Ib Hl Chemistry Assessment Statements Topic 7

Andrew Voyles IB Chemistry Assessment Statements: Topic 7: Equilibrium 7.1.1: A chemical or physical system in a state of equilibrium is characterized by a closed system at constant temperature, whose macroscopic properties (color, density, pH, etc) and concentrations of reactants and products remain constant, and whose rate of forward reaction equals the rate of the reverse reaction. 7.2.1: For a general homogeneous reaction, mA + nB  oC + pD, the equilibrium constant expression, Kc = ([A]m[B]n)/([C]o[D]p). This assumes that all reactant and products are either gases or aqueous solutions. In a reactant or product which is either a solid or a liquid does not appear in the equilibrium constant expression. The exception to this is if all reactants and products are in the liquid phase, in which case they will appear. 7.2.2: For the equilibrium constant expression Kc, if Kc is greater than one, the reaction is product favored, and equilibrium lies closer to the completion of the reaction. However, if it is less than one, the reaction is reactant favored, and equilibrium lies closer to the reaction not proceeding at all. 7.2.3: Effect of Temperature : The effect of a change of temperature on a reaction will depend on whether the reaction is exothermic or endothermic. When the temperature increases, Le Chatelier’s principle says the reaction will proceed in such a way as to counteract this change, i.e. lower the temperature. Therefore, endothermic reactions will move forward, and exothermic reactions will move backwards (thus becoming endothermic). The reverse is true for a lowering of temperature. Effect of Concentration : When the concentration of a product is increased, the reaction proceeds in reverse to decrease the concentration of the products. When the concentration of a reactant is increased, the reaction proceeds forward to decrease the concentration of reactants. Effect of Pressure : In reactions where gases are produced (i.e. there are more mols of gas on the right), an increase in pressure will force the reaction to move to the left (in reverse). If pressure is decreased, the reaction will proceed forward to increase pressure. If there are more mols of gas on the left of the equation, this is all reversed.

7.2.4: A catalyst does not affect the position of equilibrium or the value of Kc, it merely allows this position to be achieved more quickly. 7.2.5: N2(g) + 3H2(g) <=> 2NH3(g) : ΔH = -92.4 kJ mol-1 As can be seen, there are more mols of gas on the left than the right, so a greater yield will be produced at high pressure. The reaction is exothermic, therefore it will give a greater yield at low temperatures, however this is not possible as the rate of reaction becomes too low, and the temperature must actually be increased. A catalyst of finely divided iron is also used to help speed the reaction (finely divided to maximize the surface area).

17.1.1: A liquid in an enclosed chamber will form an equilibrium with it's own vapour. Fast moving particles in the liquid will escape from the surface and become part of the vapour, but slow moving particles in the vapour will be 'captured' by the liquid and become part of it. At a certain vapor pressure, the number of particles escaping (or evaporating) from the liquid will exactly equal the number being captured by it, and so a dynamic equilibrium is formed between the two. As the temperature increases, the average speed of particles is higher. As a result, more particles will have sufficient speed to escape the liquid, and fewer will be slow enough to be recaptured by the liquid. This means that as temperature increases, the equilibrium vapor pressure will also increase. This can be shown graphically with a graph of pressure against temperature, where, as temperature increases, so does pressure. 17.1.2:

As the temperature increases, the average speed of particles is higher. As a result, more particles will have sufficient speed to escape the liquid, and fewer will be slow enough to be recaptured by the liquid. This means that as temperature increases, the equilibrium vapor pressure will also increase. 17.1.3: A high boiling point and a high enthalpy of vaporization are proportional to high intermolecular forces.