The rate of a chemical reaction can be calculated by either
  • looking at the rate of appearance of a product over a change in time or
  • the rate of disappearance of the reactant over a change in time
light bulb
Key
Idea
Rates of reactions will be different depending upon which reactant or product is used to measure them
Examine the following chemical reaction. Note how the # of moles decomposed and formed are in the same ratio as the balance in the equation.

2 HI(g)
H2 (g)
+
I2(g)
When 2 moles decomposes then
reaction forms 1 mole of I2
When 1000 molecules decompose then
reaction forms 500 molecules
When HI decomposes at 2.00 mol*l -1*sec-1
reaction forms I2 at 1.00 mol*L-1*sec-1
If the rate of HI decomposition is 4.6 mol*L-1*s-1
then the rate of I2 formation is 2.3 mol*L-1*s-1
If the rate is HI decomposition is 8.4 mol*L-1*s-1
Rate of I2 formation is ???????

The balances in the equation allow us to determine the relative rate of decomposition of reactants to the formation of products.



Activities on Calculating Relative Rates of Reaction
i) Examples of calculations
eye
Check your understanding
Examine the following reaction.
N2 (g)
+
3H2 (g)
2 NH3 (g)
If the rate of decomposition of N2 (g) = 0.03 mol*L-1*s-1, what is the rate of formation of NH3 (g)?
click the eye to view the answer

ii) Complete the following assignment using this concept.