Key question: How can we calculate Kp for a reaction at equilibrium?

Equilibrium Constant, K p, at constant temperature  For reactions involving gases, measuring concentration is difficult. It is more convenient to use partial pressures. The equilibrium constant, K p, is calculated from partial pressures p at constant temperature of the species. What do you think the partial pressure of a gas is?

In general for a reaction: aA (g) + bB (g) cC (g) + dD (g) If p is the symbol for partial pressure, e.g for A, it would be p(A), suggest K p for this reaction. where a, b, c and d are the number of moles of species A, B, C and D which appear in the balanced equation for the equilibrium. K p = p(C ) c p(D ) d p(A ) a p(B) b

What reactions is Kp calculated for?  K p can be applied to any homogeneous equilibrium in the gas phase; it can also be applied to heterogeneous equilibria which involve gases and solids  Examples of heterogeneous systems to follow….

Dalton’s Law  Dalton’s Law states that the partial pressure of a component in a gas mixture is the pressure which would be exerted if that component alone filled the whole volume occupied by the mixture. Suggest how we can calculate the partial pressure of a reactant or product in an equilibrium mixture? What do we need to take into consideration?

Partial Pressures and Mole Fractions  The mole fraction, x, of a component A in a gas mixture is defined as:  The partial pressure, p A is related to mole fraction and total pressure, p tot, byp A =x A x p tot  Partial pressure terms are expressed in SI units as Pascals (Pa) or (kiloPascals) kPa. x=number of moles of A total number of moles of all components

Units for K p How would you deduce units for Kp? As for K c, the units of K p depend on the stoichiometry of the equilibrium reaction e.g. the reaction; A (g) 2B (g) + C (g) If the partial pressure for each component is kPa, deduce the units

Calculating partial pressures No. of moles ATotal no. of moles Total pressurePartial pressure of A

Calculating and using Kp  An equilibrium mixture at a pressure of 200kPa contains 13.5mol of nitrogen, 3.6mol of hydrogen and 1.0mol of ammonia. Calculate the equilibrium constant, K p, for this reaction. Question 1

Calculating and using Kp  2 moles of phosphorus(V) chloride vapour are heated to 500 K in a sealed container. The equilibrium mixture at a pressure of 665 kPa contains 1.2 mol of chlorine.  Calculate the equilibrium constant, K p, for the decomposition of phosphorus(V) chloride into phosphorus(III) chloride. Question 2

Calculating and using Kp  1 mol nitrogen and 3 mol of hydrogen are heated to a temperature of 700 K under a pressure of 5000 kPa (5.0 MPa). The equilibrium mixture under these conditions contains 0.8 mol of ammonia. Calculate the value of the equilibrium constant, K p, for the formation of ammonia from its elements under these conditions. Question 3

Calculating and using Kp  0.256g of hydrogen iodide was heated at 764K in a sealed flask. At equilibrium, the pressure in the flask was 127kPa.  The flask was cooled quickly and it was found that the mixture contained moles of iodine. Calculate a value for K p. Question 4

HW  Questions on sheet for Tue

What about a heterogeneous system? Consider, CaO (s) + SO 2 (g) CaSO 3 (s) Can the expression be simplified? Do solids have a concentration? Concentration of a solid is the same as its density