Measuring the rate of a reaction
How fast is a reaction?
The rate of a reaction tells you how quickly reactants turn into products.
A rusting bike takes months; a firework takes a fraction of a second.
You can measure how much of a reactant is used up, or how much of a product is made, over a period of time:
mean rate of reaction = quantity of reactant used ÷ time taken
mean rate of reaction = quantity of product formed ÷ time taken
Recall both: chemistry papers give no equation sheet. The quantity can be a mass in g, a volume of gas in cm³ or (Higher) an amount in mol, so the rate is in g/s, cm³/s or mol/s.
- A flask loses 1.50 g of carbon dioxide in 120 s
- mean rate = 1.50 ÷ 120 = 0.0125 g/s
- A gas syringe collects 45 cm³ of hydrogen in 30 s
- mean rate = 45 ÷ 30 = 1.5 cm³/s
Three ways to follow a reaction
- Loss of mass. Put the flask on a balance. A gas escapes through a loose cotton wool plug (which stops acid spray getting out), so the mass falls.
- Volume of gas. Collect the gas in a gas syringe, or in an upturned measuring cylinder full of water, and read the volume at regular times.
- A colour change or cloudiness. Time how long it takes for a mark under the flask to disappear as a precipitate forms.
A mean rate uses the change between two readings. If a table says 18 cm³ at 10 s and 42 cm³ at 40 s, the mean rate between them is (42 − 18) ÷ (40 − 10) = 0.8 cm³/s, not 42 ÷ 40.
Higher: rate in mol/s
To give a rate in mol/s, turn the quantity into moles first.
One mole of any gas takes up 24 dm³ (24 000 cm³) at room temperature and pressure; the paper tells you this when you need it.
- 60 cm³ of hydrogen is made in 50 s
- moles = 60 ÷ 24 000 = 0.0025 mol
- mean rate = 0.0025 ÷ 50 = 5.0 × 10⁻⁵ mol/s
Small rates are often easiest to write in standard form.
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