A solution of slaked lime Ca(OH)2 is applied on the walls and roofs. After 2 or 3 days, slaked lime reacts with atmospheric carbon dioxide and it changes into calcium carbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Calcium carbonate gives a shiny white finish to the walls.
Reactions which take place with the evolution of heat (increase of temperature) are called exothermic reactions.
For example, burning of natural gas and respiration are exothermic reactions.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) (Burning of natural gas)
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) (Respiration)
Decomposition of vegetable matter into compost is also an exothermic reaction.
Reactions which take place with absorption of heat (lowering of temperature) are called endothermic reactions.
When a single reactant breaks down to give simpler products, it is called decomposition reaction. For example, decomposition of FeSO4.7H2O, CaCO3 and lead nitrate.
Perform an activity to illustrate decomposition reaction.
Lead nitrate powder, boiling tube, test tube holder, burner.
(i) Take a clean pyrex glass boiling tube.
(ii) Add to it about 2 g powdered lead nitrate.
(iii) Using a test tube holder or pair of tongs, heat the test tube on a Bunsen burner.
(iv) Note the change that takes place.
We observe that brown fumes are emitted from the test tube. The following reaction takes place in this activity:
Similarly, we can perform the activities of heating of ferrous sulphate crystals and calcium carbonate to illustrate decomposition reactions.
Perform the electrolysis of water experiment to illustrate decomposition reaction.
A plastic mug, two graphite rods, two test tubes, 6 V battery, switch, two rubber stoppers, sulphuric acid.
Fig. 1.5 - Electrolysis of water
(ii) Fill the mug with water such that graphite rods are immersed. Add a few drops of sulphuric acid.
(iii) Connect the electrodes to 6 V battery and switch on the current. Leave the apparatus undisturbed for some time.
(iv) Formation of bubbles at both electrodes will take place.
(v) The reaction that takes place is as follow:
2H2O (l) → 2H2 (g) + O2 (g)
Hydrogen gas is collected in the test tube over the cathode while oxygen gas is collected in the test tube over the anode. It is interesting to note that volume of hydrogen collected is twice the volume of oxygen as explained by the above chemical equation. Hydrogen and oxygen gases can be identified as under : Bring a burning candle near the tube placed over the cathode. A pop sound is produced (hydrogen is a combustible gas).
Again, bring the burning candle near the tube placed over the anode. The flame gets brighter (oxygen is a supporter of combustion).
Perform an activity (decomposition of silver salt) to illustrate decomposition reaction.
China dish, silver chloride.
Fig. 1.6 Silver chloride turns grey in sunlight to form silver metal
(2) Place the china dish in sunlight for some time.
(3) You will observe that silver chloride becomes grey after some time.
This is because silver chloride undergoes decomposition by the action of sunlight (sunlight is also a form of energy).
A reaction in which one substance displaces another from the aqueous solution is called displacement reaction. A general reaction of this type may be represented as:
Thus C has displaced B in the above reaction.
Perform an activity to illustrate displacement reaction.
Test tube, thread, stand with clamp, copper sulphate solution, iron nails.
(ii) Take two test tubes. Mark them A and B. In each test tube take about 10 mL copper sulphate solution.
(iii) Tie two nails with a thread and immerse them in the copper sulphate solution in test tube B for about 20 minutes Fig. 1.7(a).
Keep one nail aside for comparison.
(iv) Take out the iron nails from the copper sulphate solution in test tube B.
(v) Compare the colour of copper sulphate solution in the two test tubes. It will be observed that the intensity of blue colour has decreased in test tube B compared to that in test tube A.
Also compare the colours on the nails.
It will be seen that nail removed from copper sulphate solution has a brown coating of Cu on it.
Iron is more reactive than copper. It. displaces copper from the solution of copper salt.
Similarly, we can perform the experiment using zinc or lead in place of Cu. Both Zn and Pb are more reactive than Cu. Zinc and lead displace copper from the solution of CuS04 as per the following equations
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Pb(s) + CuSO4(aq) → PbSO4(aq) + Cu(s)
Copper which is displaced gets deposited on zinc or lead as brown coating.
Double Displacement Reaction
Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
Perform an activity to illustrate double displacement reaction.
Two test tubes, sodium sulphate solution, barium chloride solution.
(2) Take 5 mL barium chloride solution in one test tube and 5 mL sodium sulphate solution in the other test tube.
(3) Add sodium sulphate solution to the test tube containing barium chloride solution as shown in Fig. 1.8.
(4) It will be observed that a white precipitate of barium sulphate is formed as a result of exchange of ions between the two substances.
Similarly, we can carry out the reaction between lead nitrate and potassium iodide by mixing their aqueous solutions to illustrate a double decomposition reaction.
Reactions in which a substance gains oxygen or loses hydrogen is called an oxidation reaction.
Similarly, a reaction in which a substance loses oxygen or gains hydrogen is called a reduction reaction.
If a substance gains oxygen during a reaction, it is said to be oxidised. Similarly, if a substance loses oxygen during a reaction, it is said to be reduced. One reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called reduction-oxidation reactions or redox reactions.
Perform an activity to illustrate oxidation.
China dish, copper powder, wire gauze, tripod stand, burner.
Take about 2 g copper powder in a clean china dish (Fig.1.9)
(2) Place it on a wire gauze supported on a tripod stand.
(3) Heat it for about 15 minutes.
(4) It will be observed that copper powder becomes black because of the formation of copper oxide (oxidation).
We say that copper has been oxidised to copper oxide. Now, if we pass hydrogen gas over heated CuO, we obtain Cu (Brown colour) back. This is because CuO has been reduced to Cu.
CuO + H2 → Cu + H2O
It may be schematically represented as under:
Some other examples of redox reactions are:
ZnO + C → Zn + CO
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is known as corrosion.
Iron articles are shiny when new. But with the passage of time, they get coated with reddish brown powder called rust. Silver articles become black and copper articles become green on the surface due to corrosion.
Corrosion causes damage to car bodies, bridges, iron railings, ships, etc.
When fats and oils are oxidised (or kept in open for some days), their smell and taste changes. We say that they have gone rancid. This phenomenon is called rancidity. Antioxidants are added to foods containing oils and fats to prevent rancidity. The antioxidant substances that are added to food are preferentially oxidised and thus they prevent the oxidation of foods. Keeping food in air-tight containers helps to slow down oxidation. Chips manufacturers flush bags of chips with nitrogen to prevent chips from getting oxidised.