Reaction Rates and Stoichiometry

Class Opener For the general reaction, A  B write 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) ----  H 2 (g) + I 2 (g) 2 moles of HI disappear for each mole of H 2 or I 2 that forms, so the rate of disappearance of HI is twice the rate of appearance of H 2 or I 2.

2HI(g)  H 2 (g) + I 2 (g) Rate = − 1212  [HI]  t =  [I 2 ]  t Rate = − 1212  [HI]  t =  [H 2 ]  t

To generalize, then, for the reaction aA + bBcC + dD Rate = − 1a1a  [A]  t = −= − 1b1b  [B]  t = 1c1c  [C]  t 1d1d  [D]  t =

Example 1.How is the rate at which ozone,O 3,disappears related to the rate at which oxygen appears in the reaction 2O 3 (g)  3O 2 (g) Rate = -1 /\ [O 3 ] = 1 /\ [O 2 ] 2 /\ t 3 /\ t -/\ [O 3 ] = 2 /\ [O 2 ] /\ t 3 /\ t

2. If the rate at which O 2 appears is 6.0 x M/s at a particular instant, at what rate is O 3 disappearing at this time? -/\ [O 3 ] =2 /\ [O 2 ]=2(6.0 x M/s)=4.0x10 -5 M/s /\ t 3 /\ t 3

Study Check The decomposition of N 2 O 5 proceeds according to the following equation: 2N 2 O 5 (g)  4NO 2 (g) + O 2 (g) If the rate of appearance of NO 2 at a particular instant in a reaction vessel is 8.4 x10 -7 M/s, what is the rate of disappearance of N 2 O 5 ?