Rate is a measure of how fast or slow something is. In chemistry, we speak of a rate of reaction, this tells us how fast or slow a reaction is.
Why do chemists want to know the rate of a reaction?
If you are making a product, it is important to know how long the reaction takes to complete, before the product is produced.
Rate is a measure of a change that happens over a single unit time. That unit time is most often a second, a minute, or an hour.
Reaction between zinc and dilute hydrochloric acid
What we observe over time is that gradually the zinc disappears and bubbles of gas appear. After a few minutes the bubbles of gas form less and less quickly until finally no bubbles appear because all the acid has been used up, some zinc remains.
To summarise, during this reaction zinc chloride and hydrogen gas are been formed at the same time as zinc and hydrochloric acid react.
Using the reaction between zinc and hydrochloric acid as an example, the following are methods by which you could measure the rate of that reaction.
1. Measure that amount of zinc used up per minute
2. Measure the amount of hydrochloric acid used up per minute
3. Measure the amount of zinc chloride been formed per minute
4. Measure the amount of hydrogen been produced per minute
When choosing which method to measure rate always choose the most straightforward.
In the example above, by far the easiest would be to collect the bubbles of hydrogen and measure its volume.
Methods Used for Measuring Rate
Measuring volume of gas evolved:
To measure the hydrogen gas released in the above reaction we use the apparatus as shown. As the bubbles of gas are given off, the plunger in the syringe moves out as hydrogen gas fills it. After, say every 20 seconds we read the volume of gas in the syringe. The reaction is complete when the syringe no longer moves.
To find the actual rate we plot a graph of volume of hydrogen (cm3) against time (seconds).
1. The rate is not a constant throughout the reaction - it changes!
2. The reaction is fastest at the start, gradually becoming slower as the reaction proceeds.
3. From the graph, the fastest part of the reaction is shown by the steepest curve.
4. The curve on the graph goes flat when the reaction is complete. This is because, as time goes on the volume of the gas evolved does not change.
Measuring the Rate of Loss of a Gaseous Product:
In the reaction between calcium carbonate (marble chips) and hydrochloric acid we can use the apparatus below to find the rate of reaction.
Marble chips and acid are placed in the flask but separated by a piece of card - preventing the reaction from proceeding. This apparatus is placed on a balance and the mass of the flask and its contents is read.
To start the reaction, the flask is gently lent to one side, causing the card to fall and the marble chips and acid to mix.
A piece of cotton wool is placed in the neck of the flask to allow carbon dioxide gas to escape. As the gas escapes the mass of the flask reduces. Take readings of mass loss over a time interval, e.g. 30 seconds.
To find the actual rate we plot the loss in mass (grams) against time (seconds)
As with the previous experiment, the steepest part of the curve is at the start, hence the fastest part of the reaction is at the start.
Gradually the curve becomes less and less steep as the reaction slows down. Eventually a flat curve appears indicating the end of the reaction.