Study of the First Element Hydrogen
ICSE Class 9 · Chemistry · Chapter 6
What this chapter covers
- Position of hydrogen in the periodic table
- Hydrogen from water
- Displacement of hydrogen from dilute acids
- Displacement of hydrogen from alkalis
- Laboratory preparation and collection of hydrogen
- Industrial manufacture of hydrogen: the Bosch process
- Oxidation and reduction
Key points
- Hydrogen resembles alkali metals (one valence electron, forms H+, burns to an oxide, reducing nature) and halogens (one electron short of a stable shell, forms H-, diatomic non-metal gas); its position is unique.
- K and Na react violently and Ca steadily with cold water; Mg reacts with boiling water; Al, Zn and red-hot Fe react with steam; the iron–steam reaction is reversible.
- Steam over red-hot coke gives water gas, CO + H2.
- Mg, Al, Zn and Fe displace hydrogen from dilute HCl and H2SO4; Na, K, Ca are too violent, Pb is coated by an insoluble salt, Cu and below do not react, and dilute nitric acid oxidises the hydrogen.
- Zn, Al and Pb displace hydrogen from hot concentrated NaOH and KOH, forming zincates, meta-aluminates and plumbites; they are amphoteric.
- In the laboratory, granulated zinc and dilute HCl give hydrogen, collected over water; impurities are removed with lead nitrate, potassium hydroxide and silver nitrate solutions, and air must be expelled before collection.
- The Bosch process: water gas from coke and steam at 1000 °C; CO converted by steam over Fe2O3/Cr2O3 at 450 °C; CO2 removed by water under pressure or KOH, and CO by ammoniacal copper(I) chloride.
- Oxidation is gain of oxygen, loss of hydrogen or loss of electrons; reduction is the reverse; they occur together in redox reactions.
- An oxidising agent is itself reduced; a reducing agent is itself oxidised.
Questions answered
Give two ways in which hydrogen resembles the alkali metals and two ways in which it resembles the halogens.
Like alkali metals, hydrogen has one valence electron and forms a positive ion. Like halogens, it needs one electron to complete its shell and forms diatomic molecules.
Why is the position of hydrogen in the periodic table called unique?
It shows properties of both group 1 and group 17 and differs from both, so no single group fits it perfectly.
Describe what you observe when a small piece of sodium is dropped into cold water, and write the equation.
Sodium floats, melts into a silvery ball and darts about with a hissing sound. 2Na + 2H2O → 2NaOH + H2.
Why does the solution turn milky when calcium reacts with water?
Calcium hydroxide formed is only slightly soluble in water.
Write the equation for the action of steam on red-hot iron. Why is it called reversible, and how is a good yield of hydrogen obtained?
3Fe + 4H2O rightleftharpoons Fe3O4 + 4H2. Hydrogen can reduce Fe3O4 back to iron. Steam is passed continuously so that hydrogen is swept away.
What is water gas? Write the equation for its formation.
Water gas is a mixture of carbon monoxide and hydrogen. C + H2O → CO + H2 at 1000 °C.
Why is lead not used to prepare hydrogen from dilute hydrochloric acid?
Lead forms insoluble lead chloride, which coats the metal and stops the reaction.
Why is hydrogen not obtained when zinc reacts with dilute nitric acid?
Nitric acid is a strong oxidising agent; it oxidises the hydrogen formed to water and is itself reduced to oxides of nitrogen.
Past board exam questions
ICSE 2019 sample paper · 5 marks
Give a reason for each of the following: (i) Noble gases do not combine with other elements to form molecules. (ii) Dilute nitric acid cannot be used in the preparation of hydrogen. (iii) Hydrogen although lighter than air, is not collected by the downward displacement of air. (iv) Anhydrous calcium chloride turns into a colourless solution when exposed to air. (v) Metals form positive ions.
Show answer
(i) Noble gases have a stable outermost shell (a complete octet, or a duplet in helium), so they have no tendency to lose, gain or share electrons.
(ii) Nitric acid is a strong oxidising agent even when dilute; it oxidises the hydrogen formed to water (and is itself reduced to oxides of nitrogen), so hydrogen is not obtained.
(iii) Hydrogen forms an explosive mixture with air, so it is not collected by displacement of air; it is collected by the downward displacement of water, since it is almost insoluble in water.
(iv) Anhydrous calcium chloride is deliquescent: it absorbs so much moisture from the air that it dissolves in it and forms a solution.
(v) Metal atoms have 1, 2 or 3 electrons …
ICSE 2019 sample paper · 3 marks
(i) Name the industrial method for the production of hydrogen gas. (ii) Name the catalyst used in the above process. (iii) Write the balanced chemical equation for the production of water gas.
Show answer
(i) Bosch process.
(ii) Ferric oxide (Fe₂O₃) with chromic oxide (Cr₂O₃) as promoter.
(iii) C + H₂O (steam) → CO + H₂ (at 1000 °C); the mixture CO + H₂ is water gas.