[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 ac