Measuring Reaction Rates via Observable Changes

Steps:

• Identify the definition of reaction rate as the change in concentration or amount of substance per unit time.
• Evaluate each option against standard methods for monitoring reactions.
• Check if the change directly relates to the rate law or stoichiometry.
• Note incompleteness in option D limits full analysis.

Why A is correct:

• Reaction rate is defined as the negative change in reactant concentration per unit time (-Δ[reactant]/Δt), directly measuring how fast reactants are consumed.

Why the others are wrong:

• B: While product appearance can indicate rate (+Δ[product]/Δt), it requires stoichiometric adjustment and is not the primary definition focused on reactants.
• C: Volume increase measures gas evolution but assumes constant pressure and ideal gas behavior; it indirectly relates to rate via Avogadro's law and is not universal for all reactions.
• D: Not enough information, as the option is incomplete.

Final answer: A