Making Gas...

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 reageants; normally marble chips and hydrochloric acid.

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.

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…

1. Why is carbon dioixde not collected over water?
2. What is the chemical name for marble?
3. Why would a Group 1 metal such as sodium or potassium be a bad choice for the laboratory preparation of hydrogen?