Chemical Systems & Equilibrium

Unit 4

Calculating the Equilibrium Constant The equilbrium constant, Keq, is the ratio of

equilibrium concentrations at a particular temp Kc for solution-phase systems or Kp for gas-

phase systems

Keq = [C]c[D]d for the eqn[A]a[B]b aA+bB cC+dD

Note: The equilibrium constant depends ONLY on the concentration of gases (not liquids/solids)

Questions: Equilibrium Law Expression1. Write the equilibrium law expression for the following:a) 2NO2(g) ↔ N2O4(g)

b) 2HI(g) ↔ H2(g) + I2(g)

2. A reaction vessel contains NH3, N2 and H2 gas at equilibrium at a certain temperature. The equilibrium concentrations are [NH3] = 0.25mol/L, [N2] = 0.11mol/L and [H2] = 1.91 mol/L. Calculate the equilibrium constant for the decomposition of ammonia.

K = [N2O4(g)]

[NO2(g) ]2

K = [H2(g)] [I2(g) ]

[HI(g) ]2

K = [N2(g)] [H2(g) ]3

[NH3(g) ]2K =

[0.11] [1.91 ]3

[0.25 ]2

K = 12.3

2NH3(g) ↔ N2(g) + 3H2(g)

Questions: Equilibrium Law Expression3. Nitryl chloride gas, NO2Cl, is in equilibrium at a certain temperature in a closed container with NO2 and Cl2 gases. At equilibrium, [NO2Cl] = 0.00106mol/L and [NO2] = 0.0108mol/L. If K = 0.558, what is the equilibrium concentration of Cl2?

4. Write a balanced equation for the reaction with the following equilibrium law expression:

K = [NO2(g)]2

[NO (g) ]2 [O2 (g) ]

Heterogeneous Equilibria homogeneous equilibria = equilibria in

which all entities are in the same phase Reactants and products are all gas or all

aqueous

heterogeneous equilibria = equilibria in which reactants and products are in more than one phase Reactants and products are in different

phases

Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.

N2O4 (g) 2NO2 (g)

Kc = [NO2]2

[N2O4]Kp = NO2

P2

N2O4P

Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.CaCO3 (s) CaO (s) + CO2 (g)

Kc =[CaO(s) ][CO2(g)]

[CaCO3(s)][CaCO3(s)] = constant[CaO(s) ] = constant

Kc = [CO2(g)]

The concentration of solids and pure liquids are considered to be constant and are not included in the expression for the

equilibrium constant.

Kc [CaO(s)]

[CaCO3(s)] = [CO2(g)]

PCO2 = Kp

CaCO3 (s) CaO (s) + CO2 (g)

PCO2does not depend on the amount of CaCO3 or CaO

N2O4 (g) 2NO2 (g)

Start with NO2 Start with N2O4 Start with NO2 & N2O4

equilibrium

equilibrium

equilibrium

Equilibrium favors the reactant side

CHECKPOINTThe reaction at 200C between ethanol and ethanoic acid produces ___________________ and __________________.

1. Write the equation for this reaction

2. Determine the equilibrium constant expression for the reaction

Sample Problem:When ammonia is heated it decomposes:

2NH3(g)↔ N2(g) + 3H2(g)

When 4.0 mol of ammonia is introduced in a 2.0L container and heated. Theequilibrium amount of ammonia is 2 0 mol. Determine the equilibrium concentrations of the other two entities.

STEP 1: Determine the concentration (initial and equilibrium) for known valuesSTEP 2: Setup an ICE TableSTEP 3: Determine the value of XSTEP 4: Use x value to determine the other quantities

Calculating Equilibrium Concentrations (when given one concentration)

Setup ICE Table

Determine the concentrations[NH3]initial = 4.0mol/2.0L = 2.0mol/L

[NH3]equilibrium = 2.0mol/2.0L = 1.0mol/L

Determine the value of X[NH3](g)equil = 2 0mol / L - 2x

[NH3](g)equil = 1.0mol/L (from calculations in Step 1)

2.0mol/L – 2x = 1.0mol/L-2x = - 1.0mol/Lx = 0.5mol/L

Use X to determine other quantities

constant

Reversible Reactions

For a given overall system composition, the same equilibrium concentrations are reached whether equilibrium is approached in the forward or the reverse direction

What about Keq will it be the same in fwd/rev?