Which expression correctly quantifies a reaction rate?

Which expression correctly quantifies a reaction rate?

• A. The velocity of the reaction
• B. The amount of energy released per time
• C. The total change in concentration of all species per unit time
• D. The amount of reactant or product appearing or disappearing per unit time

Answer: (D)

The key idea is that a reaction rate measures how fast the concentrations of species change as the reaction proceeds. It is defined as the change in concentration of a reactant or product per unit time, with the rate for the reactants often taken as a positive quantity when the reactant is disappearing and equal to the rate of appearance of the products when normalized by their stoichiometric coefficients. In practice, for a reaction aA → bB, the rate can be written as -d[A]/dt divided by a, which is the same as d[B]/dt divided by b. This means the rate reflects how much reactant is consumed or product is formed per unit time, typically expressed in mol/L per second. The other ideas don’t fit as the definition of reaction rate: energy released per time describes power, not how quickly concentrations change; and summing the total change in concentration of all species per unit time ignores the fact that some species are formed while others are consumed and requires normalization by stoichiometry to be meaningful as a single rate.

The key idea is that a reaction rate measures how fast the concentrations of species change as the reaction proceeds. It is defined as the change in concentration of a reactant or product per unit time, with the rate for the reactants often taken as a positive quantity when the reactant is disappearing and equal to the rate of appearance of the products when normalized by their stoichiometric coefficients. In practice, for a reaction aA → bB, the rate can be written as -d[A]/dt divided by a, which is the same as d[B]/dt divided by b. This means the rate reflects how much reactant is consumed or product is formed per unit time, typically expressed in mol/L per second.

The other ideas don’t fit as the definition of reaction rate: energy released per time describes power, not how quickly concentrations change; and summing the total change in concentration of all species per unit time ignores the fact that some species are formed while others are consumed and requires normalization by stoichiometry to be meaningful as a single rate.