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Chemistry 2 Unit 3: Chemical equilibrium

At equilibrium, forward and reverse rates are equal, and the equilibrium constant describes the balance of products and reactants.

Covers: Dynamic equilibrium, equilibrium constants K, the reaction quotient Q, ICE tables, Le Châtelier's principle, Kp and Kc

Study guide

A reversible reaction reaches dynamic equilibrium when the forward and reverse rates are equal, so concentrations stop changing even though both reactions continue. For aA + bB ⇌ cC + dD, the equilibrium constant is K = [C]^c[D]^d ÷ [A]^a[B]^b. Pure solids and liquids are left out. A large K (much greater than 1) means products are favored; a small K means reactants are favored. Kc uses concentrations and Kp uses partial pressures, related by Kp = Kc(RT)^Δn, where Δn is the change in moles of gas. Reversing a reaction inverts K (1/K), multiplying a reaction by n raises K to the nth power, and adding reactions multiplies their K values.

The reaction quotient Q has the same form as K but uses current concentrations. If Q K, it shifts left toward reactants; if Q = K, the system is at equilibrium. ICE tables (initial, change, equilibrium) are used to calculate equilibrium concentrations from initial amounts and K, sometimes using the small-x approximation when K is very small.

Le Châtelier's principle says that when a system at equilibrium is disturbed, it shifts to partly counteract the change. Adding a reactant or removing a product shifts the reaction right; adding a product shifts it left. Increasing pressure by decreasing volume shifts toward the side with fewer moles of gas; adding an inert gas at constant volume causes no shift. Changing temperature changes K itself: for an exothermic reaction, heat acts like a product, so raising the temperature shifts left and decreases K; for an endothermic reaction, raising the temperature shifts right and increases K. A catalyst doesn't change K or the equilibrium position.

To find Kp from a mixture, first find each gas's mole fraction, then its partial pressure (mole fraction × total pressure), and substitute the partial pressures into the Kp expression. On a concentration-versus-time graph, equilibrium is reached when the curves level off. A sudden jump in one species shows something was added, and the other curves then adjust gradually. A temperature change shifts all the curves to new levels with a new K, while a catalyst only makes the system reach the same equilibrium sooner.

Adding or removing a pure solid or liquid doesn't shift an equilibrium, because neither appears in K. When both sides have the same number of moles of gas, changing the volume doesn't shift the equilibrium either.

Key ideas

• Equilibrium means equal forward and reverse rates, not equal amounts.
• K > 1 favors products; K < 1 favors reactants.
• Compare Q with K to predict the direction of shift.
• Le Châtelier: systems shift to counteract disturbances.
• Only temperature changes the value of K.

Formulas and rules

Common mistakes

• Thinking equilibrium means equal concentrations of reactants and products.
• Thinking adding reactant changes K; it changes Q, and the system shifts.

Words to know

Dynamic equilibrium

Forward and reverse rates are equal.

Equilibrium constant (K)

Ratio of products to reactants at equilibrium.

Kc

Equilibrium constant using concentrations.

Kp

Equilibrium constant using partial pressures.

Reaction quotient (Q)

The ratio of products to reactants at any moment.

ICE table

Initial, change and equilibrium table for equilibrium problems.

Le Châtelier's principle

A disturbed equilibrium shifts to counteract the change.

Homogeneous equilibrium

All species in the same phase.

Heterogeneous equilibrium

Species in more than one phase.

Small-x approximation

Assuming x is negligible when K is small.

Exothermic reaction

A reaction that releases heat.

Endothermic reaction

A reaction that absorbs heat.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.