Study of the First Element: Hydrogen — ICSE Class 9 Chemistry Important Questions

Answer: (b) Granulated zinc.

Zinc is moderately reactive and reacts at a safe, steady rate: $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$Zn + H_2SO_4 → ZnSO_4 + H_2. Copper and silver are too unreactive; sodium reacts dangerously fast.

Answer: (b) Burns with a pop sound when a lighted splint is applied.

Hydrogen is combustible; a lighted splint ignites it explosively in a small volume, giving the characteristic 'pop' and forming water.

Answer: (b) Reduced.

$CuO + H_2 \xrightarrow{\Delta} Cu + H_2O$CuO + H_2 → (Δ) Cu + H_2O: oxygen is removed from $CuO$CuO, so it is reduced, while hydrogen is oxidised to water.

Answer: (c) Is a strong oxidising agent and oxidises the hydrogen formed.

Nitric acid oxidises the nascent hydrogen to water and is itself reduced to oxides of nitrogen, so hydrogen gas is not liberated (except with very dilute $HNO_3$HNO_3 and Mg/Mn).

Answer: (c) A is true - hydrogen is collected over water. But R is false: hydrogen is only slightly soluble in water and is lighter than air, which is exactly why it can be collected over water.

$Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$Zn + 2HCl → ZnCl_2 + H_2

The salt formed is zinc chloride $(ZnCl_2)$(ZnCl_2).

Oxidation: addition of oxygen to a substance or removal of hydrogen from it.

Reduction: removal of oxygen from a substance or addition of hydrogen to it.

Both always occur together in a redox reaction.

(i) Pure zinc reacts very slowly with dilute acid, so commercial (impure) granulated zinc is used to give a steady stream of hydrogen.

(ii) A little copper sulphate is added as a catalyst; the copper deposited on the zinc sets up tiny voltaic cells and speeds up the reaction.

(iii) The first sample is discarded because it is a mixture of hydrogen with the air already present in the apparatus, which would form an explosive mixture.

1. Lightest gas (much lighter than air): density about $\frac{1}{14}$1/14 that of air, so it was used to fill balloons and airships.

2. Colourless, odourless and tasteless: it is a clean, non-polluting fuel that leaves only water on burning.

3. Sparingly soluble in water: this allows it to be collected by downward displacement of water in the laboratory.

(Hydrogen also has a very low boiling point and is combustible.)

In $CuO + H_2 \xrightarrow{\Delta} Cu + H_2O$CuO + H_2 → (Δ) Cu + H_2O:

• $H_2$H_2 is oxidised (it gains oxygen to become $H_2O$H_2O).
• $CuO$CuO is reduced (it loses oxygen to become $Cu$Cu).
• $CuO$CuO is the oxidising agent (it supplies oxygen).
• $H_2$H_2 is the reducing agent (it removes oxygen).

Thus hydrogen behaves as a reducing agent towards oxides of less reactive metals.

(a) Reactants: granulated (commercial) zinc and dilute sulphuric acid, with a few drops of copper sulphate solution as catalyst. The zinc is placed in a flask fitted with a thistle funnel and delivery tube.

(b) Equation: $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$Zn + H_2SO_4 → ZnSO_4 + H_2

(c) Collection: Hydrogen is collected by the downward displacement of water (collection over water) because it is only slightly soluble in water; it may also be collected by downward displacement of air as it is lighter than air.

(d) Drying: The gas is dried by passing it through a wash bottle containing concentrated sulphuric acid, which absorbs moisture. (Anhydrous calcium chloride can also be used.)

(e) Precaution: The first sample is discarded and no flame is brought near until pure hydrogen collects, because hydrogen mixed with air forms an explosive mixture.

(i) Chemical properties:

1. Combustion in oxygen - burns with a pale-blue flame forming water:
   $2H_2 + O_2 \rightarrow 2H_2O$2H_2 + O_2 → 2H_2O

2. Reducing action - reduces heated oxides of less reactive metals:
   $CuO + H_2 \xrightarrow{\Delta} Cu + H_2O$CuO + H_2 → (Δ) Cu + H_2O and $PbO + H_2 \xrightarrow{\Delta} Pb + H_2O$PbO + H_2 → (Δ) Pb + H_2O

(ii) Uses of hydrogen:

1. Manufacture of ammonia by the Haber process: $N_2 + 3H_2 \rightleftharpoons 2NH_3$N_2 + 3H_2 2NH_3.

2. Hydrogenation of oils to make vegetable ghee (margarine): unsaturated oil $+\ H_2 \xrightarrow{Ni}$+ H_2 → (Ni) saturated fat.

3. As a fuel in the oxy-hydrogen flame and as rocket fuel, because it releases a large amount of heat and forms only water.

(i) The gas is hydrogen, prepared from a metal and a dilute acid and collected over water.

(ii) $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$Zn + H_2SO_4 → ZnSO_4 + H_2

(iii) Hydrogen is only slightly soluble in water (and lighter than air), so it can be collected by the downward displacement of water without dissolving.

(iv) Bring a lighted splint to the mouth of the jar: hydrogen burns with a 'pop' sound, confirming the gas.