1. Chapter 13 Reaction Rates and Chemical Equilibrium
13.3 Equilibrium Constants
Learning Goal Calculate the equilibrium constant for a reversible reaction, given the concentrations of reactants and products at equilibrium.
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2. Equilibrium Constants
For the reaction: aA + bB cC + dD
the equilibrium constant expression, Kc, gives the concentrations of the reactants and products, each raised to a power equal to its coefficient in the balanced equation.
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3. Equilibrium Constants
In the equilibrium expression:
the square brackets indicate the mol/liter of each substance
the coefficients a, b, c, and d are superscripts that raise the mol/liter to a specific power
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4. Guide to Writing an Equilibrium Expression
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5. Write an equilibrium expression for the following reaction:
Learning Check
Write an equilibrium expression for the following reaction:
2CO(g) + O2(g) CO2(g)
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6. Write an equilibrium expression for the following reaction:
Solution
Write an equilibrium expression for the following reaction:
2CO(g) + O2(g) CO2(g)
Step 1 Write the balanced chemical
equation.
2CO(g) + O2(g) CO2(g)
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7. Write an equilibrium expression for the following reaction:
Solution
Write an equilibrium expression for the following reaction:
2CO(g) + O2(g) CO2(g)
Step 2 Write the concentrations of the products as the numerator and the reactants as the denominator.
The square brackets indicate the mol/liter of each substance.
The coefficients a, b, c, and d are superscripts that raise the mol/liter to a specific power.
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8. Write an equilibrium expression for the following reaction:
Solution
Write an equilibrium expression for the following reaction:
2CO(g) + O2(g) CO2(g) Step 3 Write any coefficient in the equation as an exponent.
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9. Heterogeneous Equilibrium
In heterogeneous equilibrium,
gases, solids, and liquid states are part of the reaction
2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g)
solid and liquid concentrations are constant
the Kc expression is written with only the compounds that are gases
Kc = [CO2][H2O]
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10. What is the Kc expression for the following reaction?
Learning Check
What is the Kc expression for the following reaction?
2KClO3(s) KCl(s) + 3O2(g)
A B.
C D.
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11. What is the Kc expression for the following reaction?
Solution
What is the Kc expression for the following reaction?
2KClO3(s) KCl(s) + 3O2(g)
The correct answer is D, Pure solids and pure liquids have a constant concentration and are not included in the equilibrium constant expression.
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12. Equilibrium Constant Concentrations
The equilibrium constant, Kc, is the numerical value obtained by substituting experimental values of molar concentrations at equilibrium.
H2(g) + I2(g) HI(g)
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13. Equilibrium Constant Concentrations
Experimental concentrations used to calculate the equilibrium constant will always give the same value of Kc, at a specific temperature.
A reaction at a specific temperature can have only one value for the equilibrium constant.
The units of Kc depend on the specific equation. In this textbook, Kc is given without units.
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14. Equilibrium Constant Concentrations
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15. Guide to Calculating the Kc Value
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16. Learning Check
What is the numerical value of Kc at 443 oC if the equilibrium concentrations are:
H2(g) + I2(g) HI(g)
[H2 ] = 1.2 mol/L [I2] =1.2 mol/L
[HI] = 0.35 mol/L
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17. What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g)
Solution
What is the numerical value of Kc at 443 oC?
H2(g) + I2(g) HI(g)
Step 1 State the given and needed quantities.
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18. What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g)
Solution
What is the numerical value of Kc at 443 oC?
H2(g) + I2(g) HI(g)
Step 2 Write the Kc expression for the equilibrium.
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19. What is the numerical value of Kc at 443 oC? H2(g) + I2(g) 2HI(g)
Solution
What is the numerical value of Kc at 443 oC?
H2(g) + I2(g) HI(g)
Step 3 Substitute equilibrium (molar) concentrations and calculate Kc.
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