Notes on Kinetics
Reaction Rates

What is meant by reaction rate?

Consider the reaction:   2N2O5(g) -----> 4NO2(g) + O2(g)

The rate is defined as the change in concentration of N2O5 with respect to time or

rate = -DConc N2O5 / Dt

where DConc N2O5 = final conc N2O5 - orig. conc. N2O5, and

Dt = final time - initial time

Reaction rate is always positive

Concentation is expressed in mol/L, whereas time can be in seconds, minutes, hours, days, etc.

Again, the rate of this reaction can be expressed as: rate = d(conc. N2O5) / dt

The instantaneous rate = - (slope of the tangent to conc. vs. time curve)

How are Reaction Rate and Concentration Related?

The rate decreases as the concentration of the starting materials (reactants) decreases.

plot of rate vs conc - if straight line, then it is said to be a first order reaction: in other words the rate of the reacton is directly proportional to the concentration.

What is the Rate Expression?

rate = k(conc N2O5(g))

this indicates that the rate of the reaction for 2N2O5(g) -----> 4NO2(g) + O2(g) depends upon the concentration of reactant

"k" is called the rate constant. It is independent of the other quantities in the equation.

It is dependent on:

1. the nature of the reaction
"fast" reactions typically have large rate constants
if k is small, the reaction is slow at ordinary concentration
2. the temperature
ordinarily, k increases with temperature

k = rate / conc. N2O5(g)

example: k = .056 mol/L.min / .160 mol/L = 0.35/min

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 Send questions, comments or suggestions to Gwen Sibert, at the Roanoke Valley Governor's School gsibert@rvgs.k12.va.us Back to Notes Menu