Which of the following expressions represents the rate at which ClO3^- (aq) is appearing in the reaction below?2 ClO2 (aq) + 2 OH^- (aq) → ClO3^- (aq) + ClO2^- (aq) + H2O (l)

• A. rate = d[ClO3^-]/dt
• B. rate = -d[OH^-]/dt
• C. rate = -d[ClO2^-]/dt
• D. rate = (1/2) d[ClO2]/dt

Answer: (A)

Verified step by step guidance

1. Identify the species of interest in the reaction, which is ClO3^- (chlorate ion), and note that the question asks for the rate at which it is appearing (being formed).
2. Recall that the rate of appearance of a product is expressed as the change in its concentration over time, specifically the derivative of its concentration with respect to time: \(\frac{d[\text{ClO}_3^-]}{dt}\).
3. Understand that reactants decrease in concentration over time, so their rates are expressed with a negative sign, for example, \(-\frac{d[\text{OH}^-]}{dt}\) or \(-\frac{d[\text{ClO}_2]}{dt}\), to represent their consumption.
4. Use the stoichiometric coefficients from the balanced chemical equation to relate the rates of change of different species. For example, since 2 moles of ClO2 react to form 1 mole of ClO3^-, the rate of formation of ClO3^- is half the rate of consumption of ClO2, expressed as \(\frac{1}{2} \frac{d[\text{ClO}_2]}{dt}\) (with a negative sign if considering consumption).
5. Conclude that the rate of appearance of ClO3^- is best represented by \(\frac{d[\text{ClO}_3^-]}{dt}\), because it directly measures the increase in concentration of the product over time.