TO PREPARE HYDROGEN GAS IN THE LAB

Experiment 11: TO PREPARE HYDROGEN GAS IN THE LAB/LABORATORY

OBJECTIVE

TO PREPARE HYDROGEN GAS IN THE LAB/LABORATORY AND COMPARE THE PROPERTIES OF MOLECULAR AND NASCENT HYDROGEN

APPARATUS REQUIRED

Woulfe’s bottle
Thistle funnel
Delivery tube
Cork
Water trough
Gas jars
Test Tube

CHEMICALS REQUIRED

Granulated zinc
Dilute H₂SO₄
KMnO₄ solution
K₂Cr₂O₇ solution
Litmus papers
FeCl₃ solution

THEORY

Metals which occupy upper position than hydrogen in the electrochemical series such as zinc, magnesium etc can displace hydrogen gas from dilute acid (HCl or H₂SO₄). Hydrogen gas is generally prepared in the laboratory by the action of dilute H₂SO₄ on granulated zinc. The gas is produced according to the following chemical reaction:

Molecular\ reaction:\\ Zn_{(s)} + {H_{2}SO_{4}}_{(aq)}\rightarrow {ZnSO_{4}}_{(aq)}+{H_{2}}_{(g)}\\ Ionic\ reaction:\\ Zn+2H^{+}\rightarrow Zn^{2+}+H_{2}

Here Zn is oxidized by H⁺ ions, and H⁺ ions are reduced by zinc.

PROCEDURE

Some pieces of granulated zinc were placed into Woulfe’s bottle, and some water was added so that the zinc pieces were covered with it. The apparatus is shown in the figure. The apparatus was made airtight. Dilute sulphuric acid was added through the thistle funnel. The lower end of the thistle funnel dipped under the solution. Reaction occurred with effervescence. The gas was collected in a gas jar filled with water by placing it over the delivery tube with the downward displacement water. The following experiments were performed and the properties of the gas were studied.

Experiment	Observation	Inference
1. i. Color ii. Odour iii. Solubility: Invert gas jar was filled with hydrogen gas over water trough and moved it up and down for some time.	1. i. No colour ii. No odour iii. The level of water in the gas jar remained the same.	1. The gas is i. colourless ii. Odourless iii. Insoluble in water.
2. A burning candle was introduced into the jar full of H₂ gas.	2. The fire is extinguished. But gas burns at the mouth.	2. H₂ gas is combustible, but not a supporter of combustion.
3. A test tube filled with H₂ gas was inverted and was ignited.	3. The gas burns in the upper tube producing pup sound.	3. H₂ gas is lighter than air.
4. The blue and red litmus paper was wetted in water and was introduced into the jar containing H₂ gas.	4. The blue and red litmus paper was wetted in water and was introduced into the jar containing H₂ gas.	4. H₂ gas is neutral to litmus paper.
5. Hydrogen gas was passed through: i. KMnO₄ solution and dilute H₂SO₄ solution ii. FeCl₃ solution and dilute H₂SO₄ solution iii. K₂Cr₂O₇ solution and dilute H₂SO₄ solution	5. No change of colour i. No change of colour ii. No change of colour iii. No change of colour	5. Molecular hydrogen is not capable of reducing KMnO₄ ,K₂Cr₂O₇ and FeCl₃ solutions.
6. A piece of zinc was added into the above three test tubes containing acidified KMnO₄, FeCl₃ and K2Cr2O7 separately. Wait for some time.	6. i. Pink color of KMnO₄ changes to colorless. ii. Yellow color of FeCl₃ changes to light green. iii. Orange color of K₂Cr₂O₇ changes to green.	6. Nascent hydrogen can reduce all the three solutions

Reactions involved

5. Molecular hydrogen is not a powerful reducing agent:

\begin{align*} KMnO_{4} + H_{2}SO_{4} + H_{2}\rightarrow No\ reaction\\ FeCl_{3} + H_{2}\rightarrow No\ reaction\\ K_{2}Cr_{2}O_{7} + H_{2}SO_{4} + H_{2}\rightarrow No\ reaction \end{align*}

6. Reducing action of nascent hydrogen
i. With acidified KMnO₄

ii. With acidified FeCl₃

\underset{yellow}{FeCl_{3}} + [H]\rightarrow \underset{Light\ green}{FeCl_{2}} + HCl

iii. With acidified K₂Cr₂O₇

These observations show that nascent hydrogen is a much more powerful reducing agent than molecular hydrogen.

RESULT

Hydrogen gas is prepared in the lab and its properties are studied.

CONCLUSION

In this way, hydrogen gas is prepared in the laboratory and its properties are studied.

PRECAUTIONS

Pure zinc should not be used in this preparation as it reacts very slowly with dilute sulphuric acid.
Concentrated sulphuric acid should not be used instead of dilute acid as sulphur dioxide will be formed in place of hydrogen gas.
Sometimes the formation of gas bubbles at the zinc surface will prevent the further formation of gas. In such cases, a few crystals of copper sulphate can be added to the reaction mixtures.
Apparatus as fitted must be air-tight.
The lower end of the thistle funnel must be under the surface of water.