In the previous section, we saw hydrides. In this section, we will see water.
Physical properties of water
This can be written in 5 steps:
1. Water is a colorless liquid. It is also tasteless.
2. We have seen that hydrogen bonding is present between water molecules. This hydrogen bonding is responsible for the unusual properties of water. Let us see some of those properties:
(i) High freezing point of water
• Suppose that, a sample of water and a sample of ethanol are placed side by side.
• If we decrease the surrounding temperature, water will be the first one to freeze.
♦ Water will freeze when the surrounding temperature becomes 0 oC.
♦ We will have to decrease the temperature to a very low value (-114.1 oC) to achieve freezing of ethanol.
• So compared to other common liquids, water has a high freezing point. This is because of the hydrogen bonding between water molecules.
• Due to this bonding, the molecules in the liquid water are already associated together. It is easier to freeze those molecules.
• In ethanol, there is no hydrogen bonding. So freezing point is very low.
(ii) High boiling point of water
• Due to the hydrogen bonding, the liquid water molecules are associated together.
• We will have to break all those bonds before we can boil the liquid water.
• For that, large amount of heat has to be supplied. So the boiling point is high.
(iii) High heat of vaporization
• We will need to supply a huge amount of heat in order to change liquid water to water vapor.
• This is due to the hydrogen bonding. The molecules in liquid water are associated together. The high energy is required to break the bonds. Liquid will change to vapour only if those bonds are broken.
• Life on Earth is possible mainly due to this high heat of vaporization of water. Rivers, lakes and oceans are able to retain water because water do not vaporize easily.
(iv) High heat of fusion
• The water in solid state (ice) will not melt easily because, large amount of heat is required for it's fusion. This is due to the hydrogen bonding between water molecules in the solid state.
• Huge amounts of energy is required to break those bonds. That is why we do not see rapid melting of ice caps of mountains even when summer sets in.
3. Some other useful properties of water:
♦ Water has high specific heat.
♦ It has high thermal conductivity.
♦ It has high surface tension.
♦ It has high dipole moment.
♦ It has high dielectric constant.
• These properties play a major role in sustaining life on Earth.
4. Water can dissolve the nutrients and minerals which are essential for plant and animal life. Once dissolved, they can be transported easily to various parts of the body.
5. Covalent compounds usually do not dissolve in water. But some important covalent compounds like alcohol and carbohydrates dissolve in water. This is because of the polar nature of water molecules. We will see more details about this solubility in later chapters. Solubility of these important covalent compounds plays a major role in easing many of our day to day activities.
Structure of water
This can be explained in steps:
1. When water is in the gaseous form, there is not much hydrogen bonding between molecules.
• So when water is in gaseous form, we will be able to study individual molecules.
◼ Water molecule has a bent shape. We have already seen the reason for the bent. See fig.4.163 of section 4.28.
2. When water is in liquid state, the molecules are associated together due to hydrogen bonding.
• However, in this state, the molecules have some freedom of movement. The hydrogen bonds will be broken and new bonds will be formed continuously.
3. As the temperature becomes lower and lower, the kinetic energy of water molecules become lower and lower. They begin to move less.
• Now the hydrogen bonding become predominant. The molecules begin to arrange themselves in a hexagonal shape.
• When the water freezes, the molecules get fixed in the hexagonal crystal shape.
• There is lot of free space inside each hexagonal shape (remember that hexagon is a shape with six sides).
4. In the liquid state, there is not much free space between molecules because, there is no hexagonal arrangement.
◼ Since in solid state, there is much space between molecules, ice is lighter than liquid water.
• For more details about ice, see fig.11.5 of section 11.2 in physics lessons.
Chemical properties of water
• Water enters into chemical reaction with a large number of substances. Let us see some important reactions:
A. Amphoteric nature
• Water is an amphoteric substance.
• That means, it can act as both acid and base.
• We have seen the details when we discussed Brönsted-Lowry Acids and Bases in section 7.12.
B. Redox reactions involving water
This can be written in steps:
1. Consider a metal which is above hydrogen in the reactivity series.
• If that metal is made to react with water, the hydrogen will be displaced from water.
2. The H atoms thus released will combine to form dihydrogen.
• So water is a good source of dihydrogen.
3. Let us see an example:
2H2O (l) + 2Na (s) → 2NaOH (aq) + H2 (g)
♦ In H2O, the oxidation number of H is +1
♦ In H2, the oxidation number of H is 0
4. So we can write:
Water can be reduced to dihydrogen by highly electropositive metals.
• But such reactions are explosive. They can be done only in labs with advanced safety equipment.
5. Consider a non-metal which is highly electronegative. For example, F (fluorine).
• If F is made to react with water, oxygen will be released.
2F2 (g) + 2H2O (l) → 4H+ (aq) + 4F- (aq) + O2 (g)
♦ In H2O, the oxidation number of O is -2
♦ In O2, the oxidation number of O is 0
6. So we can write:
Water can be oxidized to dioxygen by highly electronegative elements.
• But such reactions are explosive. They can be done only in labs with advanced safety equipment.
C. Hydrolysis reaction
• We have seen the mechanism of hydrolysis reactions in an earlier section 7.20.
• We will see more advanced details in later sections.
D. Hydrates formation
• We have learned about aqueous solutions. For example, when we dissolve NaCl in water, we get an aqueous solution of NaCl.
• Many salts occur in nature in such aqueous solutions. Such salts can be extracted into solid forms from those solutions.
• When such salts are extracted, they will contain some water molecules also.
• The water molecules can be present in three different forms:
(i) Coordinated water
(ii) Interstitial water
(iii) Hydrogen-bonded water
We will now see each one of them in detail
(i) Coordinated water
This can be explained using an example. It can be written in 5 steps:
1. Consider the ion: [Cu(H2O)6]2+
• There are six H2O molecules around the Cu2+ ion.
2. Each of the six H2O molecules is bonded to the central Cu2+ through a coordinate bond.
• Recall that in an ordinary covalent bond between two atoms, one electron belongs to one of the two atoms and the other electron belongs to the second atom.
• But in a coordinate bond, both the electrons in the bond belong to one of the two atoms.
3. In our present case, consider any one coordinate bond.
• Both the electrons in that bond will be supplied by the O atom of H2O.
• The structure is shown in fig.9.3 below:
 |
| Fig.9.3 |
4. The structure shown in the above fig. is a positive ion with a charge of +2. It can combine with two Cl- negative ions.
• Then the crystal lattice will be composed of [Cu(H2O)6]2+ ions and Cl- ions.
♦ This is just like the crystal lattice of Na+Cl-.
5. So we can write:
In the crystal lattice formed by [Cu(H2O)6]2+ ions and Cl- ions, there are six coordinated water molecules.
(ii) Interstitial water.
This can be explained in 3 steps:
1. A crystal lattice is said to contain interstitial water if:
The H2O molecules are present in the interstitial spaces of the crystal lattice.
2. One example is: MgSO4.7H2O
• The MgSO4 crystal lattice is formed by Mg2+ ions and SO42- ions.
♦ The H2O molecules are present in the interstitial spaces of this crystal lattice.
• Seven H2O molecules are available for every Mg2+ ion and every SO42- ion.
• The H2O molecules are independent units inside the crystal lattice. They are not chemically bonded to Mg, S or O atoms.
3. Another example is (CdSO4)3.8H2O
• The CdSO4 crystal lattice is formed by Cd2+ ions and SO42- ions.
♦ The H2O molecules are present in the interstitial spaces of this crystal lattice.
• Eight H2O molecules are available for every three Cd2+ ions and every three SO42- ions.
• The H2O molecules are independent units inside the crystal lattice. They are not chemically bonded to Cd, S or O atoms.
(iii) Hydrogen-bonded water
This can be explained using an example. It can be written in steps:
1. Consider CuSO4.5H2O
• This is obtained from the [Cu(H2O)6]2+ that we saw in case (i): coordinated water.
2. Out of the six H2O molecules around the Cu2+ ion, two are removed. (one at the top and the other at the bottom)
• So [Cu(H2O)6]2+ becomes [Cu(H2O)4]2+
3. Two sulfate ions (SO42-) take those positions previously occupied by H2O molecules.
4. But the new SO42- ions are not attached by coordinate bonding to the central Cu2+ ion.
• They are attached by electrostatic force between the two opposite charges of [Cu(H2O)4]2+ and SO42-
• This is just like the attraction between Na+ and Cl- in Na+Cl-
• So our present salt can be written as: [Cu(H2O)4]2+SO42-.
5. But there is one more H2O molecule. This molecule is attached to the sulphate ion by hydrogen bonding.
• So the final structure will be as shown in fig.9.4 below:
 |
| Fig.9.4 |
6. Arrangement shown in the above fig. is one unit. There will an array of such units in the final crystal lattice.
• We can represent the crystal in two forms:
♦ CuSO4.5H2O
✰ This is just like writing the sodium chloride crystal as NaCl.
♦ [Cu(H2O)4]2+SO42-.H2O
✰ This is just like writing the sodium chloride crystal as Na+Cl-.
• The second one is more accurate because, it gives us the following details:
♦ The +ve and -ve ions that make up the crystal lattice.
♦ Four H2O molecules are attached by coordinate bonds.
♦ One H2O molecule is attached by hydrogen bonding.
Solved example 9.2
How many hydrogen bonded water molecule(s) are associated in CuSO4.5H2O ?
Solution:
1. CuSO4.5H2O is better written as: [Cu(H2O)4]2+SO42-.H2O
2. We see that:
• Four water molecules are attached to Cu2+ by coordinate bonds.
• There is only one water molecule outside the coordination sphere. This water molecule is attached by hydrogen bond.
In the next section, we
will see hard and soft water.
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