Acheiver's Dream Chemistry Energetics Notes

The Chemistry Specialist Chemical Energetics

© Achievers Dream Chemistry Energetics Standard Conditions Pressure T emperature Concentration 1 bar (105 pa) 298K 1 mol dm-3 An enthalpy change stated under standard conditions is hence called standard enthalpy change, denoted by the symbol, ΔHθ. The ‘θ’ indicates that ΔH was measured and calculated under standard conditions, standard states. Under standard conditions, substances are in their standard states. The standard state of a substance is its most stable form under standard conditions. 1 ΔHr = ∑ΔHf (products) – ∑ΔHf (reactants) 2 ΔHr = ∑ΔHc (reactants) – ∑ΔHc (products) 3 ΔHr θ = ∑BE (bonds broken) – ∑BE (bonds formed) 4 ΔHsol θ = - lattice energy + ΔHhyd θ (cations & anions) ΔGsys = ΔHsys – TΔSsys ΔG<O Spontaneous ΔG>O Not spontaneous ΔG=O both forward and backward reactions are in equilibrium (e.g. phase change) Factors affecting the entropy(s) of a chemical system 1. T emperature When T1 < T2 Temperature Particles move randomly & vigorously More ways to distribute energy of the particles in the system Greater disorder in the system Average KE of particles As Temperature S (ΔSsys >O) Number of particles with energy greater than Ea at T2 Number of particles with Kinetic Energy, E Number of particles with energy greater than E a at T2

Entropy of a substance increases as it changes from a solid to a liquid to a gas. The entropy of a dissolved solid is usually greater than the entropy of the pure solute. Dissolving of NaCl in water involves the disruption of the crystal lattice of solid NaCl and hydration of Na + and Cl- ions decreases disorder about Na+ and Cl- about the Na + and Cl- ions. disruption of the crystal lattice increases disorder. The hydration process however since it puts the hydrating water molecules into an orderly arrangement Since the ordering process is more dominant, the overall dissolution process results in a net decrease in entropy. For salts such as AlCl3 which contain more highly charged ions, the hydrating water molecules are more strongly ordered about the ions. 2. Change in Phase 4.1 Mixing of gases 4.2 Mixing of liquids 4.3 Dissolving an ionic solid Solid melts or liquid boils Particles are randomly arranged More ways to distribute energy More ways to distribute energy since there are more gas particles moving randomly Greater disorder in the system Greater disorder in the system S (ΔSsys >O) S (ΔSsys >O) S (ΔSsys >O) S (ΔSsys >O) Gas mixes Expands to occupy the whole container More ways to distribute energy among Greater disorder in the system the particles in the system When mixing liquids of similar particles More ways to distribute energy Molecules have greater volume of space to move about When products of a reaction contain more gas particles than the reactants © Achievers Dream + + - - + - + - + - - + - + - + - + Water molecules NaCl(s) Na+ (aq) + Cl- (aq) (ΔS >O) AlCl3(s) Al3+ (aq) + 3Cl- (aq) Disordering is more dominant