1. Equilibrium

2. How do we write the equilibrium constant expression for the following reaction? 2SO 2 (g) + O 2 (g)  2SO 3 (g)

3. Objectives To calculate K c, the equilibrium constant, for a reaction To define K p To calculate K p from the value of K c CLE3221.Inq.2

4. Evaluating K c When we know the equilibrium concentrations of N 2 O 4 and NO 2 in the reaction below, we can calculate K c, the equilibrium constant. N 2 O 4 (g)  2NO 2 (g)

5. If [NO 2 ] at equilibrium is.0172 M, and the [N 2 O 4 ] at equilibrium is 0.00140 M, we calculate K c with the following equation: N 2 O 4 (g)  2NO 2 (g) K c = [NO 2 ] 2 = (0.0172) 2 = 0.211 [N 2 O 4 ] (0.00140) Notice that we will not include units when we calculate K c.

6. Study Check The following reaction is at equilibrium: H 2 (g) + I 2 (g)  2HI(g) [H 2 ] = 0.012 M, [I 2 ] = 0.15 M, and [HI] = 0.30M. Calculate K c for this reaction.

7. Equilibrium Constants in Terms of Pressure, K p When the reactants and products in a chemical reaction are gases, we can use partial pressures instead of molarities to calculate the equilibrium constant. If we do this, we call the constant K p to show that we used partial pressures in our calculation.

8. Study Check What is the difference between K c and K p ?

9. Calculating K p N 2 O 4 (g)  2NO 2 (g) K p = [P NO2 ] 2 [P N2O4 ] If given the partial pressures of N 2 O 4 andNO 2, we would just substitute them into the equation.

10. When we are given the value of K c and we want to find K p, we can use the following equation: K p = K c (RT) /\n R= ideal gas constant, 0.0821L-atm/mol-K T= temperature in Kelvins /\n= (moles of gaseous product) – (moles of gaseous reactant)

11. Calculating /\ n To calculate /\ n, the change in moles when all reactants and products are gases, you subtract the moles of reactants from the moles of your product. The coefficients in the balanced equation are used to do this. PCl 5 (g)  PCl 3 (g) +Cl 2 (g) /\n= (moles of gaseous product) – (moles of gaseous reactant) /\ n= (1+1) - 1 = 1

12. Study Check For the following reaction, calculate K p when K c = 9.60 at 573 K. N 2 (g) + 3H 2 (g)  2NH 3 (g)

13. K c = 9.60 at 573 K. K p = K c (RT) /\n R= ideal gas constant, 0.0821L-atm/mol-K T= temperature in Kelvins /\n= (moles of gaseous product) – (moles of gaseous reactant)

14. Homework p. 660 in your book Problem 15.13 a,b,c,d,e Problem 15.17