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2. It is not always obvious if a mixture has reached equilibrium or not.
If it is not at equilibrium, then it is helpful to know which direction the reaction must proceed in order to reach equilibrium.
The reaction quotient, Q, measures the relative amounts of products and reactants present during a reaction at a particular point in time.
REACTION QUOTIENT, Q

3. Q helps in figuring out which direction a reaction is likely to proceed, given the concentrations of the reactants and the products.
Q can be compared to Keq to determine the direction of the reaction that is taking place.
REACTION QUOTIENT, Q

4. Suppose you are running the reaction and want to know if the reaction has reached equilibrium. Example: N2 + 3 H2 ↔ 2 NH3 Keq = You measure the concentrations to be: [NH3] = 0.15 M [H2] = 0.10 M [N2] = M The reaction quotient can be calculated by substituting these NON-EQUILIBRIUM concentrations into the equilibrium expression.
REACTION QUOTIENT, Q

5. REACTION QUOTIENT, Q SOLVE FOR “Q”: Keq = 0.105 Q = [NH3]2 = (0.15)2 =
This reaction is NOT at equilibrium. Q ≠ Keq.
The reaction proceeds towards the left, towards reactants.
REACTION QUOTIENT, Q

6. When trying to determine the direction for Q, compare Q to Keq
Then think of a number line.
Put Keq on the number line. Then put Q on it.
Determine which direction do you need to travel for Q to become Keq.
Keq = 0.105
Q = 11000
REACTION QUOTIENT, Q

7. REACTION QUOTIENT, Q STEPS: 1. Balance the chemical equation.
2. Write the equilibrium expression but put Q instead of Keq.
3. Plug in the concentrations to solve.
4. Compare Q to Keq. (greater than, less than, equal to)
5. Indicate which way the reaction needs to shift to reach equilibrium.
REACTION QUOTIENT, Q

8. REACTION QUOTIENT, Q DECIDING DIRECTION – always compare Q to Keq:
If Q is less than Keq, this means that at the time of measurement, there were too many reactants and too few products. The reaction will proceed to the right, in the direction of the products.
If Q is more than Keq, this means that at the time of measurement, there were too few reactants and too many products. The reaction will proceed to the left, in the direction of the reactants.
If Q is equal to Keq, the system is at equilibrium and there will be no shift in direction at this temperature.
REACTION QUOTIENT, Q

9. A reaction’s concentrations are measured and found to be H2 = 0
A reaction’s concentrations are measured and found to be H2 = 0.45M and H2O = 0.37M and the balanced equation is Fe2O3 (s) + 3H2 (g)  2Fe (s)+ 3H2O (g) Keq = Indicate if the reaction is at equilibrium. If it is not, indicate which direction it needs to proceed to reach equilibrium.
PRACTICE

10. 1. Write the equilibrium constant expression for any system at equilibrium. 2. Calculate Keq given the concentrations of products and reactants at equilibrium. 3. Given Keq, calculate the equilibrium concentrations of products and reactants at equilibrium (missing species). 4. Calculate the reaction quotient, Q, and relate it to Keq. Determine in which direction the reaction will proceed (shift left, shift right, at equilibrium).
QUIZ FORMAT - MONDAY

11. Reaction Quotient handout is due tomorrow.
Lab tomorrow.
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