In the previous section, we saw some basic details about dihydrogen. In this section, we will see properties of dihydrogen.
Physical properties of dihydrogen
• Following are the main physical properties of dihydrogen:
♦ Dihydrogen is colorless, odorless and tasteless.
♦ It is a combustible gas.
♦ It is lighter than air.
♦ It is insoluble in water.
• Other physical properties like melting point, boiling point etc., can be obtained from standard tables.
Chemical properties of dihydrogen
This can be explained in 9 steps:
1. Bond dissociation enthalpy plays a major role in determining the chemical properties of any molecule.
• We have seen the details about bond dissociation enthalpy in a previous section 4.8.
2. If the bond dissociation enthalpy of a molecule is high, it will not be easy to break that molecule into individual atoms. If it does not break into individual atoms, reaction with other atoms or molecules is not possible.
3. If the bond dissociation enthalpy of a molecule is low, it will be easy to break it into individual atoms. If it breaks into individual atoms, reaction with other atoms or molecules is possible.
4. In our present case, the bond dissociation enthalpy of H2 molecule is very high.
• That means, it is very difficult to break the H-H single bond.
• It is the strongest single bond between any two atoms of the same element.
• This can be explained as follows:
♦ Consider various single bonds like H-H, Cl-Cl, F-F, etc.,
♦ They are all single bonds between the same two atoms.
♦ The H-H bond will have the highest bond dissociation enthalpy among such molecules.
5. Consider a sample of H2 kept at a high temperature of 2000 K.
• Only 0.081% of that sample will dissociate into individual H atoms
6. If we want more H2 molecules of that sample to dissociate, we will have to increase the temperature to a very high value.
• If the temperature is 5000 K, 95.5% of that sample will dissociate into individual H atoms.
7. If the sample is kept at room temperature, those molecules will be inert. That is., they will not take part in reactions.
8. Due to the high bond dissociation enthalpy, we will need electric arc or ultraviolet radiations to accomplish dissociation of H2 molecules.
9. Once an individual H atom is formed, it will be having a single electron in the outer most shell.
◼ Such an atom will attain stability by any one of the following three methods:
(i) The H atom donates it’s electron to an atom of another element.
• When the donation is done, it becomes H+ ion.
• The H+ ion then combines with the atom of the other element to become a new compound.
(ii) The H atom accepts an electron from an atom of another element.
• When the acceptance is done, it becomes H- ion.
• The H- ion then combines with the atom of the other element to become a new compound.
(iii) The H atom shares it’s electron with another H atom or an atom of another element. This leads to a covalent bond.
Let us see some reactions involving hydrogen:
Reaction of dihydrogen with halogens
This can be explained in 4 steps:
1. Let us denote halogen molecules in general as X2, where X is a halogen atom. (Recall that the important halogens are: F, Cl, Br and I)
• Then the reaction can be written as:
H2 (g) + X2 (g) → 2HX (g)
2. We already know that, there will be a single covalent bond between H and X. For example, we have seen the formation of HCl in earlier sections.
3. HX is the general form of hydrogen halides.
• For example:
♦ HF is a hydrogen halide
♦ HCl is a hydrogen halide.
4. Fluorine is so reactive that, it’s reaction with dihydrogen takes place even in the dark.
• But the reaction of I with dihydrogen requires a catalyst.
Reaction of dihydrogen with dioxygen
This can be written in 3 steps:
1. The reaction between dihydrogen and dioxygen is highly exothermic.
2H2 (g) + O2 (g) → 2H2O (l) ΔH⊝ = -285.9 kJ mol-1`
2. But we will need to supply energy in the form of heat to get the reaction started.
• That is., we need to supply energy to break the bonds in H2 and O2 molecules.
3. But once the reaction starts, energy is released. This energy is sufficient to propagate the reaction.
• But the energy released is so huge that, it creates an explosion. That is the reason why we are not able to make water by burning hydrogen with oxygen.
• We can burn only very small quantities of dihydrogen with oxygen in labs. Those labs should have highly advanced safety equipment.
Reaction of dihydrogen with dinitrogen
This can be written in 3 steps:
1. The reaction between dihydrogen and dinitrogen is exothermic.
3H2 (g) + N2 (g) → 2NH3 (g) ΔH⊝ = -92.6 kJ mol-1`
2. But we will need to supply energy in the form of heat and pressure to get the reaction started.
• That is., we need to supply energy to break the bonds in H2 and N2 molecules.
3. This reaction is used in Haber process to manufacture NH3 (ammonia).
Reaction of dihydrogen with metals
This can be written in 6 steps:
1. dihydrogen reacts with many metals to form the corresponding hydrides.
2. Let us denote the alkali metal atom by the letter 'M'
• Then the reaction can be written as:
H2 (g) + 2M (s) → 2MH (s)
3. Metals are electron donors. The alkali metals are strong electron donors.
• They donate the outermost electron and become M+.
4. The H atom is forced to accept this electron. Thus it becomes H-.
(The H atom is forced to accept electrons from metals which are above it in the reactivity series. Some images of the series can be seen here)
5. The M+ and H- combines together to form the ionic compound M+H-.
6. The formation of NaH (sodium hydride) is an example.
♦ Here, Na donates it's outermost electron to become Na+
♦ H accepts this electron to become H-.
♦ Thus the ionic compound Na+H- is formed.
Reaction of dihydrogen with metal ions
This can be written in 3 steps:
1. dihydrogen reduces some metal ions in aqueous solution into the corresponding metals.
2. Metals are electron donors. But those metals which are below hydrogen in the reactivity series, will be forced to accept electron from hydrogen
3. Let us see an example:
H2 (g) + Pd2+(aq) → Pd (s) + 2H+ (aq)
Pd (Palladium) is below hydrogen in the reactivity series.
Reaction of dihydrogen with metal oxides
This can be written in 4 steps:
1. dihydrogen reduces some metal oxides into the corresponding metals.
2. Metals are electron donors. The electrons donated by them are accepted by oxygen to form metal oxides.
3. But in the presence of hydrogen, those metals in the oxides, are forced to accept electrons from hydrogen.
• When those metals accept electrons, they are reduced to pure metals.
3. The oxygen is thus released from the oxide.
• Also, hydrogen becomes H+ due to the electron donation
• These two combine together to form water.
3. If we denote the metals by the letter 'M', the general equation will be as follows:
yH2 (g) + MxOy (s) → xM (s) + yH2O (l)
This equation can be explained in 3 steps:
(i) There are y oxygen atoms on the left side. Each O atom requires 2 electrons to complete octet.
(ii) So there should be a total of 2y electrons available.
(iii) These 2y electrons can be supplied by the y number of H2 molecules because, each H can supply one electron.
4. Let us see an example:
• Dihydrogen reacts with copper(II) oxide to give pure copper and water. The equation is:
H2 (g) + CuO → Cu (s) + H2O (l)
• In Copper(II) oxide, Cu is in the oxidation state of +2.
♦ The two electrons required are supplied by the two H atoms.
♦ The Cu2+ gets reduced to Cu.
(Cu2+ is forced to accept electrons from H. Note that, H is above Cu in the reactivity series)
• The H+ thus formed will share electrons from the O atom and become H2O molecule
♦ One O atom is able to supply two electrons
♦ One electron is shared by each of the two H+ ions.
Reaction of dihydrogen with organic compounds
• Reaction of dihydrogen with some organic compounds give many commercially important products. We will see two examples:
1. Dihydrogen reacts with vegetable oils to give edible fats. (margarine and vanaspati ghee)
2. Dihydrogen reacts with olefins to give aldehydes. Aldehydes undergo reduction to give alcohols.
We will see more details in organic chemistry classes.
Uses of dihydrogen
1. Dihydrogen is used in the manufacture of ammonia. This ammonia is essential for the manufacture of nitric acid and nitrogenous fertilizers. Nitrogenous fertilizers are the only means by which we can supply the required nitrogen for food crops.
2. Dihydrogen is essential for the manufacture of edible fats from vegetable oils.
3. Dihydrogen is essential for the manufacture of methanol.
• Methanol is required for the manufacture of paints, plastics, construction materials etc.,
4. Dihydrogen is essential for the manufacture of metal hydrides.
5. Dihydrogen is essential for the manufacture of halides. Hydrogen chloride is an example.
6. We saw that dihydrogen can reduce metal oxides. So it is used in many metallurgical processes where reduction of metal oxides is involved.
7. Dihydrogen is used for cutting and welding of metals.
• First, an electric arc is used to dissociate dihydrogen into individual H atoms.
• These H atoms recombine to form dihydrogen. Huge energy is released when they recombine.
• This energy helps to raise the temperature of the metal surface up to about 4000 K. Thus welding of different metal parts can be achieved.
8. Dihydrogen is used as a fuel in rocket engines.
9. Dihydrogen has greater energy density than other fuels. For example, one kg of dihydrogen releases more energy than one kg of petrol.
10. Dihydrogen is used in fuel cells to produce electricity.
• Fuel cells are electrochemical cells in which the chemical energy of dihydrogen and dioxygen are converted into electrical energy. We will see more details about this cell in higher classes.
In the next section, we
will see hydrides.
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