Question

A reaction is said to be in equilibrium when (a) the rate of transformation of reactants to products is equal to the rate of transformation of products to the reactants (b) \(50 \%\) of the reactants are converted to products (c) the reaction is near completion and all the reactants are converted to products (d) the volume of reactants is just equal to the volume of the products.

Short answer

The correct option is (a) because equilibrium occurs when the rate of transformation of reactants to products is equal to the rate of transformation of products to the reactants.

Step-by-step solution

Understanding Equilibrium Concept

In a chemical equilibrium, the forward reaction (transforming reactants into products) proceeds at the same rate as the reverse reaction (transforming products back into reactants). This dynamic balance means that the concentrations of reactants and products remain constant over time, though they are not necessarily equal.

Evaluating the Given Options

Go through the given options one by one and check them against the definition of chemical equilibrium.

Analyzing Option (a)

Option (a) states the definition of chemical equilibrium accurately, where the rates of the forward and reverse reactions are equal.

Dismissing Option (b)

Option (b) suggests that equilibrium is reached when 50% of reactants are converted to products. This percentage does not define equilibrium; equilibrium doesn't require a specific percentage of conversion.

Dismissing Option (c)

Option (c) describes a reaction that is near completion, with all reactants converted to products. At equilibrium, however, both reactants and products are present; the transformation is not complete.

Dismissing Option (d)

Option (d) indicates that equilibrium is when the volume of reactants is equal to the volume of products, which is not a requirement for chemical equilibrium. Equilibrium considerations are about the rate of reactions and concentrations, not volumes.

Conclusion

Based on the characteristic features of chemical equilibrium, only option (a) matches the definition where the rates of the forward and reverse reactions are equal.

Key concepts

Rate of Reaction

Understanding the rate of reaction is essential when studying chemistry. It refers to how fast or slow reactants turn into products during a chemical reaction. This rate can be affected by various factors, including temperature, pressure, concentration of reactants, the presence of a catalyst, and surface area of solid reactants.

For instance, increasing the concentration of reactants generally speeds up the rate of reaction, as more reactant particles are available to collide and react. In the context of equilibrium, the rate of the forward reaction, where reactants become products, is critical because it must be equal to the rate of the reverse reaction, where products revert back to reactants, to reach a state of balance.

Effect of Temperature and Concentration

Raising the temperature usually increases reaction rates by providing more energy for collisions between particles, resulting in a higher number of effective collisions. Similarly, a higher concentration of reactants leads to more frequent collisions, which can also quicken the reaction pace. These principles help us understand how reactions can be controlled and how equilibrium can be reached or disrupted by changing the reaction conditions.

Dynamic Balance in Chemistry

When learning about chemical equilibrium, it's important to recognize that it represents a dynamic balance rather than a static one. This means that the reactions are ongoing and do not stop once equilibrium is reached. Instead, both forward and reverse reactions continue at the same rate, leading to no net change in the concentration of reactants and products over time.

Visible Stability

Even though changes are continuously occurring at the molecular level, a system at equilibrium appears stable to an observer because there is no observable change in the physical properties. It's like watching a busy street where the number of people entering and leaving a shop is the same; the crowd inside the shop seems unchanged even though individuals are constantly coming and going.

Maintaining equilibrium requires that nothing disrupts the reaction's conditions. If something does—like temperature fluctuation or pressure change—the equilibrium can shift in favor of either the reactants or the products, a concept known as Le Chatelier's principle.

Concentration of Reactants and Products

The concentration of reactants and products plays a pivotal role in chemical reactions, especially when discussing equilibrium. In a system at equilibrium, though the concentrations of reactants and products remain constant over time, it is not necessary for them to be equal to each other.

Concentration dictates how many particles of a substance are in a given volume and directly influences the rate of the reaction. At equilibrium, the balance of the forward and reverse reaction rates ensures that the relative concentrations of reactants and products stay unchanged, unless external conditions are modified.

Equilibrium Constant, K

Chemists use the equilibrium constant (K) to express the relationship between the concentrations of reactants and products at equilibrium. This value depends on the chemical reaction and the temperature and can indicate whether the reactants or products are favored in the equilibrium state. If K is large, the products are favored; if K is small, the reactants are favored. Understanding how K corresponds to the concentration ratio is key to manipulating and predicting the behavior of a chemical system at equilibrium.