Reaction Rates Using Moles
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
Worked example
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.
- 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.
Answer 1:
- Step 1: Convert the time into seconds:
- 2.5 x 60 = 150 s
- Step 2: Convert the mass lost to moles:
- The mass loss is due to carbon dioxide (Mr = 44.0) being released
- Moles = mass / Mr
- Moles = 6 / 44.0
- Moles = 0.137
- Step 3: Calculate the rate of reaction in mol / s:
- Rate of reaction = moles / time in seconds
- Rate of reaction = 0.137 / 150
- Rate of reaction = 9.1 x 10-4 mol / s
Answer 2:
- Step 1: Convert the time into seconds:
- 6.0 x 60 = 360 s
- Step 2: Convert the volume of gas to moles:
- Moles = volume (cm3) / molar gas volume (24 000 cm3)
- Moles = 15 / 24 000
- Moles = 6.25 x 10-4
- Step 3: Calculate the rate of reaction in mol / s:
- Rate of reaction = moles / time in seconds
- Rate of reaction = 6.25 x 10-4 / 360
- Rate of reaction = 1.7 x 10-6 mol / s
