Making hydrogen: metal and acid
Drop zinc, magnesium, iron or copper into dilute hydrochloric acid and collect the gas over water. Change the amounts and find out which runs out first, then test the gas with a burning splint.
Core concept
Metals above hydrogen in the reactivity series react with dilute acids to give hydrogen: Zn + 2HCl → ZnCl₂ + H₂; the amount of gas is set by whichever reactant runs out first, and hydrogen burns with a squeaky pop
Topic: Some gases
In the school laboratory, hydrogen is made by adding granules of zinc to dilute hydrochloric acid: A substance that tastes sour, like lemon juice or vinegar. It turns blue litmus red. All key words: Zn + 2HCl → ZnCl₂ + H₂. The gas goes through a delivery tube into a gas jar full of water, upside down in a trough, and pushes the water out (collection by the downward displacement of water). Hydrogen is collected this way because it hardly dissolve: To mix into a liquid so completely that you can no longer see the pieces. All key words in water. A metal must be more reactive than hydrogen to push it out of the acid: magnesium reacts fast, zinc steadily, iron slowly, and copper not at all. One mole of metal gives one mole of hydrogen, about 24000 mL at room temperature: How hot or cold something is, measured in degrees Celsius (°C). All key words, but it needs two moles of acid. When one of them is used up, the reaction stops.
What you will learn
- Describe the laboratory preparation and collection of hydrogen.
- Compare how metals in the reactivity series react with dilute acid.
- Explain which reactant limits the amount of gas made.
- Describe the test for hydrogen and some of its uses.
Before you start
- Know the order of common metals in the reactivity series.
Materials
- Zinc granules, magnesium ribbon, iron filings, copper turnings, dilute hydrochloric acid (1 mol/L)
Equipment
- A conical flask with a thistle funnel and delivery tube, a trough, a gas jar and a beehive shelf
Procedure
- Add 1 g of zinc to 40 mL of acid and record the volume of hydrogen.
- Try magnesium, iron and copper, and record each.
- Use 2 g of zinc and record.
- Use 2 g of zinc with 60 mL of acid and record.
How this simulation is simplified
- The acid is 1 mol/L hydrochloric acid; 2 moles of it give 1 mole of hydrogen.
- 1 mole of hydrogen is 24000 mL at room temperature and pressure.
- The times are rough, for small pieces of each metal.