14.5
Reaction Quotient
The status of a reversible reaction is conveniently assessed by evaluating its reaction quotient (Q). For a reversible reaction described by m A + n B…
The equilibrium constant expression is written as the molar concentrations of the products, C and D, over the reactants, A and B, at equilibrium, each raised to their respective stoichiometric coefficients. When solved, the expression is equal to the equilibrium constant, Kc.
An expression in the same form can also be written for the reactants and products at any concentration, and the calculated quantity is known as the reaction quotient, Qc.
Like Qc, the Qp expression can be written for gaseous reactions using partial pressures.
While K remains constant at a specific temperature irrespective of concentration, the value of Q changes as the reaction proceeds towards the products or the reactants.
The reaction quotient can be used to determine the direction a reaction will proceed in order to reach equilibrium.
At the start of a given reaction, if the concentration of the products is zero, the reaction quotient is zero.
Whenever the concentration of the reactants in the denominator is high, such that Q is smaller than K, the reaction will move to the right to synthesize more products until the system reaches equilibrium.
If the concentration of the reactants is zero, the reaction quotient is infinite.
Whenever the concentration of the products in the numerator is high, such that Q is larger than K, the reaction will move to the left to produce more reactants.
If Q is equal to K, the system is at equilibrium, and the rate of the forward and reverse reactions are equal.
Consider the given reaction with an equilibrium constant 50. If the reaction mixture contains 0.20 molar hydrogen, 0.20 molar iodine, and 1.7 molar hydrogen iodide, the direction of the reaction shown can be determined by calculating Q.
Substituting the given concentrations into the expression, Q equals 72, which is greater than K. Therefore, the reaction will shift towards the left.
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Q1: What is the difference between the reaction quotient and the equilibrium constant?
The reaction quotient (Q) and equilibrium constant (K) have identical mathematical forms but differ in timing and constancy. K is the fixed value of Q when a system reaches equilibrium at a specific temperature. While K remains constant regardless of concentration, Q changes continuously as the reaction proceeds. Q can be calculated at any moment using current concentrations, making it useful for determining reaction direction.
Q2: How do you calculate the reaction quotient for a gaseous reaction?
For gaseous reactions, the reaction quotient (Qp) uses partial pressures instead of molar concentrations. The expression follows the same form as the concentration-based quotient (Qc), with product partial pressures in the numerator and reactant partial pressures in the denominator, each raised to their stoichiometric coefficients. This allows assessment of homogeneous equilibria for gaseous reactions using pressure data.
Q3: What does it mean when Q is less than K?
When Q is less than K, the system is not at equilibrium and the reaction will shift toward the products (move right). This occurs when reactant concentrations are high relative to product concentrations. The reaction proceeds forward to synthesize more products until Q increases and equals K, establishing equilibrium.
Q4: How does the reaction quotient indicate which direction a reaction will proceed?
The reaction quotient determines reaction direction by comparing Q to K. If Q < K, the reaction shifts right to form more products. If Q > K, the reaction shifts left to form more reactants. If Q = K, the system is at equilibrium with equal forward and reverse reaction rates. This comparison allows prediction of how concentrations will change to reach equilibrium.
Q5: What is the initial value of the reaction quotient when a reaction starts with only reactants?
When a reaction begins with only reactants and no products present, the reaction quotient is zero. The numerator (product concentrations) equals zero while the denominator (reactant concentrations) contains measurable values. As the reaction proceeds toward equilibrium, product concentrations increase, causing Q to increase until it reaches the equilibrium constant value.
Q6: Why does the reaction quotient change as a reaction progresses toward equilibrium?
The reaction quotient changes because reactant and product concentrations continuously shift during the reaction. As reactants convert to products, the denominator decreases while the numerator increases, causing Q to increase. Conversely, if products convert back to reactants, Q decreases. These concentration changes continue until Q equals K, at which point concentrations stabilize and Q becomes constant.
Q7: What happens to the reaction quotient when Q equals the equilibrium constant?
When Q equals K, the system has reached equilibrium. At this point, the reaction quotient becomes constant and no longer changes. The forward and reverse reaction rates are equal, so concentrations of reactants and products remain stable. The system is in dynamic equilibrium homogeneous and heterogeneous equilibria, with no net change in composition.