# 6.1.2 Reaction Rates Using Moles

Higher Tier Only

### Reaction Rates Using Moles

• 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-1 or cm3 s-1 for a change in mass or volume, respectively
• However, it is often more useful tp be able to express rate of reaction in terms of moles, in which case the unit will be mol s-1
• 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-1
• 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 cmmol-1 ) = moles per unit time

#### Worked Example

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

1. 20.0 g of marble chips were added to a flask containing 100 cm3 of 2.0 mol dm-3 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.
2. A piece of magnesium ribbon, 2cm in length, was added to a flask containing 50.0 cm3 of 1.50 mol dm-3 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.

Example 1

• 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-1

• The loss in mass is due the carbon dioxide gas released
• The molar mass of carbon dioxide, CO2, is 44.0 g mol-1
• Convert to moles

0.04 g s-1 ÷ 44.0 g mol-1 = 9.1 x 10-4 mol s-1

Example 2

• Convert the time into seconds

6.0 x 60 = 360 s

• The rate of reaction is

15 cm3  ÷ 360 s = 0.0416 cm3 s-1

• Convert to moles

0.0417 cm3 s-1 ÷ 24 000 cm3 mol-1 = 1.7 x 10-6 mol s-1 ### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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