What is meant by reaction rate?

Consider the reaction:   2N₂O_5(g) -----> 4NO_2(g) + O_2(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)

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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 N₂O_5(g))

this indicates that the rate of the reaction for 2N₂O_5(g) -----> 4NO_2(g) + O_2(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. N₂O_5(g)

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

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Main Page on Kinetics | Reaction Order | Activation Energy | Catalysts | Temperature Effects | Reaction Mechanisms

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

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