WJEC Chemistry for A2: Study and Rev Guide

3.4 Chemistry of the d -block transition metals The d -block elements show similar properties such as their abilities to form variable oxidation states and complexes with ligands. The complexes of copper and cobalt are used as examples. Complexes are usually coloured and can often act as catalysts. All these compounds in solution react with sodium hydroxide, with the amphoteric metals behaving differently to the others. p24–26 3.5 Chemical kinetics In year 1 the effects of concentration and temperature were studied qualitatively, but in this topic, the effects are quantified. The effects of concentration changes are used to produce the rate equation, with the Arrhenius equation used to explain the way rate changes with temperature. The rate equation can be used to find out about the mechanism of a chemical reaction. p27–29 3.6 Enthalpy changes for solids and solutions Energy changes can be calculated using Hess’s Law. These changes can be broken down into many small steps that need to be recalled and understood such as atomisation, lattice formation, hydration, ionisation energy and electron affinity. Combining these together into an energy cycle allows the stability or solubility of ionic compounds to be calculated. p30–31 3.7 Entropy and feasibility of reactions Entropy is a measure of the freedom or disorder of a system, and it increases from ordered solids to liquids then gases. Entropy must increase overall in any change, and the effects of the entropy change of a reaction and the surroundings are combined in the Gibbs free energy. The Gibbs free energy for any change must be negative for the reaction to be feasible. p32–33 3.8 Equilibrium constants The position of any equilibrium can be described using the concentrations or partial pressures of all substances present. These are combined to form the equilibrium constants K p and K c , which can be applied to any equilibriummixtures to find the amounts of any substance present. The values of K p and K c give a qualitative idea of the position of equilibrium. p34–37 3.9 Acid-base equilibria The equilibrium constants K a and K w are key to finding the pH of weak acids and strong bases. The salts of weak acids and of weak bases are not neutral. Buffers are formed frommixtures of weak acids and their salts and their pH depends on the K a of the acid and the concentrations of the acid and salt. During titrations, the pH changes and the pH curves depend on the strength of the acids and bases used. The selection of the correct indicator depends on the shape of the titration curve. p38–43 Basic notes Good grasp Fully revised 7