2. The Extent of a Reaction
A value of K much larger than 1 means that at equilibrium the reaction system will consist of mostly products – the equilibrium lies to the right.
Reaction goes essentially to completion.
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3. The Extent of a Reaction
A very small value of K means that the system at equilibrium will consist of mostly reactants – the equilibrium position is far to the left.
Reaction does not occur to any significant extent.
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4. CONCEPT CHECK!
If the equilibrium lies to the right, the value for K is __________. large (or >1) If the equilibrium lies to the left, the value for K is ___________. small (or <1)
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5. Reaction Quotient, Q
Used when all of the initial concentrations are nonzero.
Apply the law of mass action using initial concentrations instead of equilibrium concentrations.
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6. Reaction Quotient, Q
Q = K; The system is at equilibrium. No shift will occur.
Q > K; The system shifts to the left.
Consuming products and forming reactants, until equilibrium is achieved.
Q < K; The system shifts to the right.
Consuming reactants and forming products, to attain equilibrium.
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7. Consider the reaction represented by the equation:
EXERCISE!
Consider the reaction represented by the equation:
Fe3+(aq) + SCN-(aq) FeSCN2+(aq)
Trial #1:
6.00 M Fe3+(aq) and 10.0 M SCN-(aq) are mixed at a certain temperature and at equilibrium the concentration of FeSCN2+(aq) is 4.00 M.
What is the value for the equilibrium constant for this reaction?
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8. Set up ICE Table Fe3+(aq) + SCN–(aq) FeSCN2+(aq)
Initial
Change – 4.00 –
Equilibrium
K = 0.333
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9. EXERCISE!
Consider the reaction represented by the equation: Fe3+(aq) + SCN-(aq) FeSCN2+(aq)
Trial #2:
Initial: 10.0 M Fe3+(aq) and 8.00 M SCN−(aq) (same temperature as Trial #1)
Equilibrium: ? M FeSCN2+(aq) 
5.00 M FeSCN2+
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10. EXERCISE!
Consider the reaction represented by the equation: Fe3+(aq) + SCN-(aq) FeSCN2+(aq)
Trial #3:
Initial: 6.00 M Fe3+(aq) and 6.00 M SCN−(aq)
Equilibrium: ? M FeSCN2+(aq)
3.00 M FeSCN2+
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11. Solving Equilibrium Problems
Write the balanced equation for the reaction.
Write the equilibrium expression using the law of mass action.
List the initial concentrations.
Calculate Q, and determine the direction of the shift to equilibrium.
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12. Solving Equilibrium Problems
5) Define the change needed to reach equilibrium, and define the equilibrium concentrations by applying the change to the initial concentrations.
Substitute the equilibrium concentrations into the equilibrium expression, and solve for the unknown.
Check your calculated equilibrium concentrations by making sure they give the correct value of K.
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13. EXERCISE!
Consider the reaction represented by the equation: Fe3+(aq) + SCN-(aq) FeSCN2+(aq) Fe3+ SCN- FeSCN M 5.00 M 1.00 M Find the equilibrium concentrations for all species.
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14. EXERCISE!
Answer
Trial #1: [Fe3+] = 6.00 M; [SCN-] = 2.00 M; [FeSCN2+] = 4.00 M
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