In the previous section,
we completed a discussion on redox reactions. In this chapter,
we will see Hydrogen.
• We know that:
♦ Hydrogen atom has only one electron and one proton.
✰ It is the simplest atom in the universe.
♦ Two hydrogen atoms combine together by covalent bond to attain duplet.
✰ Duplet means: two electrons in the outer most shell.
♦ So hydrogen is usually found in nature as H2. It is called dihydrogen.
Position of hydrogen in the periodic table
This can be explained in 14 steps:
1. We know that elements in the periodic table are arranged according to their electronic configurations.
2. Consider the electronic configuration of hydrogen. It is 1s1
• But group 1 elements (alkali metals) also have the general outer electronic configuration of ns1.
3. We can write:
• Alkali metals lose outer most electron to attain stability. They lose their only one outer most electron and become unipositive ions (ions with one positive charge)
For example: Li becomes Li+, Na becomes Na+
• Hydrogen also loses it’s outer most electron to attain stability. It loses it’s only one outer most electron and become unipositive ion.
That is: H becomes H+
4. Let us examine compounds formed by alkali metals and hydrogen.
• Alkali metals form oxides, halides and sulphides.
• Hydrogen also forms oxides, halides and sulphides.
5. Steps (1) to (4) shows the similarities between hydrogen and alkali metals. Seeing those similarities, we are tempted to include hydrogen in the group 1.
6. But we have to consider the similarity of hydrogen with group 17 elements (halogens) also.
• Halogens have the general outer electronic configuration of ns2np5.
♦ So they need one more electron to attain stability (octet).
• Hydrogen also needs one more electron to attain stability (duplet).
7. Halogens form diatomic molecules to attain stability. F2, Cl2 etc., are examples.
• Hydrogen also form the diatomic molecule H2 to attain stability.
8. Let us see a comparison in terms of ionization enthalpies:
♦ The ionization enthalpy of hydrogen is 1312 kJ mol-1
♦ The ionization enthalpy of fluorine is 1680 kJ mol-1
• We see that:
Ionization enthalpy of hydrogen is closer to that of fluorine.
♦ Fluorine is a halogen.
9. Steps (6) to (8) shows the similarities between hydrogen and halogens. Seeing those similarities, we are tempted to include hydrogen in
the group 17.
10. In the above steps,
♦ We have seen the similarities of hydrogen with alkali metals.
♦ We have seen the similarities of hydrogen with halogens.
• Next, we will see the differences:
11. Differences in terms of ionization enthalpies:
♦ The ionization enthalpy of hydrogen is 1312 kJ mol-1
♦ The ionization enthalpy of lithium is 520 kJ mol-1
• We see that,
ionization enthalpy of hydrogen is much different from that of lithium.
♦ Lithium is an alkali metal.
• So in terms of ionization enthalpy, we are tempted not to put hydrogen in group 1.
12. Differences in terms of reactivity:
♦ Halogens show high reactivity.
♦ But when compared to halogens, reactivity of hydrogen is very low.
• So in terms of reactivity, we are tempted not to put hydrogen in group 17.
13. We must also consider a unique behaviour of hydrogen. It can be written in 4 steps:
(i) When hydrogen loses it’s only one electron, it becomes H+ ion.
(ii) Since the only one electron is lost, H+ is a proton. It is very small in size.
Size of an H+ ion is 1.5 × 10-3 pm.
• The ionic size of other atoms in general is 50 to 200 pm.
(iii) So H+ is extremely small when compared to other ions.
• That means, the charge is concentrated on a very small space. Consequently, the charge density is very high.
• Due to this very high charge density, the H+ ion attacks any thing which comes to it’s proximity.
(iv) As a result, H+ is never found free in nature.
14. Due to these peculiarities, hydrogen is placed separately from the main body of the periodic table.
Occurrence of dihydrogen
This can be written in 3 steps:
1. Dihydrogen is the most abundant element in the universe.
2. On Earth, elemental dihydrogen is found in the atmosphere. But since hydrogen molecules have only low mass, Earth’s gravity is not able to exert enough pull to keep them confined in the atmosphere.
3. However, large quantities of hydrogen are available on Earth in combined form.
• For example,
♦ Each molecule of water has two atoms of hydrogen.
♦ Carbohydrates, proteins etc., that are found in living organisms also contain hydrogen in combined form.
♦ Fuels like petrol, kerosene etc., which are hydrocarbons also contain hydrogen in combined form.
Isotopes of hydrogen
This can be written in 9 steps:
1. We know that, isotopes are different forms of a same substance.
• We can arrange isotopes into different groups:
♦ All isotopes of hydrogen in one group,
♦ All isotopes of carbon in another group,
♦ All isotopes of chlorine in yet another group so on . . .
2. All members of a group will have the same atomic number. But their mass numbers will be different.
3. Let us examine the group containing the isotopes of hydrogen.
(i) There are three members in that group.
(ii) All the three members in the group will have the same atomic number 1.
♦ That means, each of the three members has one proton in the nucleus.
(iii) One of the three members is our ordinary hydrogen. We know that, it has no neutrons.
• So it’s mass number = (number of electrons + number of protons) = (1 + 0 ) = 1
♦ This isotope is named as Protium. It’s symbol is: 11H
(iv) The second member has one proton and one neutron.
• So it’s mass number = (number of electrons + number of protons) = (1 + 1) = 2
♦ This isotope is named as Deuterium. It’s symbol is: 12H
(v) The last member has one proton and two neutrons.
• So it’s mass number = (number of electrons + number of protons) = (1 + 2) = 3
♦ This isotope is named as Tritium. It’s symbol is: 13H
4. Protium is the predominant form. It is the ordinary hydrogen that we are familiar with.
5. If we take a sample of naturally occurring hydrogen, 99.98% of that sample will be protium. Only 0.0156% will be deuterium. In that sample, deuterium will be in the form of HD. That is., one H atom in the H2 molecule is replaced by one D atom.
6. Tritium occurs only rarely. If we take 1018 atoms of protium, only one among them will be tritium.
7. Among the three isotopes of hydrogen, only tritium is radioactive.
8. The chemical properties of the three isotopes are almost same. This is because, they have the same electronic configuration.
• But the rate of reactions will differ. Because, they have different bond dissociation enthalpies.
9. The three isotopes have different physical properties because, they have different atomic masses.
Preparation of dihydrogen in the laboratory
The following two methods are usually used for preparing dihydrogen in the laboratory.
1. Reaction of granulated zinc with dilute hydrochloric acid gives dihydrogen.
Zn + 2H+ → Zn2+ + H2
2. Reaction of zinc with aqueous alkali gives dihydrogen.
Zn + 2NaOH → Na2ZnO2 + H2
Commercial production of dihydrogen
There are four methods available to produce hydrogen commercially.
Method 1: Electrolysis of acidified water
This can be explained in 5 steps:
1. We have already seen the basics of electrolysis in our previous classes:
♦ Two electrodes are partially immersed in the electrolyte.
♦ Those electrodes are connected to a cell.
♦ One electrode is connected to the positive terminal of the cell.
♦ The other electrode is connected to the negative terminal of the cell.
♦ The circuit is completed by the flow of ions through the electrolyte.
Some images can be seen here.
2. In our present case:
• Water is the electrolyte.
• But pure water contains only very few H+ and OH- ions.
• So we add a small amount of acid (H2SO4) to the water.
• The acid dissociates into H+ and SO42- ions.
• The newly added H+ ions react with H2O molecules and produce H3O+ ions and OH- ions:
• Thus the required number of ions are produced so that, electricity can be conducted through the electrolyte.
3. Consider the negative electrode. It is the electrode connected to the negative terminal of the cell. Electrons are present in that electrode.
• The positive ions H3O+ move towards this negative electrode and accepts electrons. Thus they get reduced.
2H3O+ + 2e- → H2 + 2H2O
• Thus we get pure dihydrogen.
• We know that, the electrode at which reduction takes place is called cathode.
◼ So we can write:
dihydrogen is obtained at the cathode.
4. Consider the positive electrode. It is the electrode connected to the positive terminal of the cell.
• The negative ions SO42- and OH- ions move towards this electrode.
• The SO42- ions are stable. They do not donate electrons.
• The OH- ions donate electrons. They get oxidized.
4OH- → 2H2O + O2 + 4e-
• Thus we get pure oxygen.
• We know that, the electrode at which oxidation takes place is called anode.
◼ So we can write:
oxygen is obtained at the anode.
5. The acid added to the water will react with the metal electrodes. This will lead to the corrosion of electrodes. To avoid such a situation, we use platinum electrodes. Platinum is a noble metal like gold. It is not corroded by acids.
Method 2: Electrolysis of warm aqueous barium hydroxide solution.
• This method is similar to method 1 except that, the electrolyte here is alkaline instead of acidic. Remember that barium hydroxide is an alkali.
• As before, the H3O+ ions will get reduced at the cathode to give dihydrogen.
• The OH- ions will get oxidized at the anode to give oxygen.
• In this method, nickel is used to make electrodes. This is because, Ni is more resistant to corrosion by alkaline solutions.
Method 3: By the electrolysis of brine solution.
This can be explained in 4 steps:
1. Brine is highly concentrated solution of NaCl in water.
• It’s electrolysis is done mainly for the production of NaOH and Cl2
• During this process, dihydrogen is obtained as a byproduct.
2. The Cl- ions reach the positive electrode and get oxidized.
2Cl- → Cl2 + 2e-
• We know that electrode at which oxidation takes place is anode.
◼ So we can write:
Chlorine is produced at the anode.
3. The H3O+ reach the this negative electrode and get reduced.
2H3O+ + 2e- → H2 + 2H2O
• We know that, the electrode at which reduction takes place is called cathode.
◼ So we can write:
dihydrogen is obtained at the cathode.
4. When chlorine and hydrogen are removed, what remains in the solution is Na+ and OH- ions. Thus we get a highly concentrated solution of NaOH as the main product.
Method 4: Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst.
This can be explained in steps:
1. We know that hydrocarbons are those compounds which contain carbon and hydrogen. They have the general formula: CnH2n+2
2. Coke also contains carbon and hydrogen. It is produced by heating coal in the absence of oxygen.
• The coke thus produced can be used as a fuel. The advantage of using coke as a fuel over coal is that, it produces very little smoke during burning.
• Coke is also used in the manufacture of iron.
3. The reaction between hydrocarbon and steam is:
CnH2n+2 + nH2O → nCO + (2n+1)H2
For example:
CH4 (g) + H2O (g) → CO (g) + 3H2 (g)
• Thus we get dihydrogen.
4. We see that, on the product side, we have a mixture of CO and H2.
• This mixture is called water gas.
• This mixture can be used for the synthesis of methanol and a number of hydrocarbons. So it is also known as syngas.
5. We just saw that, syngas is produced from hydrocarbons and steam.
Coal can also be used instead of hydrocarbons. The equation is:
C (s) + H2O (g) → CO + H2 (g)
This process is called coal gasification.
6. We saw that water gas contains both CO and H2. But what we want mainly is H2. So we collect the CO from the water gas and react it with steam in the presence of iron chromate as catalyst. We get a mixture of carbon dioxide and H2. This process is called water gas shift reaction.
CO (g) + H2O → CO2 (g) + H2 (g)
• We can write:
The CO in the water gas is used to extract hydrogen from steam.
• We see that, on the product side, CO2 and H2 are present. We can remove the CO2 by scrubbing with sodium arsenite solution.
◼ After the discussion about the above four methods, we can write:
Hydrogen can be produced from:
♦ Hydrocarbons (These are readily available from the petroleum industry)
♦ Coal
♦ Electrolysis
• 77% of the industrial dihydrogen is produced from hydrocarbons.
• 18% is produced from coal
• 4% is produced from electrolysis.
• 1% is produced from other sources.
In the next section, we
will see properties of dihydrogen.
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