1. Chemical Equilibrium Chapter 18B
West Valley High School
General Chemistry
Mr. Mata
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2. The Equilibrium Expression
nA + mB <-> xC + yD
A & B = reactants; C & D = products
n, m, x, & y = coefficients
A + B -> C + D (forward reaction)
A + B <- C + D (reverse reaction)
Equilibrium: forward rate = reverse rate
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3. Predicting Equilibrium Shift
Changes in Pressure:
N H2 <-> 2NH3
Initial: 4 moles reactant : 2 moles product
Action: Increase pressure on system.
Result: equilibrium shift to right (product).
Why: increasing pressure on reactant (more moles) causes shift to product (fewer moles).
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4. Predicting Equilibrium Shift
Changes in Concentration:
A + B <-> C + D
Initial: system in equilibrium
Action: Increase concentration of A.
Result: equilibrium shift to right (product).
Why: Increase reactant [A] causes decrease in reactant [B] -> more C & D produced.
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5. Predicting Equilibrium Shift
Changes in Concentration:
A + B <-> C + D
Initial: system in equilibrium
Action: Increase concentration of D.
Result: equilibrium shift to left (reactant).
Why: Increase product D conc causes decrease in product C conc -> more A & B reactants.
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6. Predicting Equilibrium Shift
Changes in Temperature:
N H2 <-> 2NH3
Initial: system in equilibrium
Action: Increase temperature on system.
Result: equilibrium shift to right (product).
Why: Increasing temp adds energy in the form of heat (endothermic; reactant); System shifts to right (product).
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Completion Reactions
Rxns that convert all reactants to products.
Evidence for completion reactions:
Formation of a gas
Ex: gas escapes to open air
Formation of a precipitate (ppt.)
Ex: insoluble solid product is formed
Neutralization reaction
Ex: salt & water are products in reaction
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8. Equilibrium Constant Expression
Use an equilibrium constant, Keq
Abbreviated definition is
Keq = [products]
[reactants]
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9. Equilibrium Constants
aA + bB  cC + dD
Keq = [ C ]c [ D ]d
[ A ]a [ B ]b
[ ] = concentration in moles/liter (M)
a, b, c, d = coefficients
A, B = reactants; C, D = products
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Equilibrium Problem #1
If the concentration of nitrogen is 6M, the concentration of hydrogen is 3M, and the concentration of ammonia is 2M, what is the Keq value for this reaction?
__ N2 + __ H2 <-> __ NH3
Solve: Balance chemical equation
1 N H2 -> 2 NH3
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Equilibrium Problem #1
Knowns:
[N2] = 6M, [H2] = 3M, [NH3] = 2M
Unknown: Keq = ?
Keq = [products] = [NH3]2
[reactants] [N2] [H2]3
Keq = [2M] = 0.02
[6M] [3M]3
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Equilibrium Problem #2
Calculate the Keq for this reaction if concentration of hydrogen is 2M, concentration of hydrogen iodide is 6M, and the concentration of iodine is 12M.
__ H2 + __ I2 <-> __ HI
Solve: Balance chemical equation
1 H I2 -> 2 HI
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Equilibrium Problem #2
Knowns:
[H2] = 2M, [I2] = 12M, [HI] = 6M
Unknown: Keq = ?
Keq = [products] = [HI]2
[reactants] [H2] [I2]
Keq = [6M] = 1.5
[2M] [12M]
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14. Meaning of Equilibrium Constant Keq
Use letter K to represent the CONSTANT
Calculate a numerical value (no units)
If Keq < 1, reactants favored; shift to left
(little product made)
If Keq > 1, products favored; shift to right
(good way to manufacture product)

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Chapter 18 SUTW Prompt
Describe how pressure, concentration, and temperature can cause equilibrium to shift in a chemical equation.
Complete a sentence paragraph using the SUTW paragraph format. Hilight using green, yellow, and pink.
Due Date: Tuesday, February 27, 2017 (start of class).
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