Reaction quotient (q)

Definition

The reaction quotient (q) is a dimensionless number that represents the ratio of the concentrations of products to reactants at any point during a chemical reaction, not necessarily at equilibrium. It helps to determine the direction in which a reaction will proceed by comparing its value to the equilibrium constant (K). When q is compared to K, it indicates whether the system is at equilibrium, shifting towards products, or shifting towards reactants.

5 Must Know Facts For Your Next Test

1. The reaction quotient is calculated using the same expression as the equilibrium constant but uses the current concentrations of reactants and products, not just those at equilibrium.
2. If q < K, the reaction will shift to the right, favoring the formation of products until equilibrium is reached.
3. If q > K, the reaction will shift to the left, favoring the formation of reactants until equilibrium is restored.
4. The value of q changes as a reaction proceeds, reflecting how far along the reaction is at any given moment.
5. The reaction quotient can be applied to different types of equilibria, including gaseous reactions where partial pressures may be used instead of concentrations.

Review Questions

• How does the value of the reaction quotient (q) inform you about the direction a reaction will proceed?
  • The value of q provides insight into whether a reaction will shift toward products or reactants. If q is less than the equilibrium constant K, it indicates that there are more reactants than products currently present, so the reaction will shift right towards producing more products. Conversely, if q is greater than K, it suggests that there are more products than reactants present, prompting a shift left to produce more reactants.
• Compare and contrast the reaction quotient (q) with the equilibrium constant (K), highlighting their significance in understanding chemical equilibria.
  • The reaction quotient (q) and equilibrium constant (K) are both ratios of product and reactant concentrations but differ in their timing. While K reflects a system at equilibrium under specified conditions, q can be calculated at any point during the reaction. By comparing q to K, one can determine how far a reaction has progressed and predict its direction: q < K indicates a shift towards products, while q > K suggests a shift towards reactants.
• Evaluate how Le Chatelier's Principle interacts with changes in concentration as indicated by the reaction quotient (q) and its implications for dynamic equilibria.
  • Le Chatelier's Principle states that a system at equilibrium will respond to changes in concentration by shifting its position to minimize that change. When observing shifts via q, if additional reactants are introduced and q decreases relative to K, the system will adjust by favoring product formation until a new equilibrium is established. This illustrates how dynamically equilibrated systems respond actively to concentration shifts, ensuring that they strive to maintain balance despite external changes.

Related terms

Equilibrium Constant (K): A specific value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature.

Le Chatelier's Principle:

A principle stating that if an external change is applied to a system at equilibrium, the system adjusts itself to counteract that change and re-establish equilibrium.

Dynamic Equilibrium: A state in which the rates of the forward and reverse reactions are equal, resulting in no net change in concentrations of products and reactants over time.