2 Characteristics of Equilibrium Closed system – nothing in, nothing outForward and reverse reactions are occurring at the same rate – known as ‘dynamic equilibrium’Macroscopic properties (e.g. Colour, concentration, pH) remain constantChanges in temp, pressure, volume and concentration can change the equilibrium position. Catalysts do not.
3 Equilibrium Graph - Concentration A BThis reaction starts with A only, with the [A] (concentration of A) at a maximum at Time = 0 s.As the reaction proceeds, A gets used up and [B] steadily increases until equilibrium is reached which can be seen as no change in either concentration.In this graph, [B] > [A] which means the forward reaction is more favoured than the reverse.What would the graph look like if the reverse was favoured?
4 Equilibrium Graph – Rate of Reaction A BThis reaction starts with A only, so the rate of the forward reaction is at a maximum and slows down as [A] decreasesAs the reaction proceeds and [B] steadily increases, the rate of the reverse reaction increases until equilibrium is reached when the rates of the forward and reverse reactions are equal.
5 Equilibrium constant - Kc productsreactantsIndices are from the coefficients in the balanced chemical equation
6 Equilibrium constant - Kc Units for Kc will vary depending upon the reaction. In fact, there may be no units.The value of Kc for a particular reaction is only affected by changes in temperatureIn a homogeneous reaction, all of the states of matter are the sameIn a heterogeneous reaction, there are different states of matter. Solids do not take part in the equilibrium constant
7 Equilibrium constant - Kc When Kc >> 1 When Kc << 1Equil goes far right (forward rxn almost to completion) Equil goes far left (forward rxn hardly proceeds)What happens when Kc = 1? Kc = 0?
8 Le Chatelier’s Principle “An equilibrium system that is exposed to a stress will shift the equilibrium position to oppose that stress”Note: a stress can be a change in temperature, pressure, volume or concentration. Catalysts do not affect equilibrium; they simply affect the rate of the forward and reverse reactions.
9 Le Chatelier’s Principle – Concentration Consider the following reaction:A + B C + DAdding a reactant:This will stress the systemTo relieve the stress, the system can produce more productsEquilibrium shifts rightAdding a product:This will stress the systemTo relieve the stress, the system can produce more reactantsEquilibrium shifts leftRemoving a chemical substance is a stress that is relieved by producing more of that substance, shifting the equilibrium towards that direction. For example, removing a product will shift the equilibrium towards the right, making more products.What happens if you remove some reactant or product?
10 Le Chatelier’s Principle – Temperature (exothermic) Consider the following exothermic reaction:A + B C + D + heatIncreasing the temperature:This is like adding a productTo relieve the stress, the system can reduce the heat by producing more reactantsEquilibrium shifts leftDecreasing the temperature:This is like removing a productTo relieve the stress, the system can produce more productsEquilibrium shifts right
11 Le Chatelier’s Principle – Temperature (endothermic) Consider the following endothermic reaction:heat + A + B C + DIncreasing the temperature:This is like adding a reactantTo relieve the stress, the system can reduce the heat by producing more productsEquilibrium shifts rightDecreasing the temperature:This is like removing a reactantTo relieve the stress, the system can produce more reactantsEquilibrium shifts left
12 Le Chatelier’s Principle – Pressure Consider the following reaction:Assume all species are gases2A + B C + DNotice that there are 3 moles of gas on the left and 2 moles on the rightIncreasing the pressure:This means there is less room for the particlesTo relieve the stress, the system can reduce the pressure by producing less moles of gasEquilibrium shifts rightDecreasing the pressure:This means there is more space for particlesTo relieve the stress, the system can increase the pressure by producing more moles of gasEquilibrium shifts leftIncreasing the volume is the same as decreasing the pressure and decreasing the volume is the same as increasing the pressureWhat happens if you increase or decrease the volume? How does this relate to pressure?
13 Le Chatelier’s Principle – Catalysts Consider the following reaction:A + B C + DAdding a catalystThis will not affect the equilibrium position or KcCatalysts reduce the activation energyThis speeds up the forward and reverse reactions equallyEquilibrium is reached fasterSource of graph:
14 Production of Ammonia – The Haber process In 1912, German scientist, Fritz Haber developed a process for manufacturing ammonia from nitrogen and hydrogen.N2 (g) + 3H2 (g) 2NH3 (g) + 92kJSource:Notice that this reaction is reversible which can establish equilibrium. This means that Le Chatelier’s Principle applies to the chemistry of this process.Also, note that the forward reaction is exothermic
15 Ammonia and Le Chatelier N2 (g) + 3H2 (g) 2NH3 (g) + 92kJUsing your knowledge of Le Chatelier’s Principle, describe what the optimum conditions (in terms of yield and rate) for this reaction will be in relation to the following:TemperaturePressureUse of a catalyst
16 Optimum ammonia production RateYieldCostSource: Chemistry Contexts 2, 2006
17 Conditions for Haber Process Pressure - high(250 atm) – to shift equilibrium right and increase rateTemperature – moderate (4500C) – low favours equilibrium, high favours rate. This temperature is a trade-offCatalyst – use of an iron catalyst helps to increase the rate and overcome the relatively low temperature required.Removal of ammonia – shifts the equilibrium towards the products.
18 Production of Sulfuric Acid – The Contact Process Sulfuric acid is made in 3 steps:Diagram source:Steps to make sulfuric acidSulfur dioxide is madeSulfur trioxide is made from sulfur dioxideSulfuric acid is made from sulfur trioxide
19 Contact Process Chemistry Step 1:Sulfur is roasted in oxygen to produce sulfur dioxideS(s) + O2(g) SO2(g)Step 2:Sulfur dioxide is reacted with oxygen using a Vanadium catalyst to produce sulfur trioxide in a reversible exothermic reactionSO2(g) + O2(g) SO3(g) + heatStep 3:Sulfur trioxide is converted to sulfuric acid through a series of reactionsSource of animation:
20 Contact Process conditions Using Le Chatelier's principle, the equilibrium yield of sulfur trioxide (step 2) should increase when: - temperatures are low, since the reaction is exothermic; - pressure is high; - excess reactants are present.However the rate of the reaction is high when: - temperature is high, hence an obvious conflict exists with the equilibrium yield; - the pressure is high; - a catalyst is used.Predict the conditions that would be used in this process. Justify your answer
21 Contact Process conditions Pressure - low(1-2 atm) – the process already favours the foward reaction. The extra pressure is not cost effectiveTemperature – moderate (4500C) – low favours equilibrium, high favours rate. This temperature is a trade-off same as HaberCatalyst – use of an vanadium catalyst helps to increase the rate and overcome the relatively low temperature required.Excess oxygen– shifts the equilibrium towards the products.