The lightest element: where it occurs, how it is made in the lab, and what it does.
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Hydrogen is the lightest element (atomic number 1). It is made in the laboratory by the action of dilute sulphuric or hydrochloric acid on granulated zinc: Zn + H₂SO₄ → ZnSO₄ + H₂↑. A few drops of copper sulphate can speed it up. Concentrated or nitric acid is not used because they give other products. Hydrogen is almost insoluble in water, so it is collected by downward displacement of water. Industrially it is obtained from steam and heated iron or from the electrolysis of acidified water.
What mass of hydrogen forms from 6.5 g of zinc (Zn = 65, H = 1)?
65 g Zn gives 2 g H₂, so 6.5 g gives 6.5 × 2 ÷ 65 = 0.2 g.
What volume of hydrogen at STP is that? (1 mole of gas = 22.4 L)
0.2 g is 0.1 mole, so 0.1 × 22.4 = 2.24 L.
Why is hydrogen not collected by upward displacement of air?
It is lighter than air and would escape; and it is nearly insoluble in water so water collection works.
Hydrogen is colourless, odourless, tasteless and neutral to litmus, and it is the lightest gas. It burns in air with a pale blue flame to give water, and a mixture with air explodes, so a test tube of it gives a pop on a lighted splint. It does not support combustion. It is a good reducing agent: it removes oxygen from heated metal oxides (CuO + H₂ → Cu + H₂O). With steam, red-hot iron gives Fe₃O₄ and hydrogen.
How much oxygen is needed to burn 4 g of hydrogen, and how much water forms?
2H₂ + O₂ → 2H₂O: 4 g H₂ needs 32 g O₂ and gives 36 g water.
What is observed when hydrogen is passed over heated copper oxide?
Black CuO turns reddish-brown copper and droplets of water form.
Give the test for hydrogen.
A lighted splint brought to the gas burns it with a pop.
Hydrogen is used to make ammonia (Haber process) and hydrochloric acid, to make vanaspati ghee by hydrogenating vegetable oils, as rocket fuel and in the oxy-hydrogen flame for cutting and welding metals, and in fuel cells. Its low density once made it popular for balloons, but it is flammable, so helium is now used instead.
Why is helium used in balloons instead of hydrogen?
Hydrogen is highly flammable; helium is inert and safe, though slightly heavier.
What is hydrogenation?
Adding hydrogen to unsaturated vegetable oils with a nickel catalyst to turn them into solid fats.
Name two uses of hydrogen as a fuel.
Rocket fuel and fuel cells (also in the oxy-hydrogen flame).
1. Name the gas formed when zinc reacts with dilute H₂SO₄.
2. Why is granulated zinc used?
3. Why is concentrated sulphuric acid not used?
4. Name the method of collecting hydrogen.
5. Write the equation for the burning of hydrogen.
6. Is hydrogen oxidising or reducing?
7. What happens to red-hot iron in steam?
8. State two physical properties of hydrogen.
9. Find the volume of H₂ at STP when 13 g of zinc reacts with excess acid.
10. State two uses of hydrogen.