Equilibria G11.2C

Learning objectives recall and be able to use ideas from previous teaching on equilibria, including the concept of dynamic equilibrium be able to predict the effects change of temperature, concentration and pressure on equilibria understand why a catalyst does not affect the position of equilibrium understand the importance of equilibria in industrial chemistry understand (with examples) why reaction conditions for industrial processes involving equilibria are often a compromise

Lecture outline Dynamic equilibria Effects of change of pressure, temperature, concentration (qualitative) Industrial equilibria Equilibrium constant Kc Effects of concentration and temperature on Kc (qualitative)

What is the equilibrium state?

RECAP

LE CHÂTELIER’S PRINCIPLE

Disturbing a chemical equilibrium THREE WAYS TO DISTURB A CHEMICAL EQUILIBRIUM CHANGE IN TEMPERATURE CHANGE IN THE CONCENTRATION OF REACTANT OR PRODUCT CHANGE IN VOLUME/PRESSURE

Le Châtelier’s Principle If a system in dynamic equilibrium is subjected to a change, processes will occur to counteract this change and to restore equilibrium. Henry Le Chatelier ( )

Effect of changing concentration A change in the concentration of product or reactant will cause the shift of the reaction in order to restore the equilibrium state. Fe(H 2 O) 6 3+ (aq) + SCN - (aq) Fe(H 2 O) 5 (SCN) 2+ (aq) + H 2 O (l) CONCENTRATIONCHANGE Fe(H 2 O) 6 3+ (aq) SCN - (aq) Fe(H 2 O) 5 (SCN) 2+ (aq) ↑ ↓ ↑ ↑↑ ↑ ↑ ↑ ↓

Effect of temperature changes Equilibrium Constant, K c Temperature [K] 4.5 × × × heat is a “reactant” increase in the temperature causes increased production of NO and consumption of N 2 & O 2 the endothermic reaction is favoured

Effect of temperature changes Equilibrium Constant, K c Temperature [K] heat as a reaction “product” decrease in the temperature causes increased production of N 2 O 4 the exothermic reaction is favoured brown gas pale yellow gas

Effect of pressure changes N 2 (g) + 3H 2 (g) 2NH 3 (g) Equilibrium % of NH 3 at 723 KTotal pressure [atm]

Effect of pressure changes N 2 (g) + 3H 2 (g) 2NH 3 (g) 1 molecule of N 2 reacts with 3 molecules of H 2 to form 2 molecules of NH 3 4 molecules of gas give 2 molecules of gas Fewer gaseous molecules decreases total pressure ↑ pressure shifts reaction to → (fewer molecules) ↓ pressure shifts reaction to ← (more molecules) H 2 (g) + I 2 (g) 2HI(g) 1 molecule of H 2 reacts with 1 molecule of I 2 to form 2 molecules of HI 2 molecules of gas give 2 molecules of gas Pressure has no effect on a gaseous reaction when there is not change in the number of molecules

Homogenous equilibria Haber Process N 2 (g) + 3H 2 (g) 2NH 3 (g) ∆H = - 92 kJ mol -1 the equilibrium needs to be shifted towards products as far as possible. Low temperature - favouring the exothermic forward reaction. High pressure - favouring the forward reaction, T = 723 K, P = 250 atm, catalyst = iron mixed with K 2 O and Al 2 O 3 This gives a 15% yield of ammonia Optimal conditions

Summary of Le Châtelier’s Principle DISTURBANCE CHANGE AS MIXTURE RETURNS TO EQUILIBRIUMEFFECT ON EQUILIBRIUMEFFECT ON K Addition of reactant Addition of product Some of added reactant is consumed Some of added product is consumed Shift to right Shift to left No change Increase in pressure Decrease in pressure Pressure decreases Pressure increases Shift toward fewer gas molecules Shift toward more molecules No change Rise in temperature Drop in temperature Heat energy is consumed Heat energy is generated Shift in the endothermic direction Shift in the exothermic direction Change CatalystNO EFFECT on equilibrium position (but the equilibrium is reached faster

Video links and websites A very useful video clip: A video illustration of Le Chatelier’s Principle using Fe 3- and SCN - : A detailed explanation for learners: Two very short videos of the NO 2 /N 2 O 4 equilibrium: Learner explanation of atom economy: chemcalc_higherrev5.shtml Learner experiment sheet for the determination of an equilibrium constant: levelchemistry.co.uk/AQA%20A2%20Chemistry/Unit%206/PSA%20(PSV)/AQA-2420-W- TRB-PSA14.pdf Useful explanation here: