6.1 - Rate of Reaction & Measuring It

Rate of reaction

• The rate of a reaction is how quickly it happens.
• Reactions can be very slow (e.g. iron rusting, over months or years), fast (e.g. burning), or over in a fraction of a second (e.g. a firework exploding).
• A rate can be followed by measuring either how fast a reactant is used up or how fast a product is formed - either can be used.

Calculating average rate

Average rate of reaction = quantity of reactant used (or product formed) ÷ time taken

The unit of rate depends on how the quantity is measured: g/s for a mass in grams, cm³/s for a volume of gas in cm³, and mol/s for an amount in moles.

Rate graphs

• A rate graph shows how fast a reaction is going at any particular time.
• The steeper the line, the faster the reaction at that point.

The line changes shape as the reaction runs

1. Steepest at the start, when there's most reactant and the reaction is going fastest.
2. Less steep as the reactants get used up and the reaction slows down.
3. Flat once the reactants are used up and the reaction has stopped.

Measuring a rate of reaction

1. Measuring the volume of gas given off

• Used for reactions that produce a gas by measuring the volume collected at regular time intervals.
• A gas syringe measures the volume directly as the gas pushes the plunger out.
• The gas can also be collected over water in an upturned measuring cylinder.
• Typical example: marble chips + dilute hydrochloric acid, collecting the carbon dioxide gas produced.

2. Measuring the loss in mass

• Used when a produced gas escapes into the room rather than being collected.
• As gas escapes, the flask and its contents lose mass, so the balance reading falls over time.
• A loose plug of cotton wool in the neck lets gas escape while stopping liquid spitting out.

3. Following a visible change

• Used for reactions that produce a precipitate or colour change in solution.
• Standard example: sodium thiosulfate + dilute hydrochloric acid forms a cloudy yellow precipitate of sulfur.
• Time how long a cross marked on paper under the flask takes to disappear through the cloudy mixture - the shorter the time, the faster the reaction.
• For a reaction that changes colour, time how long the colour change takes.
• Either change can be judged by eye, or timed with apparatus:
  • A colorimeter for a colour change.
  • A light sensor connected to a data logger for the cloudiness of a precipitate.

Calculating rate from time

• Whichever method is used, timing begins the moment the reactants are mixed.
• Gas volume and mass loss give continuous measurements to plot against time.
• When timing a single end point (like the disappearing cross), the rate is proportional to 1 ÷ time (in s^-1): the shorter the time, the faster the reaction.