A Level Chemistry OCR Practice Exam

What type of equilibrium exists when the rate of the forward reaction equals the rate of the reverse reaction in a closed system?

1. Static equilibrium

2. Dynamic equilibrium

3. Partial equilibrium

4. Reversible equilibrium

The correct answer is: Dynamic equilibrium

The correct choice is dynamic equilibrium, which occurs in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction. In such an equilibrium, the concentrations of reactants and products remain constant over time, even though both reactions are still occurring. This means that the system is not static; rather, there is continuous activity at the molecular level, and the system is in a state of balance. Static equilibrium, on the other hand, describes a situation where there are no moving parts or no change happening within the system, which does not apply to chemical reactions where reactions are happening simultaneously. Partial equilibrium refers more generally to a scenario where some parts of a system may reach a balance, but this doesn't specifically apply to the context of chemical reactions as the term could imply only a temporary state. Reversible equilibrium is a term that suggests the capability of the reaction to move back and forth between reactants and products. While this can describe a dynamic equilibrium, it doesn't accurately imply the specific balance of rates that is a hallmark of dynamic equilibrium. Thus, dynamic equilibrium is the most precise term that signifies the continual, yet balanced activity of both the forward and reverse reactions within a system.