Rate

Not all chemical reactions go to completion. Sometimes, as the products accumulate the products being to react and remake the reactants. We will talk about equilibrium reactions later. To simplify our discussion of kinetics we will be examining reactions where the reverse reaction does not occur, or we will be examining reactions before the products have a chance to accumulate; that is, early in the reaction. Making this simplification lets us describe reaction rates based on reactant consumption or product production based on the concentrations of the reactants. If we did not make this simplification the rate would be a function of the concentrations of the reactants and the products.

Rate: the change in concentration of the molecules per unit time.

When we talk about rate and cars, we talk about the distance the car covers per unit time. A chemical reaction starts with a certain concentration of material and is done when that material is gone, so rate is change in conc w/r/t time (speed is change in distance w/r/t time)

Lets look at the following reaction...

When talking about the rate of this reaction we would describe it with the following expressions

the change in concentration of NO2 w/r/t time. In order to keep the rate positive a negative sign is added since the concentration of NO2 is decreasing.

the change in concentration of NO w/r/t time.
NO is increasing

the change in concentration of O2 w/r/t time. No negative sign because the concentration of O2 is increasing

Are all these rates equal? no not really...look at the stoichiometry.

For every 2 moles of NO2 consumed 1 mole of O2 is produced. That means if NO2 is being consumed at a rate = Ratea then O2 is beging produced at half that rate;

 or

The rate of production of O2 is 1/2 the rate
consumption of NO2.

From the stoichiometry of the reaction it also follows that

 

So, two things here...

The rate will always be positive: that is why the negative sign is needed because reactants are being consumed and products are being produced
a positive sign proceeds the concentration for rate of product formation

a negative sign proceeds the concentration for rate of reactant consumption

The word rate must be used carefully; the actual rates for the consumption and production of reactants and products will be related through stoichiometry. So, we must be careful to say things like, the rate of production of O2 is, or the rate of consumption of NO2 is.

continue