Rates and calculating reaction rates
Calculating reaction rates from change in concentration over time
The reaction rate is equal to the amount of reaction during a specified time interval.
The amount of reaction can be calculated from the change in concentration of a product or reactant during that time interval by dividing that change by the coefficient of the substance in the balanced equation for the reaction.
If the concentration of a reactant or product is known at two times (from a graph, for example). The rate of reaction can be calculated by dividing the difference in concentration by the time interval.
The change (Δ) in the relationship above is equal to the final minus initial
Therefore for the graph at the right, because a mass is plotted.
A negative sign is used iin calculating the rate of reaction from the change in concentration of a reactant because the rate of reaction is always positive, and the concentration of a reactant will be smaller at t2.
What is the unit of rate?
The reaction rate is equal to the amount of reaction during a specified time interval.
The amount of reaction can be calculated from the change in concentration of a product or reactant during that time interval by dividing that change by the coefficient of the substance in the balanced equation for the reaction.
For aA + bB 
cC + dD
A, B, C and D are substances.
a, b, c and d are coefficients.
cC + dDA, B, C and D are substances.
a, b, c and d are coefficients.
| reaction rate = | –Δ[A] | = | –Δ[B] | = | Δ[C] | = | Δ[D] |
| a × Δt | b × Δt | c × Δt | d× Δt |
If the concentration of a reactant or product is known at two times (from a graph, for example). The rate of reaction can be calculated by dividing the difference in concentration by the time interval.
graph
The change (Δ) in the relationship above is equal to the final minus initial
Therefore for the graph at the right, because a mass is plotted.
ΔA = m2(A) - m1(A)
Δt = t2 - t1
Δt = t2 - t1
A negative sign is used iin calculating the rate of reaction from the change in concentration of a reactant because the rate of reaction is always positive, and the concentration of a reactant will be smaller at t2.
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.