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Reaction Rates Using Moles | Chemistry for Grade 10 PDF Download

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  • 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 triangleThe 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

Example 1: Calculate the rates of reaction, in mol/s, in the following experiments:

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
The document Reaction Rates Using Moles | Chemistry for Grade 10 is a part of the Grade 10 Course Chemistry for Grade 10.
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