Reaction Rates

In my previous post I posted the factors affecting reaction rates, it means you know already how to increase or decrease the rate of chemical reaction if in case you are the one manipulating the reaction.  In this post you will learn how to write the expression for the rate of disappearance of reactants and rate of appearance of products and to calculate the rate of reaction using both the disappearance of reactants and appearance of products.

Reaction rates is defined as the change in the concentration of  reactants or products per unit time. It units is molarity per second (M/s).  molarity is for the change in the concentration and seconds is for the change in time.

The rate of reaction can be expressed either in the rate of disappearance of the reactants or the rate of appearance of the product.  Let us have an example the simple reaction A →  B, where A is the reactant and B the product.  Rate can be calculated either

Let us see have an example:

a. Calculate the rate of appearance of B over the time interval from 0 to 20s.
b. Calculate the rate of disappearance of A overt the time interval from 20 to 40s.

Solution:
'

a.

 

b.  

What if the stoichiometric relationship of the reaction is not one is to one ratio?  Like for example

2HI(g)  →  H₂(g)  +   I₂(g)

How do we calculate the rate of reaction?  As you can see there are two moles of HI in every 1 mole of H₂ and I₂, therefore the rate of disappearance of HI is twice the rate of appearance of either H₂ and I₂.  To equate  the rates, we must therefore divide the rate of disappearance of HI by 2 as shown below:

In general, for the reaction

aA  +  bB  →  cC  +  dD

the rate is given by

The coefficient of the reactants and products is the denominator in the fraction for each reactant and product.

Sample Exercise:

a) How is the rate of disappearance of ozone related to the rate of appearance of oxygen in the following equation:  2O₃(g)  → 3O₂(g) ?  b) If the rate of appearance of O₂, ∆[O₂]/∆t, is 6.0 x 10^-5 M/s at a particular instant, what is the value of the rate of disappearance of O₃, -∆[O₃]/∆t, at this same time?

Solution:

a) Using the balance equation the rate is 

b)  Solving the equation from a:

TRY THIS:

1.  The decomposition of N₂O₅ proceeds according to the following equation:

2N₂O₅(g)  →  4NO₂(g)  +  O₂(g)

If the rate of decomposition of N₂O₅ at a particular instant in a reaction vessel is 4.2 x 10^-7 M/s, what is the rate of appearance of a) NO₂;  b) O₂?

2. Consider the reaction,     4NO₂(g) +  O₂(g)   →  2N₂O₅(g)

Suppose that, at a particular moment during the reaction, molecular oxygen is reacting at the rate of 0.024 M/s. (a) At what rate is N₂O₅ being formed? (b) At what rate is NO₂ reacting?