# Which substance does rate in a rate equation refer to?

## Main Question or Discussion Point

For instance, consider the reaction X + 2Y --> Z.

Suppose the reaction is first order with respect to both X and Y. We can write that "rate" = k[X][Y].

However, we could measure the rate of reaction as the (negative of the) rate of change of [X] OR that of [Y], however the rate of change of [Y] is double that of [X] due to the reaction stoichiometry.

So what does this notion of rate refer to? It must have a fixed definition since from the rate and concentrations we determine k, which is part of the Arrhenius equation.

Thank you

BvU
Homework Helper
What is the unit of rate ?

 never mind, the definition (you did look it up ?) is clear

Chestermiller
Mentor
For instance, consider the reaction X + 2Y --> Z.

Suppose the reaction is first order with respect to both X and Y. We can write that "rate" = k[X][Y].

However, we could measure the rate of reaction as the (negative of the) rate of change of [X] OR that of [Y], however the rate of change of [Y] is double that of [X] due to the reaction stoichiometry.

So what does this notion of rate refer to? It must have a fixed definition since from the rate and concentrations we determine k, which is part of the Arrhenius equation.

Thank you
It would be X. Y would disappear twice as fast.

mjc123