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}} + HCliii. 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.
