3.2.3 Chemical Equilibrium All reversible reactions reach an dynamic equilibrium state. Many reactions are reversible N2 + 3H2 2NH3 Dynamic equilibrium occurs when forward and backward reactions are occurring at equal rates. The concentrations of reactants and products stays constant and the reaction is continuous. We use the expression ‘position of equilibrium’ to describe the composition of the equilibrium mixture. If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants. Le Chatelier’s Principle We use Le Chatelier’s principle to work out how changing external conditions such as temperature and pressure affect the position of equilibrium Le Chatelier’s principle states that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it). Effect of Temperature on equilibrium If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try and reduce the temperature by absorbing heat. N2 + 3H2 2NH3 ∆H = -ve exo If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia. Exam level answer : must include bold points Typical Exam question: What effect would increasing temperature have on the yield of ammonia? If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try and increase the temperature by giving out heat. And its reverse Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate Effect of Pressure on equilibrium Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure. CO (g) + 2H2(g) CH3OH (g) If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure . The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol. Exam level answer : must include bold points Typical Exam question: What effect would increasing pressure have on the yield of methanol? Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure. And its reverse If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium H2 + Cl2 2HCl Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the equipment is expensive (to contain the high pressures) Effect of Concentration on equilibrium Increasing the concentration OHions causes the equilibrium to shift to oppose this and move in the forward direction to remove OHions. The position of equilibrium will shift towards the right, giving a higher yield of I- and IO- . ( The colour would change from brown to colourless) Adding H+ ions reacts with the OHions and reduces their concentration so the equilibrium shifts back to the left giving brown colour. Effect of Catalysts on equilibrium A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved. It does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
/
~
~
~
/
3.2.3 Chemical equilibrium
Dynamic equilibrium and le Chatelier’s principle (a) explanation that a dynamic equilibrium exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change (b) le Chatelier’s principle and its application for homogeneous equilibria to deduce qualitatively the effect of a change in temperature, pressure or concentration on the position of equilibrium Definition for le Chatelier’s principle not required. HSW1,2,5 Use of le Chatelier’s principle to explain effect of factors on the position of equilibrium. (c) explanation that a catalyst increases the rate of both forward and reverse reactions in an equilibrium by the same amount resulting in an unchanged position of equilibrium (d) the techniques and procedures used to investigate changes to the position of equilibrium for changes in concentration and temperature. Qualitative effects only. HSW4 Opportunities to carry out experimental and investigative work.