CHEMICAL E

QUILIBRIU

M &

K C

PL N

13

& P

L N 1

4

IMPORTANT CONCEPTS

Equilibrium Constant

Reaction Quotient

INTRODUCTION TO CHEMICAL EQUILIBRIUMNo Reaction “Goes to Completion” Reactants are forming products However, products are also forming reactants Reactants are formed at such a slow rate, it’s barely noticeable

usually

When the Reaction Appears to Stop Product and reactant concentrations level off An equilibrium is reached

DYNAMIC EQUILIBRIUM

The Reaction Only Appears to Have Stopped The forward reaction is going just as fast as the reverse reaction

A Balance is Reached, but the Reactions are Still Occurring Dynamic equilibrium

EQUILIBRIUM EXAMPLES

Ionization of Weak Acids or Weak Bases

Dissolution/Precipitation

Biogeochemical Cycles Carbon Cycle

Acetic Acid Water Acetate Ion Hydronium Ion

POSITION OF EQUILIBRIUM

The Position is How Far the Reaction Proceeds Before Reaching Equilibrium

An Equilibrium that Lies to the Right:

Reaction almost completed System is mostly products

Equilibrium Lies to the Left:

Reactants barely reacted System is mostly reactants

EQUILIBRIUM CONSTANT, KC

For a Specific Temperature, the Concentration Ratio of Products to Reactants is a Constant. Where:

Shows the Relationship Between the Concentrations

Illustrates How Each Product or Reactant Concentration Can be Manipulated to Affect the Others

(KC Has No Units)

KC

  Equilibrium Concentrations and the Value of the Mass Action Expression

Experiment

Experiment Concentrations

      

I 0.0222 0.0222 0.156  49.4

II 0.035 0.045 0.28 49.8 

III 0.015 0.0135 0.1  49.4

IV 0.0442 0.0442 0.311  49.5

CATALYST

Speeds up the Rate of Reaction Does NOT alter the equilibrium constant The equilibrium does not change, the system just reaches equilibrium

quicker

GASES

The “Concentration” of a Gas in a Mixture is Measurable by its Partial Pressure

Therefore, While:

The Equilibrium Constant:

CONVERTING KP TO KC

Where:

SIZE OF KC OR KP

Remember That Equilibrium Constants are Related to the Position of the Equilibrium

Small Numerical Value:

More Reactants Fewer Products

Large Numerical Value:

Fewer Reactants More Products

MANIPULATING EQUILIBRIUM EQUATIONS

Flipping:

Multiplying by a Coefficient:

Adding Equations Together:

REACTION QUOTIENTS

The Reaction Quotient is Calculated Using the Same Equation as for the KC

Is frequently compared to the equilibrium constant to determine if the reaction is at equilibrium

If not, then they may be compared to determine which direction the reaction is going at that certain point

Q = K The reaction has reached equilibrium

Q < K Reaction will proceed forward More products will form

Q > K Reaction will continue backwards More reactants will form

CONSIDER THIS PROBLEM

A mixture of 0.0100 moles each of carbon monoxide and water were added to a 100-mL flask at 900 K and the reaction allowed to come to equilibrium. The experimental value of K = 1.56 at this temperature.

Calculate the concentrations of all substances at equilibrium.

CALCULATING EQUILIBRIUM CONCENTRATIONSICE Method (Initial Change Equilibrium) Write out equilibrium equation:

Write expression for equilibrium constant:

Make an ICE chart:

[CO2] [H2] [CO] [H2O]

Initial 0.0 M 0.0 M 0.100 M 0.100 M

Change +x +x -x -x

Equilibrium x x 0.100 - x 0.100 - x

CALCULATING EQUILIBRIUM CONCENTRATIONS (CONTINUED) Substitute the equilibrium values into the equilibrium constant

expression:

Solve:

PLUG AND CHECK!

SYSTEMS WITH SMALL EQUILIBRIUM CONSTANTSConsider:

Now, Calculate all equilibrium concentrations when (1) 5.00 moles of COCl2 and (2) 0.100 moles of COCl2 decompose in a 10.0-L flask.

PART 1 – 5.00 MOLES OF COCl2 K expression:

ICE chart:

Since K is very small, what can we say about x?

[COCl2] [CO] [Cl2]

Initial 0.500 M 0.0 M 0.0 M

Change -x +x +x

Equilibrium 0.500 - x x x

PART 1 (CONTINUED)

If K is so small, then the reaction must not produce a lot of its products, therefore we can assume x is relatively small and won’t make a large impact on the concentration of the COCl2

Let’s rewrite the K expression, substituting our equilibrium values in and making the appropriate assumption:

PART 2 – 0.100 MOLES OF COCl2Not Fair to Assume that x is Unimportant in This Case

That Assumption May Only be Made if: K is very small

~OR~ The initial concentration is very high compared to the value of K

As a General Rule: If 100×K < [A]0, then you can approximate x to be unimportant to the

overall value of [A]0

ANOTHER TRICK

Similar to the Method Used in the Previous Example, if K is Very Large, then You Can Assume (When Calculating Equilibrium Concentrations) that the Reaction Goes to Completion

Example:

If given the number of moles of the reactants, just add them to calculate the moles of the products and assume there will be no reactants remaining at equilibrium (keep in mind that there may be a limiting reactant!)

THE METHOD OF SUCCESSIVE APPROXIMATIONSIf, After Plugging in Equilibrium Values from an ICE Chart, You

Get Something that Looks Like this:

Where K is Small and Therefore We Can Assume x is Small:

Now Plug this Back in For x in the Original Equation:

Repeat:

Keep Repeating Until x ‘Stabilizes at a Fixed Value (at Step 4 this Case stabilizes at 3.4 × 10‐4)This Method Works as Long as K < 4C Where: