I am sure you are familiar with the processes involved in making and collecting gases in the laboratory. The two most common ones we encounter are carbon dioxide and hydrogen.
You will know that carbon dioxide is produced when carbon combusts and when carbonates are heated, but we tend to use the action of an acid on a carbonate as a more controllable method in the laboratory. The rate of CO2 production can be varied by changing either of the two reagents; normally marble chips and hydrochloric acid. The carbon dioxide can be collected like this because it is heavier than air. If you need to know the volume of gas produced you should use a gas syringe. It is not collected over water because carbon dioxide is quite soluble in water.
Word Equation…
You could speed up the rate of the reaction by…
- decreasing the size of the marble chips
- increasing the concentration of the hydrochloric acid
- increasing the temperature
Perhaps you could suggest in the comments below some ways to decrease the speed of the reaction?
Hydrogen is usually produced by reacting a metal with an acid such as sulphuric acid. Several metals could be used but zinc has the right level of reactivity to produce a steady stream of hydrogen bubbles.
Word Equation…
The gas is collected over water because it is lighter than air and insoluble. The reaction can be speeded up in a number of ways…
- decreasing the particle size of the zinc
- using a more reactive metal such as magnesium or calcium
- increasing the concentration of the acid
- increasing the temperature
- adding a catalyst such as copper sulphate
Questions…
- Why is carbon dioxide not usually collected over water?
- What is the chemical name for marble?
- Why would a Group 1 metal such as sodium or potassium be a bad choice for the laboratory preparation of hydrogen?
- What is the chemical test for hydrogen?
- How could you make a sample of pure oxygen in the laboratory?
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