CHM 2046C Module 10-Chapter 16 Sample Test Answers

Part D: Equilibrium Constant, K & The Reaction Quotient, Q 10 points

Make three generalizations concerning the composition of equilibrium mixtures:

(McMurry p 509)

a) If K_c > 10⁺³, products dominate over reactants. If K_c is very large, the reaction proceeds nearly to completion.

b) If K_c < 10^-3, reactants predominate over products. If K_c is very small, the reaction proceeds hardly at all.

c) If K_c is in range 10^-3 10⁺³, appreciable concentrations of both reactants and products are present

Determining a reaction quotient is useful for two reasons, state the reasons:

a) It tells you whether a system is at equilibrium (when Q=K) or is not at equilibrium (when Q ≠ K).

b) By comparing Q and K, we can predict what changes will occur in reactant and product concentrations as the reaction proceeds to equilibrium.

Q < K If Q is less than K, some reactants must be converted to products for the reaction to reach equilibrium

Q > K If Q is greater than K, some products must be converted to reactants for the reaction to reach equilibrium.

Demonstrate the Usefulness of the reaction Quotient:

The brown gas nitrogen dioxide, NO₂, will exist in equilibrium with the colorless gas, N₂O₄, K = 170 at 298 K.

2 NO₂ (g) ß à N₂O₄ (g) K = 170

Suppose that, at a specific time, the concentration of NO₂ is 0.015M, the concentration of N₂O₄ is 0.025M.

a) Is the Quotient Q larger than, or smaller than, or equal to K?

[N₂O₄]¹ 0.025 0.025

Q = ------------ = ----------- = ------------- = 111.1

[NO₂]² [0.015]² 0.000225

Q at 111.1 is smaller than K = 170

b) If the system is not a equilibrium, in which direction will the reaction proceed to achieve equilibrium?

The system is not at equilibrium, reaction proceeds to the right toward equilibrium

Picture Interpretation:

The Reaction A₂ + B₂ <====> 2 AB has an equilibrium constant K_c = 4. The following pictures represent reaction mixtures that contain A₂ molecules (red), B₂ molecules (blue) and AB molecules:

a) Which reaction mixtures is at equilibrium?

b) For the reaction mixtures that are not at equilibrium, will the net reaction go in the forward or the reverse direction to react equilibrium?

Is the Quotient Q larger than, or smaller than, or equal to K?

Picture 1:

[AB]² [6]² 36

Q = ------------- = ------- = ------ = 36

[A₂]¹[B₂]¹ 1x1 ¹

Q at 36 is larger than K = 4; therefore the quotient is larger, reaction is not at equilibrium and the reaction proceeds to the left (or reverse)

Picture 2:

[AB]² [4]² 16

Q = ------------- = ------- = ------ = 4

[A₂]¹[B₂]¹ 2x2 4

Q at 4 is equal to the K = 4; therefore the system is at equilibrium

Picture 3:

[AB]² [2]² 4

Q = ------------- = ------- = ------ = 0.44

[A₂]¹[B₂]¹ 3x3 9

Q at 0.44 is smaller than K = 4; therefore the quotient is smaller, reaction is not at equilibrium and the reaction proceeds to the right (or forward)

See McMurry Section 13.5 pages 508-511.Look at Worked Example 13.8 p510, then work Problem 13.9 p 511 Work Problems 13.62-13.79 for D, E, F

See pages Kotz 767-9. Work Problems #3-#6 page 789