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
Worked example
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.
Calculate the rates of reaction, in mol/s.
Answer:
- 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
- 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
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.
Calculate the rates of reaction, in mol/s.
Answer:
- 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
