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Chapter 7: Equilibrium
7.1 Dynamic equilibrium
- Equilibrium state is when rate of forward reaction = rate of backward reaction.
- The equilibrium mixture contains a fixed concentration of reactants and products.
7.2 The position of equilibrium
- For a reaction aA + bB ® cC + dD
- Kc, the equilibrium constant, is a constant for a given reaction at a specified temperature.
- The higher the value of Kc the further to the right the equilibrium mixture lies.
- When Kc is very small, [R]equilibrium ~ [R] initial.
- When a change is applied to an equilibrium mixture, the composition will change to minimize the effect of the change. The new equilibrium mixture will have different concentrations of reactant and product, but the value of Kc will be unchanged at the same temperature.
- Catalysts do not change the yield or the equilibrium mixture because they have an equal effect on the forward and backward reactions.
- Optimum conditions for an industrial process are based on equilibrium, kinetic and economic considerations.
17.1 Liquid–vapour equilibrium
- Vapour pressure depends only on:
- the nature of the substance
- the temperature.
- Boiling occurs when the vapour pressure reaches the external pressure.
- Lower intermolecular forces ® higher vapour pressure ® lower boiling point.
Higher intermolecular forces ® lower vapour pressure ® higher boiling point.
Get it straight
- The only thing that changes the value of Kc for a reaction is the temperature.
- Vapour pressure does not depend on the surface area or the volume of the liquid. Unit 7: Equilibrium important definitions
Autoclave A pressure cooker used to sterilize equipment. It operates at high pressure and so raises the boiling point of water inside.
Backward reaction Also known as the reverse reaction. It is the reaction in which products are converted into reactants and is written from right to left.
Boiling Change of state from liquid to gas that occurs when the vapour pressure reaches the external pressure. It is characterized by bubbles of gas forming within the volume of the liquid.
Catalyst A catalyst is a substance that increases the rate of a reaction without itself undergoing chemical change. Most catalysts work by providing an alternate reaction route of lower activation energy. Catalysts speed up the attainment of equilibrium, but they do not affect the equilibrium position or the value of Kc.
Closed system A system in which matter cannot be exchanged with the surroundings. Equilibrium can only be established in a closed system.
Condensation The change of a vapour into its liquid.
Contact process The industrial process for the production of sulfuric acid, H2SO4. The step in the reaction in which SO2 is oxidized to SO3 is subject to equilibrium considerations.
Dew point The saturation point of a vapour. At temperatures at and below the dew point the vapour condenses. If the dew point remains constant and the temperature increases, the relative humidity decreases.
Dynamic equilibrium Refers to the fact that at equilibrium both forward and backward reactions are still occurring.
Endothermic reaction A reaction in which heat energy is taken in from the surroundings. When the forward reaction is endothermic, an increase in temperature will cause the equilibrium position to shift to the right, in favour of products.
Enthalpy of vaporization The energy required to convert one mole of a substance in its liquid state into one mole of gas.
Equilibrium constant expression The product of equilibrium concentrations of products, each raised to the power of their coefficient in the equation, divided by the product of concentrations of reactants, each raised to the power of their coefficient in the equation.
Equilibrium constant, Kc The constant value obtained when equilibrium values of concentration of reactant and product are substituted into the equilibrium expression. The value of Kc is a constant for a particular reaction at a specified temperature.
Equilibrium mixture The concentrations of reactants and products present together at equilibrium.
Equilibrium position This refers to the relative amounts of reactants and products in the equilibrium mixture. If the equilibrium ‘lies to the left’ it has a high concentration of reactants; if it ‘lies to the right’ it has a high concentration of products.
Evaporation The change of a liquid into its vapour. This is a surface event, which takes place at temperatures below the boiling point of the liquid.
Exothermic reaction A reaction in which heat energy is given off into the surroundings. When the forward reaction is exothermic, an increase in temperature will cause the equilibrium position to shift to the left, in favour of reactants.
Forward reaction A reaction in which reactants are converted into products; it is written from left to right.
Haber process The industrial process for the synthesis of ammonia, NH3, from nitrogen and hydrogen. It is named for Fritz Haber who devised the optimum conditions for the reaction in 1913.
Homogeneous equilibria Equilibria in which all reactants and products are in the same phase, e.g. all gases. All the examples of equilibria discussed in this chapter are homogeneous equilibria.
Le Chatelier’s principle This states that when a system at equilibrium is subjected to a change, it responds in such a way as to minimize the effects of that change. The principle makes it possible to predict qualitative responses to changes in equilibrium.
Macroscopic properties These are properties which depend on the concentrations of species in a mixture, such as colour. These remain constant at equilibrium.
Vapour pressure Also known as the saturated vapour pressure or equilibrium vapour pressure.
Volatile A liquid which is close to its boiling point and so has a high tendency to vapourize.