Enthalpy changes involving ionic compounds 4.1 Ionisation energy 4.2 Electron affinity 4.3 Lattice energy 4.4 Standard enthalpy change of hydration 4.5 Standard enthalpy change of solution Part 5: Hess’ Law 5.1 Hess’ Law of constant heat summation 5.2 Born-Haber cycles

2024 JC1 H2 CHEMICAL ENERGETICS I (THERMOCHEMISTRY) 2024/ASRJC/Chemistry 3 1 INTRODUCTION • What is enthalpy change? • What is an exothermic reaction and an endothermic reaction? Thermodynamics is the study of the interaction between matter and energy. It considers both the energy change and flow of energy from one substance to another. This is used to predict the behaviours of chemical systems – to determine whether or not a change is possible. Chemical energy of a matter is made up of kinetic and potential energy. How do particles in a matter possess kinetic energy (K.E.)? K.E. is the energy of motion arising from rotations, vibrations and translational movements of particles. The temperature of a system is a measure of the average kinetic energy of that system. How do particles in a matter possess potential energy (P.E.)? P.E. is the energy that particles store within the electrostatic attractions or repulsions that they experience with one another. According to the First Law of Thermodynamics, energy can neither be created nor destroyed; it can only be transferred. The ways to change K.E. and P.E. from one form to another are through heat or work. Most chemical reactions or phase changes are accompanied by the absorption or release of heat. Thermochemistry is thus t he study of heat energy changes that occur during these changes. 1.1 System and Surrounding To study heat energy changes, we focus on a particular part of the universe, which we call the ‘system’. Everything else that can exchange energy with the system is called the ‘surrounding’. For a chemical reaction, at the start the system is the reactants, and at the end the system is the products. e.g. For a neutralisation reaction between aqueous NaOH and aqueous HCl: system NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) surrounding everything else, including the solution, beaker, air, etc. System: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Surrounding Surrounding Surrounding Surrounding

2024 JC1 H2 CHEMICAL ENERGETICS I (THERMOCHEMISTRY) 2024/ASRJC/Chemistry 4 1.2 Heat transfer Heat is the energy that flows into or out of a system due to a difference in temperature between the system and its surroundings. When a system and its surroundings are in thermal contact, energy flows from the region of higher temperature to the one of lo wer temperature, until they reach the same temperature (thermal equilibrium). 1.3 Enthalpy and Enthalpy Change Enthalpy, H • The energy content of a substance under constant pressure. ✓ The energy content includes the kinetic energy of the particles in the substance and the potential energy stored in the arrangements of the particles in the substance. ✓ Enthalpy of a substance reflects its stability. A substance with higher enthalpy has higher energy and is thus less stable. For your info: Why is a substance with higher energy content less stable? Energy content is in the form of K.E. and P.E. If the particles have high K.E., they are moving faster and thus there are minimal attractive forces to keep them bonded. Similarly, if the particles have high P.E., they are further apart from one another with weak attractive forces. As a result, the substance is unstable. ✓ Absolute enthalpy cannot be measured, but we can measure the change in enthalpy accompanying a process by measuring the flow of heat into or out of the system. Enthalpy Change, ∆H: • Enthalpy change of reaction (Hrxn) is the energy change in a chemical reaction when the molar quantities of reactants specified in the chemical equation react to form products. Hrxn = Hpdts - Hrxts where Hrxn = enthalpy change of reaction Hpdts = sum of enthalpies of products Hrxts = sum of enthalpies of reactants Units of Hrxn : kJ mol–1 All ∆H have accompanying signs, + or –, that indicate the direction of heat flow into or out of the system respectively. 1.4 Endothermic and Exothermic Reactions In a chemical reaction, bonds in reactants are broken and bonds in products are formed. The breaking of bonds requires energy and is an endothermic process. The formation of bonds evolves energ