Equilibrium

How do we write the equilibrium constant expression for the following reaction?2SO2(g) + O2 (g) 2SO3(g)

Objectives

• To calculate Kc, the equilibrium constant, for a reaction

• To define Kp

• To calculate Kp from the value of Kc

CLE3221.Inq.2

Evaluating Kc

When we know the equilibrium concentrations of N2O4 and NO2 in the reaction below, we can calculate Kc, the equilibrium constant.

N2O4 (g) 2NO2 (g)

If [NO2] at equilibrium is .0172 M, and the

[N2O4] at equilibrium is 0.00140 M, we calculate Kc with the following equation:N2O4 (g) 2NO2 (g)

Kc = [NO2]2 = (0.0172)2 = 0.211

[N2O4] (0.00140)

Notice that we will not include units when we calculate Kc.

Study Check

The following reaction is at equilibrium:H2(g) + I2(g) 2HI(g)

[H2] = 0.012 M, [I2] = 0.15 M, and [HI] = 0.30M. Calculate Kc for this reaction.

Equilibrium Constants in Terms of Pressure, Kp

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 Kp

to show that we used partial pressures in our calculation.

Study Check

What is the difference between Kc and Kp?

Calculating Kp

N2O4 (g) 2NO2 (g)

Kp = [PNO2]2

[PN2O4]

If given the partial pressures of N2O4 andNO2 , we would just substitute them into the equation.

When we are given the value of Kc and we want to find Kp, we can use the following equation:

Kp = Kc(RT)/\n

R= ideal gas constant, 0.0821L-atm/mol-KT= temperature in Kelvins/\n= (moles of gaseous product) – (moles of gaseous reactant)

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.

PCl5(g) PCl3(g) +Cl2(g)

/\n= (moles of gaseous product) – (moles of gaseous reactant)/\ n= (1+1) - 1 = 1

Study Check

For the following reaction, calculate Kp when Kc = 9.60 at 573 K.

N2(g) + 3H2(g) 2NH3(g)

N2(g) + 3H2(g) 2NH3(g)

Kc = 9.60 at 573 K.

Kp = Kc(RT)/\n

R= ideal gas constant, 0.0821L-atm/mol-KT= temperature in Kelvins/\n= (moles of gaseous product) – (moles of gaseous reactant)

Homework

p. 660 in your bookProblem 15.13 a,b,c,d,eProblem 15.17