What does K c (or K p ) tell us? Consider the following reactions: 2H 2 (g) + O 2 (g) 2H 2 O (g) K c = 9.1 x mol -1 dm 3 N 2 (g) + O 2 (g) 2NO (g) K c = 4.8 x mol -1 dm 3 Write out Kc expressions and explain what the values are showing

Summary Value of KcExtent of reaction Very large K c > 1x10 10 Around 1 Very small Kc < 1 x Reaction proceeds almost to completion, leaving very small amounts of reactants at equilibrium; equilibrium lies on the RHS Concentrations of reactants and products are nearly the same at equilibrium Reaction hardly occurs, producing very small quantities of products at equilibrium; equilibrium lies on the LHS Position of equilibrium Does Kc tell us anything about rate?

Example AgCl (s) + Br-(aq) AgBr(s) + Cl-(aq) The equilibrium constant for this reaction is 360 at 298K. If 0.1moldm -3 Br-(aq) is added to solid AgCl, what will be the equilibrium concentrations of Br-(aq) and Cl-(aq)? Answer: [Cl-(aq] = moldm -3, [Br-(aq)] = moldm -3

Answer AgCl (s) + Br-(aq) AgBr(s) + Cl-(aq) Initial a = Eqm a-x = 0.1-x x Kc = x 0.1-x 360 = x 0.1-x 360(0.1-x) = x 36 – 360x = x 36 = x + 360x 36 = x ( ) 36 = x 361 x = = [Cl-] and [Br-] = 0.1 – =

Key questions: How is an equilibrium constant deduced for heterogeneous equilibria? What does K c (or K p ) tell us?

Specification  Knowledge of the concepts introduced in Unit 2, Topic 2.9: Chemical equilibria will be assumed and extended in this topic. Students will be assessed on their ability to: a) demonstrate an understanding of the term dynamic equilibrium as applied to states of matter, solutions and chemical reactions b) recall that many important industrial reactions are reversible c) use practical data to establish the idea that a relationship exists between the equilibrium concentrations of reactants and products which produces the equilibrium constant for a particular reaction, e.g. data on the hydrogen ‑ iodine equilibrium 4.5 Equilibria

Students will be assessed on their ability to: a) demonstrate an understanding of the term dynamic equilibrium as applied to states of matter, solutions and chemical reactions b) recall that many important industrial reactions are reversible c) use practical data to establish the idea that a relationship exists between the equilibrium concentrations of reactants and products which produces the equilibrium constant for a particular reaction, e.g. data on the hydrogen ‑ iodine equilibrium d) calculate a value for the equilibrium constant for a reaction based on data from experiment, e.g. the reaction of ethanol and ethanoic acid (this can be used as an example of the use of ICT to present and analyse data), the equilibrium Fe2+(aq) + Ag+ (aq) > Fe3+(aq) + Ag(s) or the distribution of ammonia or iodine between two immiscible solvents e) construct expressions for Kc and Kp for homogeneous and heterogeneous systems, in terms of equilibrium concentrations or equilibrium partial pressures, perform simple calculations on Kc and Kp and work out the units of the equilibrium constants f) demonstrate an understanding that when ΔStotal increases the magnitude of the equilibrium constant increases since ΔStotal = RInK g) apply knowledge of the value of equilibrium constants to predict the extent to which a reaction takes place h) relate the effect of a change in temperature on the value of ΔStotal.

Key question: How can we maximise yield and profit, making a product on an industrial scale?

What factors do we need to consider?

Temperature Temperature / K Equilibrium constant, K N 2 + 3H 2 2NH 3 H 2 + I 2 2HI N 2 O 4 2NO 2

Temperature - summary ReactionTemperatureKc

Pressure Does a change in pressure have an effect on Kc? N 2 + 3H 2 2NH 3 H 2 + I 2 2HI N 2 O 4 2NO 2

Change in total pressure  Changes in total pressure have a significant effect on the composition of a mixture at equilibrium only if the reaction involves gases. The changes observed are due to changes in the concentrations of the reactants and products. At a fixed temperature, the values of K c and K p are not affected.  According to Le Chatelier’s principle, the constraint of additional pressure can be relieved if the equilibrium moves in the direction of fewer gaseous molecules, which exert less pressure and thus lower the overall pressure.  If there are more moles of gaseous reactant than there are moles of gaseous product, an increase in total pressure will displace the reaction to the right to oppose the increase in pressure.  The system responds by trying to reduce the pressure by reducing the number of moles of gas present, and more product is formed. The converse of this also applies.  In general, under increased pressure, the system moves to the side of the equation containing fewer moles of gas.  The fact that the equilibrium constant K p does not alter when the total pressure changes cannot be predicted without having further information. At this stage all that has to be remembered is that; Although the position of equilibrium shifts, the value of K p and K c remains the same, whatever the pressure.

Effect of change in pressure More gas molecules on Change in pressure Shift of equilibrium Yield of product Equilibrium constant product sideincrease product sidedecrease reactant sideincrease reactant sidedecrease

Effect of change in pressure More gas molecules on Change in pressure Shift of equilibrium Yield of productEquilibrium constant product sideincreaseto leftreducedunchanged product sidedecreaseto rightincreasedunchanged reactant sideincreaseto rightincreasedunchanged reactant sidedecreaseto leftreducedunchanged

What can you deduce about the reaction?

Catalyst Does it have an effect on Kc?

Concentration  How is concentration used to improve the yield of a product?  Is there an effect on Kc?

HW for THU 8 th Oct – 10min group presentations  In your group, research and summarise how product yield is maximised for a reaction carried out on an industrial scale  Haber process  Methanol production  Urea production  Contact process (first reaction)  Esterification You may choose your own format for presentation,e.g. power-point presentation / poster / video, etc