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7.6Equilibrium

Properties of the Equilibrium Constant — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 19 1 quiz

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Lesson summary

7.6 Properties of the Equilibrium Constant — AP CHEMISTRY notes: Calculate K for a reversed, scaled or combined equation.Specific — K applies to a particular balanced equation at one temperatureAlgebraic changes — K can be changed for new equations while keeping the same temperat…

  • Multiply by 3
  • Reverse and halve
  • Equation 1 multiplied by 3
  • Equation 2 reversed
  • Reversed equation
  • Scaled equation

Section 1 of 9K belongs to one equation

Specific
  • K applies to a particular balanced equation at one temperature
Algebraic changes
  • K can be changed for new equations while keeping the same temperature

Section 2 of 9Rules

Change to equationNew K
Reverse1/K
Multiply coefficients by cKc
Halve the coefficientsK1/2
Sum of equationsK1 × K2

Section 3 of 9Pattern: 2BrCl ⇌ Br₂ + Cl₂, K = 32

Reverse
  • K = 1/32 = 0.031
Multiply by 3
  • K = 323 = 3.3 × 104
Reverse and halve
  • K = 1/321/2 = 0.18

Section 4 of 9Combining equations

1
Reverse or scale each given equation to match the target.
2
Calculate the new K for each.
3
Add the equations and cancel common species.
4
Multiply the new K values.

Section 5 of 9Pattern: C₆H₁₂ ⇌ 3C₂H₂ + 3H₂

Pattern: C₆H₁₂ ⇌ 3C₂H₂ + 3H₂
Equation 1 multiplied by 3
  • K = (0.333)3
Equation 2 reversed
  • K = 1/13.3
Product
  • Product about 2.8 × 10-3

Section 6 of 9Contrast with ΔH

Scaling
  • ΔH is multiplied by the factor, but K is raised to that power
Example
  • ½N2 + 3/2 H2 ⇌ NH3 has ΔH = −46 kJ and K = 1.20; doubling gives −92 kJ and K = 1.44

Section 7 of 9More from the specification

1Temperature dependence of Kc
  • Temperature dependence of Kc altering reaction temperature shifts Kc values according to van 't Hoff equations in an endothermic or exothermic system.
2Forward reaction
  • Reaction converting reactants into products.
3Reaction quotient
  • The ratio of product to reactant concentrations at any moment.
4Reactants
  • The substances that react together at the start of a reaction.
5Pressure
  • Force of gas particles colliding with a surface.
6Concentration
  • Moles ÷ volume in dm³, or g/dm³.

Section 8 of 9Key terms

TermMeaning
Reversed equationEquation with products and reactants exchanged.
Scaled equationEquation with coefficients multiplied by a factor.
Combined equationSum of two or more equations.
KcNew constant for an equation multiplied by c.

Section 9 of 9Exam technique

Avoid these mistakes
Multiplying K by the factor instead of raising it to the power.
Not inverting K for a reversed equation.
Adding K values for summed equations instead of multiplying.

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