The common properties of ionic compounds have been described as follows:
1. Physical State. Ionic compounds usually exist in the of crystalline solids. It has been shown by X-ray analysis ionic compounds that they do not exist as independent molecules but exist in the form of ions. These ions are arranged in a well defined geometric pattern to constitute ionic crystal. Ionic crystal is a giant network of ions in three dimensions. For example, in sodium chloride crystal, there is no single unit of the Na+ CJ- ion pair but a large number of them arranged in definite pattern in three dimensions. Each sodium ion (Na+) in the crystal is surrounded by six chloride (CI-) ions and each chloride (Cl-) ion is surrounded by six sodium ions. The simplest unit within the crystal which repeats itself in three dimensions to form the entire space lattice is called unit lattice cell. The unit lattice cell of sodium chloride is shown in Fig. 7 .5. It may be noted that pattern of arrangement of ions in the ionic crystal depends on the size of the ions and also on magnitude of charge on the ions.
Fig. 7.5. Unit lattice cell of sodium chloride crystal.
ACTIVITY 7.1 To build a model of unit lattice cell of sodium chloride crystal
1 . Sodium chloride is ionic solid. It has a giant network structure of oppositely charged ions.
• Write the Lewis symbols of cation and anion.
• Which ion has a larger size? Why?
2. Take 14 coloured polystyrene balls of large size and 1 3 white polystyrene balls of small size. The size of coloured ball is approximately double ( 1 .9 times actually) the white ball.
3 . Arrange 5 coloured balls and 4 white balls with the help of tooth picks or glue as shown in layer 1 . Make another similar arrangement to constitute layer 3.
4. Now make layer 2 by arranging five white balls and four coloured balls as shown .
5. Now place layer 2 just above layer 1, and layer 3 above layer 2. This will result in the formation of unit lattice cell of sodium chloride.
Answer the following from your observations:
(i) Which ball represents Na+ ion and which represents CJ- ion?
(ii) How many CI- ions surround one Na+ ion?
(iii) How many Na+ ions touch one Cl- ion?
(iv) What is co-ordination number of CI-and Na + ions?
(v) How many CI- ion touch one CJ- ion?
(vi) What is the shape of the unit lattice cell?
2. High Melting and Boiling Points. Ionic compounds generally have high melting and boiling points. It is because of very strong electrostatic force of attraction between oppositely charged ions. Consequently, a large amount of energy is required to overcome strong attractive inter-ionic forces. For example, melting point of sodium chloride is 1074 K and that of copper (II) chloride is 1085 K.
3. Solubility. Ionic compounds are soluble in water and other polar solvents which have high dielectric constants. It is due to the relatively strong electrostatic interactions between the ions and the polar solvent molecules. However, the ionic compounds ate insoluble in non-polar solvents.
In order to understand these interactions let us study the dissolution of sodium chloride in water. Water is a polar solvent because H2O molecules have oppositely charged ends with partial negative charge on 0 atom and partial positive charged on H atoms as shown in Fig. 7 .6.
water dipoles. The oxygen end gets attracted towards cation while hydrogen end gets attracted towards anion. If ion-di attractive forces are stronger enough to overcome the in1 ionic attraction, the ions are pulled out of crystal lattice they pass into solution. In the solution, the ions behave independent entities, _but they remain surrounded by envelop of water dipoles and are called hydrated ions (Fig. 7. 7). This process is referred to as hydration of ions.
The energy released by hydration of ions is call hydration energy. The hydration energy released compensates the lattice energy required to dismental the crystal structure In short, for dissolution of ionic substance in water Hydration energy > Lattice energy.
For example, many ionic compounds such as AgCl(s), PbC12(s), BaS04(s), etc., do not dissolve in water because high value of their lattice energies.
Non-polar solvents do not have polar sites, hence do not get attracted towards the ions. Therefore, ionic compounds are not soluble in non-polar solvents such as benzene tetrachloromethane, etc.
4. Electrical Conductivity. Ionic compounds do not conduct electricity in the solid state because the ions occupy fixed positions. and are not able to have free movement. however, they are able to conduct electricity in the molten state or in their aqueous solutions because in that case ions free to move. Thus, ionic compounds are bad conductors electricity in the solid state but are good canductors of electricity in the molten or dissolved state. Solid, molten and solved state of PbI2 and NaCl are shown in Fig. 7.8.
Fig. 7.8. (a) Freeing the ions by melting
Fig . 7.8. (b) Freeing the ions by dissolution.
ACTIVITY 7.3 To study the movement of ions in electrical field
1. Take a filter paper strip, soak it in potassium trioxonitrate (V) solution. Wrap the wet filter paper at its ends around the graphite electrodes (Use graphite rods of dry cell) as shown.
2. Prepare an aqueous solution of (CuCrO4 copper (II) tetraoxochromate (VI). With the help of dropper put a drop of this aqueous solution in the centre of the filter paper.
3. Pass electrical current through the graphite rods and make the observations.
Watch the splitting of CuCr04 spot into blue and yellow spots. Blue spot moves towards cathode and yellow moves towards anode.
Try to answer the following:_
(i) What is the colour of CuCr04 solution?
(ii) Blue colour of the spot is due to ….. .
(iii) Yellow colour of the spot is due to ….. .
(iv) Why blue colour moves towards anode?
ACTIVITY 7.4 To study the effect of electrical field on lead (II) iodide and sodium chloride
(i) Four 1.5 V dry cells or 6 V power pack
(ii) 6 V Bulb
(iii) Two graphite rods (from used up dry cells)
(iv) Connecting wires (v) Silica crucible
(vi) Heating arrangement
A. LEAD (IT) IODIDE
1. Set up the apparatus as shown in diagram.
2. Fill the crucible half way with solid Lead (II) iodide (Pbl2) and press the switch. Observe whether the bulb glows or not.
3. Heat the crucible strongly to melt the lead (II) iodide. This should only be done in a very well ventilated place or a fume cupboard.
4. After lead iodide melts, again press the switch and note whether bulb is lit or not.
5. Also observe what happens at the electrodes.
Try to answer the following from your observations:
(i) What happens at cathode?
(ii) What happens at anode?
(iii) What is the colour of the gas evolved or solid formed?
(iv) Give the formulae of cation and anion of the salt used.
(v) Give chemical equation for the process.
B. SODIUM CHLORIDE
1 . In a new crucible add sodium chloride, press the switch. See whether the bulb glows or not.
2. Heat strongly for about two minutes. Note whether the salt melts or not.
3. Add distilled water to a beaker and insert clean electrodes into it. Now, press the switch to see
whether it conducts electricity or not.
4. Cool the salt and add it to water in a beaker. Stir to dissolve the salt. Insert the electrodes in the solution so formed and press the switch. Note whether the bulb is lit or not.
Answer the following on the basis of observations:
(i) Does the aqueous solution of the sodium chloride conduct electricity?
(ii) Is it a gas or solid which has been formed at the electrodes?
(iii) Is it the water or the aqueous solution of the salt which conducted the electricity? Give reasons.
(iv) Which species are taking part in the reaction?
(v) What conclusion can be drawn about the effect of electricity on solid and an aqueous solution of sodium chloride?
(vi) Predict whether molten sodium chloride willVconduct electricity. How would you melt it?
(vii) Which out of lead (II) iodide and sodium chloride has higher melting point?
5. Ionic Crystal are Brittle. Ionic crystals are generally brittle. It means that when strong force or stress is applied on them they break into pieces along definite planes. This is because of the fact that stress or force causes displacement of ions along the planes and brings similarly charged ions near each other. The repulsive force between similarly charged ions cause them to fall apart along the plane as shown in Fig. 7.9.