Reaction Rates Using Moles | Chemistry for Grade 10 PDF Download

Higher Tier Only

• We have seen that the rate of reaction is a measure of a changing property per unit time
• This will frequently be expressed in units of g/s or cm3/s for a change in mass or volume, respectively
• However, it is often more useful to be able to express rate of reaction in terms of moles, in which case the unit will be mol/s
• There is no direct way to measure moles, so first it is necessary to calculate the rate using the rate formula triangle:

The rate formula triangle

• After calculating the rate of reaction it can then be converted into mol/s
• If you are working in mass per unit time then the process is:
  mass per unit time ÷ molar mass of the substance = moles per unit time
• If you are working in volume per unit time then you have to use the molar gas volume
  volume per unit time ÷ molar gas volume (24 000 cm³/mol ) = moles per unit time

Solved Examples

20.0 g of marble chips were added to a flask containing 100 cm³ of 2.0 mol/dm³ hydrochloric acid. The flask was placed on a balance and the mass was measured at the start of the reaction. After 2.5 minutes the mass was found to have decreased by 6.0 g.

• Convert the time into seconds
  2.5 x 60 = 150 s
• The rate of reaction is
  6.0 g ÷ 150 s = 0.04 g/s
• The loss in mass is due the carbon dioxide gas released
• The molar mass of carbon dioxide, CO₂, is 44.0 g/mol
• Convert to moles
  0.04 g s-1 ÷ 44.0 g/mol = 9.1 x 10^-4 mol/s

Example 2: Calculate the rates of reaction, in mol/s, in the following experiments:
A piece of magnesium ribbon, 2 cm in length, was added to a flask containing 50.0 cm³ of 1.50 mol/dm³ sulfuric acid. The flask was connected to a gas syringe and the volume of gas measured every minute. 15 cm³ of gas had been collected after 6 minutes.

• Convert the time into seconds
  6.0 x 60 = 360 s
• The rate of reaction is
  15 cm³  ÷ 360 s = 0.0417 cm³/s
• Convert to moles
  0.0417 cm³/s ÷ 24 000 cm³/mol = 1.7 x 10^-6 mol/s