Chemical equilibrium is attained when

Step 1: Analyze the question The question asks what defines chemical equilibrium, which applies to reversible reactions.

Step 2: The concept In a reversible reaction, reactants form products (forward reaction) and products also convert back to reactants (reverse reaction). Equilibrium is reached when:

• The rate of the forward reaction equals the rate of the reverse reaction.
• As a result, the concentrations of reactants and products stay constant (they do not change with time).

This is called dynamic equilibrium. The reactions have not stopped, but they are happening at equal rates, so there is no net change.

Step 3: Evaluate each option

• A. All the reactants have been completely used up: Wrong. At equilibrium, both reactants and products are present.
• B. The reaction goes to completion: Wrong. A reaction that goes to completion is not in equilibrium. Equilibrium involves a reversible reaction that does not go to completion.
• C. The concentration of reactants and products remain constant: Correct. This is the macroscopic sign of equilibrium.
• D. The concentration of reactants and products are equal: Wrong. The concentrations are constant, but not necessarily equal. Their ratio is given by the equilibrium constant, Kc, which can be much greater or less than 1.

Step 4: Answer

Exam tip: Remember the phrase "constant, not equal." At equilibrium, concentrations are constant (unchanging), and the rates of the forward and backward reactions are equal. Do not mix these up. Examiners often use "equal concentrations" as a trap.