Which substance does rate in a rate equation refer to?

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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
 

Answers and Replies

BvU
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What is the unit of rate ?

[edit] never mind, the definition (you did look it up :rolleyes: ?) is clear
 
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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
Science Advisor
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Rate of reaction = 1/a*d[A]/dt where A is any reagent; a is the stoichiometric coefficient of A in the equation, and has a positive sign for products, negative for reactants.
 

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