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The Reaction Quotient, “Q”

The Reaction Quotient, “Q”. Lecture 3 Part 2. What is “Q”?. Q is a value we can use to determine if a reaction is at equilibrium.

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The Reaction Quotient, “Q”

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  1. The Reaction Quotient, “Q” Lecture 3 Part 2

  2. What is “Q”? • Q is a value we can use to determine if a reaction is at equilibrium. • If a reaction is NOT at equilibrium, we can predict which direction (LEFT or RIGHT) the reaction will shift in order to reach equilibrium by comparing the value of “Q” to the value of “Keq”.

  3. How Do We Get the Value of “Q”? • It is calculated exactly like Keq, EXCEPT the concentrations we plug in are NOT equilibrium concentrations. • They are just the concentrations of reactants and products at whatever time Q is calculated.

  4. Example: at 472oC, Keq = 0.105 N2(g) + 3H2(g)  2NH3(g) • 2 minutes after this reaction starts, you want to know if it’s at equilibrium so you measure the concentrations and find: [N2] = 0.0020M [H2] = 0.10M [NH3] = 0.15M These are not necessarily Equilibrium concentrations

  5. Compare “Q” to “Keq” • If Q = Keq, the reaction is at equilibrium • Q = 1x104 • Keq =0 .105 • So this reaction is NOT at equilbrium

  6. Which Way Will the Reaction Shift to Reach Equilibrium? • If Q < Keq, the reaction will shift RIGHT • In this case, the denominator of the Q expression is too large (lots of reactants), so the reaction shifts RIGHT to use up reactants and the value of Q increases until finally it equals Keq.

  7. Which Way Will the Reaction Shift to Reach Equilibrium? • If Q > Keq, the reaction will shift LEFT • In this case, the numerator of the Q expression is too large (lots of products), so the reaction shifts LEFT to use up some products and the value of Q decreases until finally it equals Keq.

  8. Which Way Will the Reaction Shift to Reach Equilibrium? • If Q = Keq, the reaction is already at equilibrium.

  9. Practice Problem: Keq = 170 for CoCl2(g)  Co(g) + Cl2(g) • If [Co] = [Cl2] = 0.15M and [CoCl2] = 1.1x10-3, is the reaction at equilibrium? If not, which way will it shift to reach equilibrium?

  10. Practice Problem: Keq = 170 for CoCl2(g)  Co(g) + Cl2(g) Q = [Co][Cl2] = (.15)(.15) = 20 [CoCl2] (1.1x10-3) So… Q < Keq 20 < 170 The reaction will shift RIGHT to reach equilibrium

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