Thermodynamics

• Standard enthalpy change is the change in enthalpy when reactants in their standard states form products also in their standard states
• Standard enthalpy of formation is the enthalpy change under standard conditions when one mole of a compound is formed from its elements with all reactants and products in their standard states EXOTHERMIC
• Electron affinity is the standard molar enthalpy change for the addition of an electron to an isolated atom in the gas phase eg. Cl(g) + e- -> Cl-(g) EXOTHERMIC
• Enthalpy of atomisation is the standard enthalpy change for the formation of one mole of gaseous atoms from the atomic element in its standard state eg. Na(s) -> Na(g) ENDOTHERMIC
• Enthalpy change is the amount of heat released or absorbed when a chemical or physical change occurs at constant pressure
• Enthalpy of fusion is the standard molar enthalpy change when a solid forms a liquid at its melting point eg. H2O(S) -> H2O(l)
• Enthalpy of hydration is the standard molar enthalpy change for the formation of aqueous ions from gaseous ions eg. Mg2+(g) -> Mg2+(aq) EXOTHERMIC
• Enthalpy of lattice dissociation is the standard molar enthalpy change for the separation of a solid ionic lattice into its gaseous ions eg. NaCl(s) -> Na+(g) + Cl-(g) ENDOTHERMIC
• Enthalpy of lattice formation is the standard molar enthalpy change for the formation of a solid ionic lattice from its gaseous ions eg. Na+(g) + Cl-(g) -> NaCl(s) EXOTHERMIC
• Enthalpy of solution is the standard enthalpy chasnge that occurs when an ionic solid dissolves in enough water to ensure that the dissolved ions are well separated and do not interact with one another eg. NaCl(s) -> Na+(aq) + Cl-(aq)
• Enthalpy of sublimation is the standard molar enthalpy change that occurs on sublimation, when a solid changes directly to a gas without forming a liquid phase
• Enthalpy of vaporisation is the standard molar enthalpy change when a liquid forms a gas at its boiling point eg. H2O(l) -> H2O(g)
• Mean bond enthalpy is the average of several values of the bond dissociation enthalpy for a given type of bond, taken from a range of different compounds
• Bond dissociation enthalpy is the standard molar enthalpy change for the breaking of a covalent bond in a gaseous molecule to form two gaseous free radicals
• First ionisation enthalpy is the standard molar enthalpy change for the removal of an electron from one mole of an atom in the gas phase to form one mole of a positive ion and an electron, both in the gas phase ENDOTHERMIC
• Reducing agents donate electrons and are themselves oxidised
• Oxidising agents take electrons and are themselves reduced
• Down group 7, Atomic radius increases, EN decreases, Bpt increases, Reactivity decreases, Bond energy decreases
• Fluorine bond energy is weaker than expected as short distance results in repulsion between lone pairs
• Oxidising power decreases down group 7, harder to gain electron as atoms get larger, increased shielding, weaker attraction
• Reducing power increases down group 7, more shells, greater shielding, weaker attraction, electrons lost easier
• Cl2 + H2O = Cl- + ClO- + 2H+ (disproporation) If more conc HCl added system moves left so more Cl2 appropriated, would not happen with dilute HCl as water is a constant. Takes place in swimming pools
• In presence of sunlight: 2Cl2 + 2H2O --> 4H+ + 4Cl- + O2, chlorine rapidly lost from pools and needs to be added frequently
• Cl2 + 2OH- = Cl- + ClO- + H2O, with more alkali added the equilibrium moves right and becomes a normal forwards reaction
• Advantages of adding oxygen to water: chlorate ions kill bacteria, prevents algae growth, removes bad taste, removes discolouration from organic compounds
• Dis advantage of adding oxygen to water: chlorine is toxic, chlorine is a respiratory poison, chlorine can react with organic compounds in water to form chlorinated hydrocarbons (cancer causing)
• In exothermic reactions, if temp increases, Kc will decrease
• In endothermic reactions, if temp increases, Kc will increase