Finding Average Rate of Reaction from measurable quantities

  1. During a chemical reaction, two things happen
    1. The quantities of reactants reduce.
    2. The quantities of products increase.
  2. Therefore, the rate of the reaction can be determined by
    1. measuring the decrease of the amount of the reactants over time.
    2. measuring the increase of the amount of the products over time.
  3. However, the quantity (or change) of the reactants/product may be measurable or immeasurable.
  4. The easily measured quantity changes include
    1. Mass
    2. Concentration (Conductivity)
    3. Volume of gas

Example
Reaction between calcium carbonate and hydrochloric acid.
CaCO3 + 2HCl → CaCl2 + CO2 + H2O
The rate of the reaction can be determined by
a. Measuring the reduces of the mass of the calcium carbonate (reactants) over time
Rate of Reaction =  Mass of CaCO 3  reacted Time taken
b. Measuring the increases of the volume of the carbon dioxide gas produced over time.
Rate of Reaction =  Volume of CO 2  produced Time taken
Example:
In a reaction, 5 g of calcium carbonate takes 250 seconds to completely react with solution of hydrochloric acid. Calculate the average rate for this reaction in units
(a) g s-1 and
(b) mol s-1
[ Relative atomic mass: C 12; O, 16; Ca, 40]

Answer:
a.
Rate of Reaction = 5g 250s =0.02g s -1
b. Relative Formula Mass of calcium carbonate (CaCO 3 ) =40+12+3(16)=100 Number of mole of CaCO 3 = 5g 100gmo l -1 =0.05mol Rate of Reaction = 0.05mol 250s =0.0002mol s -1
We shall discuss the immeasurable quantity (or changes) in next section.