[H2 CHEM] Chapter 7 - Chemical Energetics (Thermochemistry and Thermodynamics)

CHAPTER ANALYSIS FOCUS EXAM WEIGHTAGE ELAINE PEH (COPYRIGHTED) © Chapter 7 Chemical Energetics: Thermochemistry and Thermodynamics

CHAPTER ANALYSIS FOCUS EXAM WEIGHTAGE ELAINE PEH (COPYRIGHTED) ©

Enthalpy Change, Activation Energy & Energy Profile Diagram Standard Enthalpy Changes Calculating Enthalpy Change of a Reaction Predicting the Spontaneity of a Reaction KEY CONCEPT ELAINE PEH (COPYRIGHTED) ©

ENTHALPY CHANGE • Enthalpy change = energy change that occurs from energy absorbed in bond breaking and released in bond forming. • Units = J mol-1, kJ mol-1 Note: +/- must be written in front of the ΔH EXOTHERMIC REACTION, ΔH<0 Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. Energy is released because the total energy of the products < total energy of the reactants. ENDOTHERMIC REACTION, ΔH>0 Endothermic reactions are reactions that require external energy, usually in the form of heat, for the reaction to proceed. Note: Exothermic reactions are energetically more favourable than endothermic reactions as a system with lower heat content is more stable ACTIVATION ENERGY, Ea Minimum energy which reacting particles must possess in order to overcome the activation energy barrier before becoming products. ELAINE PEH (COPYRIGHTED) © Enthalpy Change, Activation Energy

ELAINE PEH (COPYRIGHTED) © ENERGY PROFILE DIAGRAM • It is a diagram representing the energy changes that take place during a chemical reaction. • ΔH = Energy level of products – Energy level of reactants Note: Endothermic reaction pathway is energetically more unfavorable due to the higher Ea as compared to an exothermic reaction pathway. Energy Profile Diagram

STANDARD CONDITION • 298K (25oC), 1 bar (100,000Pa), 1.0 mol dm-3 Note: ≠ s.t.p! s.t.p = 273K (0oC), 1 bar (10,000Pa), 22.7 dm3mol-1 Note: ≠ r.t.p! r.t.p = 293K (20oC), 1 atm (101,325Pa), 24.0 dm3mol-1 THERMOCHEMICAL EQUATIONS Includes ΔH (with units and sign +/-). State symbols of the reactants and products must be written. ΔHr (reaction) Enthalpy change when molar quantities of reactants as specified by the chemical equation react to form products at 1 bar and 298K (standard conditions) ΔHf (formation) • Enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 1 bar and 298K (standard conditions). • Can be both ΔH<0 and ΔH>0 ELAINE PEH (COPYRIGHTED) © Standard Enthalpy Change

ELAINE PEH (COPYRIGHTED) © Standard Enthalpy Change ΔHc (combustion) • Heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298K and 1 bar (standard conditions). • Is always ΔH<0. ΔHneut (neutralisation) • Heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base at 298K, 1 bar (standard conditions). • Is always ΔH<0 • ΔHneut (strong acid & strong base) = -57.0 kJ mol-1 as strong acid/base ionised completely in water • ΔHneut (weak acid or base) < -57.0 kJ mol-1. Less exothermic as some energy is absorbed to ionise the weak acid/base that has not been fully ionised, hence the net heat evolved is lesser. (Recap: weak acid/base only dissociates partially in water)

ELAINE PEH (COPYRIGHTED) © Standard Enthalpy Change ΔHatom (atomisation) • ΔHatom for an element is the energy required (absorbed) when 1 mole of gaseous atoms is formed from the element at 298K and 1 bar (standard conditions) • Is always ΔH>0 Note: It is 1 2 x Bond Energy of diatomic molecules BOND ENERGY • Bond energy (of dissociation) is the average energy required to break 1 mole of a covalent bond between two atoms in the gaseous state • Is always ΔH>0 Note: It is 2 x ΔHatom for diatomic molecules Note: The greater the magnitude of the bond energy, the stronger the covalent bond

ELAINE PEH (COPYRIGHTED) © Standard Enthalpy Change I.E. (ionisation energy) • First IE is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous (cat)ions • Is always ΔH>0 • Second IE is the energy required to remove 1 mole of electrons from 1 mole of singly charged gaseous (cat)ions to form 1 mole of doubly charged gaseous (cat)ions • Is always ΔH>0 E.A. (electron affinity) • First EA is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous (an)ions • Is always ΔH<0 • Second EA is the enthalpy change when 1 mole of electrons is added to 1 mole of singly charged gaseous (an)ions to form 1 mole of doubly charged gaseous (an)ions • 2nd E.A. onwards is always ΔH>0 Note: This is due to the electrostatic repulsion between the negatively charged gaseous (an)ions and electrons. Energy taken in to overcome the electrostatic repulsion > Energy given out during bond formation

ELAINE PEH (COPYRIGHTED) © Standard Enthalpy Change LE (Lattice energy) • Heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions • Is always ΔH<0 Note: LE provide a measure of the ionic bonds present. Hence the more exothermic LE is, the stronger the ionic bonding Note: If both the charge and radius increase, it will result in an overall increase in the LE due to the multiplication of the charge of the ions as compared to the addition of the radii of the ions ΔHhyd (hydration) • Heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar (standard conditions) • Is always ΔH<0 due to the formation of ion-dipole interactions with water and ions • Higher charge density = Stronger ion-dipole interaction hence more exothermic ΔHsol (solution) • Enthalpy change when 1 mole of a solute (solid) is completely dissolved in an infinite volume of solvent at 298K and 1 bar (stp) • When ΔHsol very endothermic, compound is insoluble in water • When ΔHsol is exothermic, compound is soluble in water Note: For ionic compounds, it will dissociate into its respectively ions when dissolved in solvent