Reaction Rates andStoichiometry

Class Opener

For the general reaction,A Bwrite an equation relating the average rate of the appearance of B with the average rate of the disappearance of A

Objective: To determine how to calculate reaction rates when the stoichiometric relationships in a chemical reaction are not one to one

How do we calculate reaction rates when the stoichiometric relationships in a chemical reaction are not one to one? For example, consider this reaction:2HI(g) ---- H2(g) + I2(g)

2 moles of HI disappear for each mole of H2 or I2 that forms, so the rate of disappearance of HI is twice the rate of appearance of H2 or I2 .

2HI(g) H2(g) + I2(g)

Rate = −12

[HI]t =

[I2]t

Rate = − 12

[HI]t =

[H2]t

• To generalize, then, for the reaction

aA + bB cC + dD

Rate = −1a

[A]t = −

1b

[B]t =

1c

[C]t

1d

[D]t=

Example

1.How is the rate at which ozone ,O3,disappears related to the rate at which oxygen appears in the reaction 2O3(g)3O2(g)

Rate = -1 /\ [O3] = 1 /\ [O2]

2 /\ t 3 /\ t-/\ [O3] = 2 /\ [O2]

/\ t 3 /\ t

2. If the rate at which O2 appears is 6.0 x 10-5 M/s at a particular instant, at what rate is O3 disappearing at this time?

-/\ [O3] =2 /\ [O2]=2(6.0 x 10-5M/s)=4.0x10-5M/s

/\ t 3 /\ t 3

Study Check

The decomposition of N2O5 proceeds according to the following equation:2N2O5(g) 4NO2(g) + O2(g)

If the rate of appearance of NO2 at a particular instant in a reaction vessel is 8.4 x10-7 M/s, what is the rate of disappearance of N2O5?