Rate is a measure of change over time. The rate of reaction for a chemical reaction occurring in a closed system is defined as shown below in terms of changes in concentrations of reactants and products over a time interval.
For aA + bB
cC + dD
A, B, C and D are substances.
a, b, c and d are coefficients.
dt is change in time (tfinal - tinitial)
dt is always positive.
** d[A] is change in concentration of A ([A]final - [A]initial)
d[A] negative for a reactant.
As reaction rates must always be positive, rate is calculated using the negative of a change in concentration of a reactant.
What is the unit of rate?
For aA + bB
cC + dDA, B, C and D are substances.
a, b, c and d are coefficients.
| reaction rate | = – | 1 | d[A] | = – | 1 | d[B] | = | 1 | d[C] | = | 1 | d[D] |
| a | dt | b | dt | c | dt | d | dt | |||||
| always positive | use negative sign for reactants** | use positive sign for reactants | ||||||||||
dt is change in time (tfinal - tinitial)
dt is always positive.
** d[A] is change in concentration of A ([A]final - [A]initial)
d[A] negative for a reactant.
As reaction rates must always be positive, rate is calculated using the negative of a change in concentration of a reactant.
Note that in calculating the rate of reaction, the change in concentration of any reactant or product is divided by its coefficient just as it is necessary to divide the amount of product consumed or reacted by its coefficient to calculate the amount in moles of reaction.
How is reaction rate related to the rate of appearance of products and disappearance of reactants? The rates of appearance and disappearance of various reactants and products depends on their coefficient in the balanced equation and can be calculated by multiplying the reaction rate by this coefficient.
This is analogous to calculating the amount of reactants or products consumed for a given amount in moles of reaction by multiplying the amount in moles of reaction by the coefficient associated with that reactant or product.
What is the unit of rate?
The unit associated with reaction rate is concentration per unit time.
If concentration is measured in and time in seconds, rate has the unit s–1.
If concentration is measured in and time in seconds, rate has the unit s–1.