Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

A reversible reaction is said to have attained equilibrium, when (a) backward reaction stops (b) both backward and forward reactions take place at equal speed (c) both backward and forward reactions stop (d) concentration of each of the reactants and products becomes equal

Step-by-Step Solution

Answer

Option (b): Both backward and forward reactions take place at equal speed.

Step 1: Understanding Equilibrium in Chemistry

In a chemical reaction, equilibrium is a state where the rate of the forward reaction equals the rate of the backward reaction. It doesn't mean the reactions stop; instead, they occur at the same rate so that the concentrations of reactants and products remain constant.

Step 2: Analyzing the Options

Let's go through each option: (a) Backward reaction stops: This would mean the system is not in equilibrium. (b) Both backward and forward reactions take place at equal speed: This matches the definition of equilibrium. (c) Both backward and forward reactions stop: This does not define equilibrium. (d) Concentration of each reactants and products becomes equal: Equilibrium does not require equal concentrations, just constant concentrations over time.

Step 3: Choosing the Correct Option

Based on the analysis, option (b) correctly describes a reaction at equilibrium, where the forward and backward reactions proceed at the same rate.

Key Concepts

Reversible Reactions

A reversible reaction is a chemical process where the products can react to regenerate the original reactants. In simpler terms, these reactions can go forward and backward. Unlike irreversible reactions, reversible reactions don't go to completion in one direction. Instead, they can shift between the reactant and product sides depending on certain conditions. Each direction of the reaction involves changes in energy and molecular interactions. In reversible reactions:

• The reaction begins with a forward reaction (reactants to products).
• As products form, a backward reaction starts (products reverting to reactants).
• An equilibrium state is eventually achieved when both reactions reach the same rate.

Reversible reactions are fundamental to many biological systems and industrial processes, making their understanding crucial in chemistry.

Forward Reaction Rate

The forward reaction rate is the speed at which reactants are converted into products. This rate can be influenced by several factors, including temperature, concentration of reactants, and the presence of a catalyst.

Factors affecting the Forward Reaction Rate

Here's how these factors can influence the forward reaction rate:

• **Temperature:** Generally, increasing the temperature speeds up the reaction. Particles gain kinetic energy and collide more frequently and more energetically.
• **Concentration of Reactants:** Higher concentration means more particles in a given volume, which increases the likelihood of collision and thus speeds up the reaction.
• **Catalyst:** A catalyst can lower the activation energy required for the reaction, allowing it to proceed faster without being consumed in the process.

The forward reaction rate is crucial in determining how quickly a reaction reaches equilibrium. Understanding these influencing factors helps in predicting and controlling the outcomes of chemical reactions.

Backward Reaction Rate

The backward reaction rate describes the speed at which products revert to reactants. Just like the forward reaction, this rate is subject to change based on similar variables.

Influencing Factors of Backward Reaction Rate

The backward reaction rate depends on:

• **Concentration of Products:** A higher concentration of products can increase the rate at which they transform back into reactants due to more frequent collisions.
• **Temperature:** It can either increase or decrease the backward reaction rate depending on whether the reaction is endothermic or exothermic.
• **Catalyst:** Catalysts facilitate the forward and backward reactions, balancing the rates to reach equilibrium more quickly.

In equilibrium, the backward reaction rate matches the forward reaction rate. A clear understanding of this rate helps predict how adjustments in reaction conditions can shift equilibrium, like in the application of Le Chatelier's principle. By effectively managing these rates, chemists can optimize reaction efficiency for desired outcomes.