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

At equilibrium, both backward and forward reactions take place at equal speed, so the answer is (b).

Step 1: Understanding the Equilibrium Concept

A reversible chemical reaction can proceed in both the forward and backward directions. Equilibrium is achieved when the rate of the forward reaction equals the rate of the backward reaction, meaning that there is no net change in the concentrations of reactants and products over time.

Step 2: Analyzing the Given Options

Let's evaluate the given choices: (a) backward reaction stops - incorrect, equilibrium involves ongoing reactions; (b) both backward and forward reactions take place at equal speed - correct, equilibrium is defined by equal reaction rates; (c) both backward and forward reactions stop - incorrect, reactions continue at equilibrium; (d) concentration of reactants and products becomes equal - incorrect, their concentrations need not be equal, just constant over time.

Step 3: Choosing the Correct Option

Since at equilibrium, the forward and backward reaction rates are equal resulting in constant concentrations of reactants and products, the correct answer is (b) both backward and forward reactions take place at equal speed.

Key Concepts

Reversible Reactions

Reversible reactions are a unique type of chemical reaction where the products can revert back into reactants. This means that the reaction doesn't just proceed in one direction but can also occur in the opposite direction.
In simple terms, the substances that are being transformed can recreate the original compounds that they were formed from.

These reversible reactions are common in everyday chemical processes. For example:

• In water, carbon dioxide reacts with water to form carbonic acid, and carbonic acid can decompose back into carbon dioxide and water.
• When an acid reacts with a base to create salt and water, this is also partially reversible under the right conditions.

With reversible reactions, it’s important to note that not all are equally reversible. Some reactions may favor the formation of products, whereas others might favor maintaining the reactants in their form. External conditions like temperature and pressure can influence the direction and extent of these reactions.

Reaction Rates

Reaction rates play a critical role in understanding chemical equilibrium. They describe how quickly or slowly a reaction proceeds.
In the context of reversible reactions, both the rate at which the forward reaction (reactants converting to products) and the rate at which the backward reaction (products converting back to reactants) are important.

A few factors affect reaction rates:

• Concentration: Higher concentrations of reactants or products can increase the rate of a reaction.
• Temperature: Typically, increasing the temperature speeds up the reaction rate.
• Catalysts: These are substances that increase the reaction rate without being consumed in the process.
• Surface Area: Finely divided materials react faster due to the larger area available for the reaction.

By balancing and analyzing these factors, chemists can control reaction rates to achieve desired chemical products efficiently and safely. At equilibrium, the rate of the forward reaction will match the rate of the backward reaction.

Dynamic Equilibrium

Dynamic equilibrium is a fascinating concept in chemistry, especially in reversible reactions. At dynamic equilibrium, reactions don't stop; instead, they occur continuously but with no net change in the concentration of reactants or products.
Thus, the system appears static, but it is, in fact, dynamically balanced.

Key characteristics of dynamic equilibrium include:

• Equal Rates: The rate of the forward reaction is equal to that of the backward reaction.
• Constant Concentrations: The concentrations of reactants and products remain constant over time, though they are not necessarily equal.
• Reversible: It can be reached only in reversible reactions where both directions are possible.
• Response to Changes: Any changes to the system, like pressure or temperature, can shift the equilibrium position according to Le Chatelier's principle.

Understanding dynamic equilibrium is crucial for designing efficient chemical processes, as it allows chemists to determine the conditions needed to optimize and control the production of desired compounds.