AQA A level Chemistry 3.1.8 – Thermodynamics Already Passed

AQA A level Chemistry 3.1.8 – Thermodynamics Already Passed What are standard conditions? 298K/25°C 100kPa/1 atm 1 moldm-3 Enthalpy change of formation The enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions, with all reactants and products in their standard states. Enthalpy change of combustion The enthalpy change when 1 mole of a substance is burned completely in oxygen. Enthalpy change of atomisation The enthalpy change that accompanies the formation of 1 mole of gaseous atoms from the element in its standard state. First ionisation enthalpy The enthalpy change when 1 mole of gaseous atoms loses 1 electron per atom to form 1 mole of gaseous 1+ ions. Second ionisation enthalpy The enthalpy change when 1 mole of gaseous 1+ ions loses 1 electron per atom to form 1 mole of gaseous 2+ ions. First electron affinity The enthalpy change when 1 mole gaseous atoms gains 1 electron per atom to form 1 mole of gaseous 1- ions. Second electron affinity The enthalpy change when 1 mole of gaseous 1- ions gains 1 electron per atom to form 1 mole of gaseous 2- ions. Mean bond enthalpy The enthalpy change when 1 mole of gaseous molecules each breaks a covalent bond to form 2 gaseous ions or free radicals. Lattice enthalpy of formation The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions. Lattice enthalpy of dissociation The enthalpy change when 1 mole of a solid ionic compound dissociates into its gaseous ions. Enthalpy of hydration The enthalpy change when 1 mole of gaseous ions dissolves completely in water. Enthalpy of solution The enthalpy change when 1 mole of an ionic solid dissolves in enough solvent to ensure that the dissolved ions are well separated and do not interact with one another. Enthalpy of vaporisation The enthalpy change when 1 mole of a liquid evaporates to form 1 mole of a gas. Enthalpy of sublimation The enthalpy change when 1 mole of a solid sublimes to form 1 mole of a gas. Why are ionisation enthalpies always positive (endothermic)? Energy needs to be put in to pull an electron away from the nucleus to overcome the electrostatic attraction. Why are first electron affinities always negative (exothermic)? Energy is given out when an electron is attracted to the positively charged nucleus. Why are second electron affinities and onwards always positive (endothermic)? Energy needs to be put in to overcome the repulsion between an electron and a negatively charged ion. As the charge of ions increases, the lattice enthalpy... Increases. As the ionic radius increases, the lattice enthalpy... Decreases. If there is "good agreement" between the experimental value (Born-Haber Cycle) and the electrostatic attraction theory (Perfect Ion Theory) then the bonding is... Pure ionic bonding. If there is "significant difference" between the experimental value (Born-Haber Cycle) and the electrostatic attraction theory (Perfect Ion Theory) then the bonding is... Ionic bonding with some covalent character. How much is a significant difference in the perfect ion theory? Over 7%. As charge of CATION (+) increases, covalent character... Increases. As ionic radius of CATION (+) increases, covalent character... Decreases. As charge of ANION (-) increases, covalent character... Increases. As ionic radius of ANION (+) increases, covalent character... Increases. What is the symbol for entropy and what is it measured in? S JK⁻¹mol⁻¹ When the number of molecules increases, entropy... Increases. When the temperature increases, entropy... Increases. Exothermic reactions are _________ likely to happen. More Endothermic reactions are _________ likely to happen. Less Why are exothermic reactions more likely to happen, and endothermic reactions less likely to happen? The universe favours lower enthalpies, so reactions that result in a lower enthalpy are favoured over reactions that result in a higher enthalpy. What is the symbol for Gibb's free energy and what is it measured in? G kKmol-1 If a process has ΔG ≤0 then it is... Feasible. If a process has ΔG >0 then it is... Not feasible. Gibb's free energy equation ΔG = ΔH - T x ΔS ΔS must ÷ 1000 to be in kJK⁻¹mol⁻¹ An ENDOthermic reaction with an INCREASE in entropy is... Feasible ABOVE a critical temperature. An ENDOthermic reaction with an DECREASE in entropy is... Never feasible. An EXOthermic reaction with an INCREASE in entropy is... Always feasible. An EXOthermic reaction with an DECREASE in entropy is... Feasible BELOW a critical temperature. At 0K the Gibb's free energy equation is ΔG = ΔH At the critical temperature, the Gibb's free energy equation is Tc = ΔH ΔS