Question

The concept of chemical equilibrium is very important. Which one of the following statements is the most correct way to think about equilibrium? (a) If a system is at equilibrium, nothing is happening. (b) If a system is at equilibrium, the rate of the forward reaction is equal to the rate of the back reaction. (c) If a system is at equilibrium, the product concentration is changing over time. [Section 4.1\(]\)

Short answer

The most correct statement about chemical equilibrium is: \( (b) \) If a system is at equilibrium, the rate of the forward reaction is equal to the rate of the back reaction.

Step-by-step solution

(a) If a system is at equilibrium, nothing is happening.

A common misconception is that at equilibrium, the reaction stops. In reality, a system at equilibrium is still dynamic, meaning the forward and reverse reactions continue to occur, but at the same rate. This statement is not the most correct.

(b) If a system is at equilibrium, the rate of the forward reaction is equal to the rate of the back reaction.

Chemical equilibrium occurs when the rate of the forward reaction is equal to the rate of the reverse reaction. This means that the concentrations of the reactants and products remain constant over time, and the overall reaction appears to have stopped. However, the forward and reverse reactions are still occurring at the same rate. This statement correctly describes the concept of chemical equilibrium.

(c) If a system is at equilibrium, the product concentration is changing over time.

If a system is at equilibrium, the concentrations of reactants and products remain constant, because the forward and reverse reactions are occurring at the same rate. Therefore, this statement is not accurate. Based on the analysis above, the most correct statement about chemical equilibrium is:

Most correct statement:

(b) If a system is at equilibrium, the rate of the forward reaction is equal to the rate of the back reaction.

Key concepts

Reaction Rates

Reaction rates refer to how fast or slow a chemical reaction occurs. In the context of chemical equilibrium, understanding reaction rates is crucial. A chemical reaction doesn't stop when it reaches equilibrium. Instead, both the forward and reverse reactions continue to occur. However, they do so at equal rates, which is why we say the system is at equilibrium.

When the rate of the forward reaction equals the rate of the back reaction, the concentrations of reactants and products remain constant. This balance is what defines equilibrium. It’s important to realize that equilibrium does not mean a reaction has stopped, but that its rates are balanced. By studying reaction rates, chemists can predict how quickly a reaction will reach equilibrium under given conditions.

Factors affecting reaction rates include:

• Temperature: Higher temperatures generally increase reaction rates because particles move faster and collide more frequently.
• Concentration: More reactants can lead to more collisions and thus a faster reaction rate.
• Catalysts: These substances speed up reactions by lowering the activation energy needed.

Dynamic Equilibrium

Dynamic equilibrium is a state where a chemical system achieves balance, even though reactions are still happening. It is essential to understand that equilibriums are dynamic in nature. This means that in a dynamic equilibrium, the processes of the forward and reverse reactions continue to happen without any net change in concentrations of products and reactants.

This equilibrium is called "dynamic" because although it appears stable and unchanging from an outside perspective, internally, the molecules continue to react and transform between reactants and products. The continuous nature of these reactions maintains the equilibrium, ensuring that concentrations remain constant over time.

Dynamic equilibrium is an important concept not only in chemistry but in various fields such as biology and economics, wherever balance is required between competing processes. Recognizing dynamic equilibrium helps us understand the stability of a system, despite the ongoing processes taking place within it.

Concentration

Concentration refers to the amount of a substance in a given volume. In chemical equilibrium, the concept of concentration is crucial. When a system is at equilibrium, the concentrations of the reactants and products do not change over time, even though chemical reactions are continuing.

This means the system has achieved a steady state where the concentrations remain constant. It's important to remember that equilibrium doesn't imply equal concentrations; instead, it means that the concentrations are stable and maintained at a certain ratio.

Knowing how changes in concentration affect equilibrium is part of Le Chatelier's principle, which helps predict how a system at equilibrium will respond to changes in conditions, such as:

• Adding or removing reactants or products
• Changing the temperature or pressure

Changes in concentration can shift the equilibrium position, causing the system to adjust in a way that counteracts the change and re-establishes equilibrium. Understanding concentration's role in equilibrium helps chemists control and predict reaction outcomes.