7/27/2019 lecture 1001_30_2013(final)

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Lecture 29: Chemical Equilibrium (2)

Lecture 30: Chemical Equilibrium Calculations Worksheet on equilibrium calculation methods

Lecture 31: Catalysis and Industrial Processes

Chemistry 1001 - Semester 1, 2013 Solving for Equilibrium Concentrations

COF2 with an initial concentration of 0.0250 M is allowed to come toequilibrium at 1000 C according to the reaction and equilibriumconstant below. What are the equilibrium concentrations of COF2and CO2?

2COF2(g) CO2(g) + CF4(g) K = 2.00

We will use the ICE approach to solve this. First, create a tableshowing initial compositions, and how each changes to reach

equilibrium in terms of a single unknown quantity, x.

Initial 0.0250 0 0

Change -2x +x +x

Equilibrium 0.0250-2x +x +x

22

2

42

]20250.0[00.2

]COF[

]CF][CO[

x

xxK

Second, write out the equilibrium expression and substitute in theequilibrium concentrations from the table

Solving for Equilibrium Concentrations

Solving this quadratic equation gives two answers: x =9.235 x 10-3 or0.019 M. The second answer is mathematically correct butunphysical, as it leads to a negative [COF2] at equilibrium. Theequilibrium concentrations are:

[CO2] =[CF4] =9.235 x 10-3 M [COF2] = 6.53 x 10

-3 M

This is easy when somebody else does it but now it is up to you andtodays worksheet!

In every example today we started with reactants only. That is, the reactantquotient, Q =0. This is not always the case. Recall the example from lastlecture:-

Predicting Change Using the Reaction Quotient

What happens when solid silver is added to a solution initially containing[Ag+] =0.200M, [Fe2+] =0.100M and [Fe3+] =0.300 M?

The following reversible reaction occurs:

Ag+(aq) +Fe2+(aq) Ag(s) +Fe3+(aq) K =2.98 L/mol

3

2

[ ] 0.30015.00

[ ][ ] 0.100 0.200

FeQ

Fe Ag

As Q >K; reaction will move to the left.

Here its convenient to use +x to denotethe change in the reactant, [Ag+].

Ag+(aq) + Fe2+(aq) Ag(s) + Fe3+(aq)

Initial: 0.200 0.100 0.300 mol/L

Change: +x

Equilibrium

7/27/2019 lecture 1001_30_2013(final)

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1. Complete the calculation and determine the equilibrium concentrations in thesystem Ag+(aq) +Fe2+(aq) Ag(s) +Fe3+(aq) shown on the previous slide.

2. Calculate the exact equilibrium compositions that result if 0.0100 atm H2, 0.0050atm I2 and 0.500 atm HI are initially combined in sealed flask. The system comesto equilibrium by the reaction

H2(g) +I2(g) 2HI(g) for which K =1.00 x 102

(Hint: pV =nRT, so you can use change in pressure h=the same way as changein concentration to determinex.)

3. Calculate the approximate equilibrium compositions for the reaction

2NOCl(g) 2NO(g) +Cl2 (g)

For which Kc =1.6 x 10-5, given the initial condition of 1.0 mole of NOCl placed

in 2.0L flask.

Homework Questions

You should now be able to Calculate the equilibrium composition for a chemical reaction from

appropriate equilibrium constant and initial composition data.

Recognise and use the small Kapproximation for more complex and other

appropriate equilibrium composition problems.

Next Lecture

Chemical Equilibrium, Catalysis and Mineral Extraction

Summary