Question

Which of the following is not true for a chemical equilibrium? (1) When the experimental conditions arc not changed, no change in the concentration of any of the substance occurs. (2) No reaction, neither forward nor reverse occurs at cquilibrium. (3) The ratio of the rate constants of forward and reverse reactions is constant. (4) The rclative amounts of reactants and products at cquilibrium vary greatly for different chemical reactions.

Short answer

Option (2) is not true for a chemical equilibrium.

Step-by-step solution

Understand the Concept of Chemical Equilibrium

Chemical equilibrium is the state in a chemical reaction where the concentrations of reactants and products remain constant over time because the rates of the forward and reverse reactions are equal.

Analyze Option (1)

Option (1) states: 'When the experimental conditions are not changed, no change in the concentration of any of the substance occurs.' This is consistent with the definition of chemical equilibrium since the concentrations remain constant over time if conditions do not change.

Analyze Option (2)

Option (2) states: 'No reaction, neither forward nor reverse occurs at equilibrium.' This is incorrect. At equilibrium, both the forward and reverse reactions occur, but at the same rate, which results in no net change in concentrations.

Analyze Option (3)

Option (3) states: 'The ratio of the rate constants of forward and reverse reactions is constant.' This is true because the equilibrium constant, K, is equal to the ratio of the rate constants of the forward and reverse reactions: \( K = \frac{k_1}{k_{-1}} \) where \(k_1\) is the rate constant for the forward reaction and \(k_{-1}\) is the rate constant for the reverse reaction.

Analyze Option (4)

Option (4) states: 'The relative amounts of reactants and products at equilibrium vary greatly for different chemical reactions.' This is true because the equilibrium position depends on the specific chemical reaction, the conditions, and the equilibrium constant.

Identify the Incorrect Statement

Based on the previous steps, identify the statement that is not true about chemical equilibrium. Option (2) is incorrect because at equilibrium, the forward and reverse reactions continue to occur but at equal rates.

Key concepts

Equilibrium State

Chemical equilibrium refers to the point in a chemical reaction where the concentrations of all reactants and products no longer change with time. This happens because the rates of the forward and reverse reactions have become equal.
When a reaction reaches this state, although individual molecules continue to react, there is no net change in the amounts of reactants and products. This means that in a closed system, with no change in experimental conditions like temperature or pressure, the system will remain in a state of equilibrium as long as those conditions are maintained.

• It is important to note that equilibrium does not mean that the reactants and products are in equal amounts, but that their concentrations remain constant over time.
• Equilibrium can be reached from either direction, whether starting with reactants or products.

Forward and Reverse Reactions

In a chemical reaction, two types of reactions occur: the forward reaction and the reverse reaction.
The forward reaction is where reactants turn into products, while the reverse reaction is where products revert to reactants.
At equilibrium, these two reactions occur at identical rates, balancing each other out so that no overall change in concentration of reactants and products is observed. This can often be misunderstood to mean no reaction is occurring, but in reality, both reactions are continually happening.

• Dynamic Equilibrium: This term captures the idea that the reactions continue to occur, but the total amounts of reactants and products remain constant.
• Net Change: At equilibrium, the forward and reverse reaction rates are equal, leading to no net change in the concentrations.

Equilibrium Constant

The equilibrium constant, denoted as K, is a vital concept when discussing chemical equilibrium.
It is a number that provides the ratio of the concentration of products to reactants, each raised to the power of their coefficients in the balanced chemical equation.
Mathematically, it is expressed as:
\[ K = \frac{{[C]^c[D]^d}}{{[A]^a[B]^b}} \]
Here, \[ [A], [B], [C], [D] \] are the concentrations of the reactants and products, and \[ a, b, c, d \] are their respective coefficients in the balanced equation.

• A constant at equilibrium: K provides valuable information on the extent of a reaction and remains constant at a given temperature.
• Forward and Reverse Rate Constants: K can also be defined using the rate constants of the forward \[ k_1 \] and reverse \[ k_{-1} \] reactions as \[ K = \frac{{k_1}}{{k_{-1}}} \].
• The value of K helps predict which direction the reaction will proceed to reach equilibrium.

Reaction Rates

Reaction rates refer to how quickly reactants turn into products or vice versa.
At the beginning of a reaction, the forward reaction rate is typically higher because of the higher concentration of reactants. Conversely, the reverse reaction rate is relatively low due to the low concentration of products.
As the reaction progresses, the concentration of reactants decreases, and the concentration of products increases.

• Dynamic shift: This causes the rate of the forward reaction to decrease and the rate of the reverse reaction to increase.
• Equal rates at equilibrium: Eventually, the rates of the forward and reverse reactions become equal, achieving equilibrium.
• Influencing factors: Reaction rates can be influenced by various factors like temperature, concentration, and the presence of a catalyst.

Understanding these dynamics is crucial to mastering chemical equilibrium concepts and solving related problems effectively.