Chemical Equilibrium & K C
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Transcript of Chemical Equilibrium & K C
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: