Equilibrium expression

Definition

An equilibrium expression is a mathematical representation of the relationship between the concentrations of reactants and products at chemical equilibrium. It is derived from the balanced chemical equation and is crucial for calculating the equilibrium constant, which quantifies the extent of a reaction. This expression helps in understanding how changes in conditions affect the position of equilibrium, making it a key concept in chemical thermodynamics.

5 Must Know Facts For Your Next Test

1. The equilibrium expression is written as $$K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$$ for a general reaction $$aA + bB \rightleftharpoons cC + dD$$.
2. Only gaseous and aqueous species are included in the equilibrium expression; solids and liquids do not appear because their concentrations remain constant.
3. The value of the equilibrium constant (K) indicates whether the products or reactants are favored at equilibrium: K > 1 favors products, while K < 1 favors reactants.
4. Changes in temperature will alter the value of K, while changes in pressure or concentration can shift the position of equilibrium without changing K.
5. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, leading to constant concentrations of reactants and products.

Review Questions

• How does the equilibrium expression reflect the balanced chemical equation and what information does it provide about a chemical system?
  • The equilibrium expression is directly derived from the balanced chemical equation, incorporating the coefficients as exponents corresponding to each reactant and product. This representation quantifies how concentrations relate at equilibrium, indicating whether products or reactants are favored. By examining this expression, one can gain insights into reaction dynamics and predict how changes in concentration or pressure might shift equilibrium.
• Discuss how Le Chatelier's Principle applies to shifts in equilibrium based on changes in concentration or pressure within a chemical reaction.
  • Le Chatelier's Principle states that if an external change, such as increased concentration or pressure, is imposed on a system at equilibrium, the system will respond by shifting in a direction that counteracts that change. For instance, increasing reactant concentration will drive the reaction toward product formation to restore balance. Similarly, increasing pressure favors the side with fewer moles of gas, affecting the equilibrium position without altering the value of K.
• Evaluate how understanding the relationship between reaction quotient (Q) and equilibrium constant (K) can aid in predicting reaction direction and achieving desired outcomes in chemical processes.
  • By comparing the reaction quotient (Q) with the equilibrium constant (K), one can determine whether a reaction will proceed forward or reverse to reach equilibrium. If Q < K, products are favored and the reaction shifts right; if Q > K, reactants are favored and it shifts left. This evaluation enables chemists to manipulate conditions strategically to maximize product yield or optimize conditions for specific reactions, making it crucial for industrial applications and research.

Related terms

equilibrium constant (K): A numerical value that represents the ratio of concentrations of products to reactants at equilibrium, raised to the power of their coefficients in the balanced equation.

Le Chatelier's Principle:

A principle stating that if an external change is applied to a system at equilibrium, the system will adjust to counteract that change and restore a new equilibrium.

reaction quotient (Q): A ratio similar to the equilibrium constant but calculated using the current concentrations of reactants and products, used to determine the direction a reaction will shift to reach equilibrium.