Chapter 7 Hydrogen
Hydrogen is the lightest element known. It was first prepared by Robert Boyle by diluting sulphuric acid on iron nails. Cavendish studied the gas and called it inflammable air, as the gas burns when kindled
Occurrence And Preparation
The occurrence of Hydrogen Stars (including our sun) is mainly composed of hydrogen. Hydrogen is the most abundant element in the universe. On the earth, however, hydrogen occurs in very small amounts in the free state —in the air, in volcanic gases and the earth’s crust.
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But there is plenty of hydrogen on Earth in combination with other elements, i.e., as part of compounds. Water is an important source of hydrogen. Every nine parts by mass of water contains one part of hydrogen. In combination with carbon, hydrogen is present in natural gas and petroleum as well as in all living things. Acids and alkalis also contain hydrogen.
Preparation of Hydrogen
Hydrogen can be obtained by the following methods.
1. By displacement reactions:
Since hydrogen is available in the combined state, it can be obtained by displacement reactions. You have learnt earlier that
- Metals, along with hydrogen,n are arranged in order of their activity in the activity series, and
- A more active metal displaces a less active one (including hydrogen) from its compounds.
Thus, metals above hydrogenin the activity series can displace hydrogen from water and acids. However, remember that tin and lead do not undergo this reaction with water and nor does lead with acids.
NCERT Solutions for Hydrogen Class 8
From water:
Highly active metals like sodium and calcium vigorously react with water, displacing hydrogen from it even in cold conditions.
2 Na + 2H2O → 2NaOH (sodium hydroxide) + H2↑
Ca + 2H2O → Ca2 (OH)(Calcium hydroxide) + H2↑
A less active metal, like magnesium, does so when steam is passed over it.
⇒ \(\mathrm{Mg}+\underset{\text { (steam) }}{\mathrm{H}_2 \mathrm{O}} \quad \rightarrow \underset{\text { magnesium oxide }}{\mathrm{MgO}}+\mathrm{H}_2 \uparrow\)
A still less active metal like iron, when steam is passed over the red-hot metal.
3 Fe + 4H2O(steam) ⇌ Fe3O4 (ferrosoferric oxide) +4H2 ↑
From an acid:
All metals above hydrogen (except lead) in the activity series displace hydrogen from acid, like dilute hydrochloric or sulphuric acid. (We do not consider nitric acid for this reaction because it gives the oxides of nitrogen.)
⇒ \(\left.\begin{array}{c}
2 \mathrm{Na}+2 \mathrm{HCl} \longrightarrow 2 \mathrm{NaCl}+\mathrm{H}_2 \dagger \\
\mathrm{Mg}+2 \mathrm{HCl} \longrightarrow \mathrm{MgCl}_2+\mathrm{H}_2 \uparrow
\end{array}\right] \text { Vigorous }\)
⇒ \(\left.\begin{array}{l}
\mathrm{Zn}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \underset{\text { zinc sulphate }}{\mathrm{ZnSO}_4}+\mathrm{H}_2 \uparrow \\
\mathrm{Fe}+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \underset{\text { iron(II) sulphate }}{\mathrm{FeSO}_4}+\mathrm{H}_2 \uparrow
\end{array}\right] \text { Moderate }\)
2. By the electrolysis of water:
You have learnt earlier that acidulated water decomposes into hydrogen and oxygen when an electric current is passed through it
- The decomposition of a substance in the molten are forms. state or in solution, when an electric current is passed through it is called electrolysis.
- The substance taken in the molten state or solution is called the electrolyte. And the terminals—made of metals or graphite— through which the current enters or leaves the electrolyte are known as electrodes.
- The electrode connected to the negative pole of the battery is called the cathode, and the one connected to the positive pole of the battery, the anode. The vessel in which electrolysis is carried out is called an electrolytic cell
- An electric current passes through a compound in the molten state or solution only when the liquid contains ions. So, only such substances will act as electrolytes which are made up of ions
Example:
NaCl, which is made of Na+ and Cl– ions) or form ions in solution. Pure water does not allow an electric current to pass through it. But water acidulated with (a very small amount of) dilute hydrochloric or sulphuric acid does because it dissociates.
NCERT Class 8 Chemistry Chapter 7 Hydrogen
⇒ \(\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OH}^{-}\)
During electrolysis, the H+ ions move towards the negative electrode, i.e., the cathode, and get discharged (lose their charge) there by the negative charge of the electrode. And the OH” ions move to the positive electrode, i.e., the anode, and get discharged to give oxygen
You will learn in higher classes that, instead of the H+ ion, it is the H3O+ ion (i.e., H++ H2O), called the hydronium ion, that exists in an aqueous solution
The laboratory method
Principle:
Hydrogen is prepared in the laboratory by the action of dilute hydrochloric or sulphuric acid on granulated zinc. We do not use nitric acid, as the oxides of nitrogen are formed
⇒ \(\mathrm{Zn}+2 \mathrm{HCl} \longrightarrow \mathrm{ZnCl}_2+\mathrm{H}_2 \uparrow\)
⇒ \(\mathrm{Zn}+\mathrm{H}_2 \mathrm{SO}_4 \longrightarrow \mathrm{ZnSO}_4+\mathrm{H}_2 \uparrow\)
As the gas is almost insoluble in water, it is collected by the displacement of water.
Procedure:
A conical flask is fitted with a thistle funnel and a delivery tube. The other end of the delivery tube passes through a beehive shelf placed in a water trough. A gas jar full of water is inverted over the beehive shelf. Some granulated zinc is placed in the conical flask.
Dilute hydrochloric or sulphuric acid is added through the thistle funnel till the lower end of the funnel dips in the liquid. Hydrogen then begins to evolve.
The gas is collected by the downward displacement of water. It is not collected by the downward displacement of air since a mixture of hydrogen and air is explosive.
Initially, the air inside the flask and the delivery tube is driven out. So, whatever is collected in the first one or two jars is air, and is rejected. The gas collected afterwards is hydrogen.
Why we prefer zinc to other metals
In the above method, we could have used any metal more active than hydrogen (except lead). We do not use highly active metals like sodium, calcium and magnesium because the reactions of these metals with an acid are too vigorous to control and these metals are expensive too.
The moderately active metals—zinc and iron, which are also cheaper—should be more suitable. We do not use iron either, because it forms some foul-smelling poisonous gases of silicon, sulphur and phosphorus present in it as impurities. So, we prefer zinc to other metals.
Hydrogen Class 8 NCERT Notes
Large-scale preparation
Hydrogen is prepared on a large scale, i.e., in the industry, by the following methods.
- The electrolytic method
- The Bosch method
Let us discuss the principles of the Bosch method.
The Bosch method:
When steam is passed over red-hot coke, what is known as water gas is formed. Water gas is a mixture of mainly carbon monoxide (CO) and hydrogen (H2), containing some carbon dioxide (CO2).
⇒ \(\underset{\text { (steam) }}{\mathrm{H}_2 \mathrm{O}}+\underset{\text { (red hot) }}{\mathrm{C}} \longrightarrow \underbrace{\mathrm{CO}+\mathrm{H}_2}_{\text {water gas }}\)
The water gas is mixed with excess steam and heated to 450 °C. The mixture is passed over a catalyst consisting of iron(III) oxide (Fe2 O3) mixed with some chromium(III) oxide (Cr2O3). The CO abstracts oxygen from Thistle funnel H2O to form CO2 and releases more H2
⇒ \(\underbrace{\mathrm{CO}+\mathrm{H}_2}_{\text {water gas }}+\underset{\text { (steam) }}{\mathrm{H}_2 \mathrm{O}} \frac{450^{\circ} \mathrm{C}}{\mathrm{Fe}_2 \mathrm{O}_3 \text { catalyst }} \mathrm{CO}_2+2 \mathrm{H}_2\)
The CO2 is removed from the mixture by dissolving it in water under pressure. (Remember that ordinarily, CO2 is only slightly soluble in water but highly soluble under pressure.)
Properties
Physical Properties
- Hydrogen is a colourless and odourless (having no smell) gas.
- It is the lightest element and the lightest gas known. It is 14.6 times lighter than air.
- It is almost insoluble in water
Chemical Properties
1. Reaction with oxygen (or air):
When kindled, hydrogen bums in air or oxygen to form water
⇒ \(2 \mathrm{H}_2+\mathrm{O}_2 \rightarrow 2 \mathrm{H}_2 \mathrm{O}\)
A large amount of heat is produced in this reaction. So, a mixture of hydrogen and oxygen may explode.
2. Reaction with chlorine:
When kindled, hydrogen burns in chlorine to form hydrogen chloride gas. Also, a mixture of hydrogen and chlorine, when placed in sunlight, explodes to form the same product
⇒ \(\mathrm{H}_2+\mathrm{Cl}_2 \longrightarrow \underset{\text { hydrogen chloride gas }}{2 \mathrm{HCl} \uparrow}\)
Hydrogen chloride gas can be dissolved in water to obtain hydrochloric acid.
Properties of Hydrogen Class 8
3. Reaction with sulphur:
Hydrogen reacts with molten sulphur to give hydrogen sulphide gas, which smells like rotten eggs.
⇒ \(\mathrm{H}_2+\underset{\text { (molten) }}{\mathrm{S}} \longrightarrow \underset{\text { hydrogen sulphide }}{\mathrm{H}_2 \mathrm{~S} \uparrow}\)
4. Reaction with nitrogen:
Hydrogen reacts with nitrogen only under special conditions to form an appreciable amount of ammonia.
The ammonia gas formed, in turn, decomposes back to hydrogen and nitrogen. So, this reaction occurs both ways. Such reactions are called reversible reactions.
5. Reactions with some metal oxides:
When passed over some hot metal oxides like zinc oxide, iron oxide and copper oxide, hydrogen gas converts them into the corresponding metals.
The oxides of metals like potassium, calcium, sodium and magnesium are not converted into the metals.
Hydrogen is a reducing agent. The addition of hydrogen to or the removal of oxygen from a substance is called reduction.
On the other hand, the addition of oxygen to or the removal of hydrogen from a substance is called’ oxidation. Also, a substance causing reduction is known as a reducing agent, and one causing oxidation is called an oxidising agent.
You have just seen that hydrogen
- Adds itself to oxygen, chlorine, sulphur and nitrogen, and
- Removes oxygen from some metal oxides.
Thus, hydrogen is a reducing agent. It reduces
By adding H:
- O to H2O
- Cl to HCl
- S to H2S
- N to NH3
By removing O:
- ZnO to Zn
- PbO to Pb
- CuO to Cu
- FeO to Fe
- Fe2O3 to Fe
Hydrogen as a Fuel Class 8
Test
Hydrogen burns with a characteristic sound, or ‘pop’. This property is used as a test for hydrogen
Uses
1. For manufacturing ammonia:
Hydrogen is used in large quantities to manufacture ammonia.
- When a mixture of hydrogen and nitrogen is passed over an iron catalyst at 500 °C and 200 atm, ammonia is formed.
- (A catalyst is an element or a compound that hastens a reaction without taking part in it.)
- This process is known as the Haber process. Ammonia is used in large quantities in the manufacture of nitrogenous fertilisers.
2. For the hydrogenation of oils:
Vegetable oils react with hydrogen in the presence of a catalyst (like nickel) to form solid fats. Such addition of hydrogen is called hydrogenation.
3. As a fuel:
On the combustion of hydrogen, i.e., when hydrogen is burnt, heat is produced along with water
So, hydrogen can be used as a fuel. The product of the reaction is water, which does not pollute the environment. Hence, hydrogen is a clean fuel.
Hydrogen produces the maximum heat among all the known fuels. Liquid hydrogen is used as rocket fuel. However, it is difficult to handle and store.
4. The oxyhydrogen flame:
Hydrogen is used to produce an oxyhydrogen flame, which is employed for welding and cutting metals. The two gases—oxygen and hydrogen—passing through different pipes, mix at a point where the mixture is kindled. A high-temperature (2800 °C) flame, called an oxyhydrogen flame, is produced. The metal melts at this temperature, enabling it to be cut or welded.
5. As a reducing agent:
Hydrogen is used as a reducing agent in the burning laboratory and industry
6. For filling balloons:
As hydrogen is lighter than air, a balloon filled with hydrogen rises in the air and drifts in the wind. If a weather instrument is placed in it, the balloon can be used for studying weather conditions.
There was a time when such weather balloons were much in use. But the practice was stopped as such weather balloons often caught fire due to the inflammability of hydrogen. As helium, the next lightest gas, is available in plenty, it is preferred to hydrogen. Helium does not catch fire.
Hydrogen and its Compounds Class 8
Weather balloons:
Fill balloons:
In the 1930s, airships using hydrogen to float were used for transport. But they were discontinued after the airship Hindenburg caught fire in 1937, killing 36 people.