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7.7Equilibrium

Calculating Equilibrium Concentrations — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 19 1 quiz

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Lesson summary

7.7 Calculating Equilibrium Concentrations — AP CHEMISTRY notes: Use K, initial amounts and an ICE table to find equilibrium concentrations or partial pressures.Required data — the balanced equation, the initial concentration or partial pressure of each species, and KDirection of…

  • Direction of change
  • Perfect square
  • Change row
  • Equilibrium value
  • Write the balanced equation and the equilibrium expression
  • Enter the initial concentrations or partial pressures

Section 1 of 8What is needed

Required data
  • Required data the balanced equation, the initial concentration or partial pressure of each species, and K
Direction of change
  • Direction of change if no products are present, the net change must convert reactants to products

Section 2 of 8ICE table method

1
Write the balanced equation and the equilibrium expression.
2
Enter the initial concentrations or partial pressures.
3
Write the change row: −(coefficient)x for consumed species, +(coefficient)x for produced species.
4
Add initial and change to give the equilibrium row.
5
Substitute the equilibrium row into K and solve for x.
6
Calculate the equilibrium values and check by recalculating K.

Section 3 of 8Pattern: cis ⇌ trans (Kc = 1.40)

Initial
  • [cis] = 2.00 M and [trans] = 0
Equilibrium
  • [cis] = 2.00 − x and [trans] = x
Solution
  • 1.40 = x/(2.00 − x) gives x = 1.17 M; [cis] = 0.83 M and [trans] = 1.17 M

Section 4 of 8Pattern: I₂ + H₂ ⇌ 2HI (Kp = 160 at 500 K)

Initial
  • Initial equimolar gases at total 12.00 atm give PI2 = PH2 = 6.00 atm and PHI = 0
Equilibrium
  • PI2 = PH2 = 6.00 − x and PHI = 2x
Perfect square
  • Perfect square taking the square root of 160 = (2x)2/(6.00 − x)2 gives 12.65 = 2x/(6.00 − x)
Result
  • Result x = 5.18 atm and PHI = 2x = 10.4 atm

Section 5 of 8Practical points

Perfect squares
  • Perfect squares take the square root of both sides to avoid a quadratic
Check
  • Check substituting the answers should return the original K
Units
  • Units use mol L-1 in Kc and atm in Kp

Section 6 of 8More from the specification

More from the specification
ICE table equilibrium calculations
  • ICE table equilibrium calculations tabulating initial, change,
    • and equilibrium molarities allows exact concentration determination in any closed chemical system.
Equilibrium constant
  • The reaction quotient at equilibrium.
Particulates
  • Tiny solid particles, such as soot, in the air.
Reversible
  • Able to change back.

Section 7 of 8Key terms

TermMeaning
ICE tableInitial, change, equilibrium table used to solve equilibrium problems.
Change rowAmounts consumed or produced, written using x and coefficients.
Equilibrium valueInitial value plus change.
Perfect squareExpression whose square root can be taken to simplify the algebra.

Section 8 of 8Exam technique

1
Using +x for a reactant that is consumed.
2
Not multiplying x by the coefficient in the change row.
3
Using initial values in the K expression instead of equilibrium values.
4
Forgetting that solids and liquids are not included.

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