CHEMICAL REACTIONS & EQUATIONS
CaO (s) + H2O (l) → Ca(OH)2(aq) + Heat
(Quick lime) (Slaked lime)
Formation of slaked lime
Slaked lime solution is used for whitewashing. Calcium hydroxide reacts slowly with CO2 in air to form a thin layer of calcium carbonate (CaCO3) after 2-3 days of whitewashing giving a shiny finish to the walls. Marble is also CaCO3.
Other examples of combination reactions:
(i) Burning of coal:
C(s) + O2(g) → CO2(g)
(ii) Formation of water from H2 and O2:
2H2(g) + O2(g) → 2H2O(l)
Reactions in which heat is released along with formation of products are called exothermic chemical reactions.
Other examples of exothermic reactions:
(i) Burning of natural gas:
CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
(iii) Decomposition of vegetable matter into compost.
(iv) Burning magnesium to form magnesium oxide.
These are the reactions in which single reactant breaks down to give simpler products.
- Take 2 g ferrous sulphate crystals in a dry boiling tube. Heat it over flame. Green colour of ferrous sulphate is changed with characteristic odour of burning sulphur. When heated, ferrous sulphate (FeSO4. 7H2O) lose water. It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3).
- Decomposition of CaCO3 to calcium oxide (CaO) & CO2 on heating is used in industries. CaO has many uses (e.g. manufacture of cement). When a decomposition reaction is carried out by heating, it is called thermal decomposition.
At the base of a plastic mug, drill two holes and fit rubber stoppers. Insert carbon electrodes in rubber stoppers. Connect electrodes to a 6 volt battery.
When we bring a burning candle to the gas at cathode, it burns immediately. But gas at anode does not burn.
Silver bromide also behaves in the same way.
Ba(OH)2 +NH4Cl → BaCl2 + NH4OH
Take 3 iron nails cleaned by rubbing with sand paper.
Take two test tubes A and B. Add 10 mL copper sulphate (CuSO4) solution in each test tube.
Immerse two iron nails in CuSO4 in test tube B. Keep one iron nail aside for comparison.
After 20 minutes, take out the iron nails. They become brownish in colour. Blue colour of CuSO4 solution fades.
This is due to the following chemical reaction:
Here, iron has been displaced by copper from the CuSO4 solution.
Other examples of displacement reactions:
Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
These are the reactions in which there is an exchange of ions between the reactants.
Take 3 mL sodium sulphate solution in a test tube. In another test tube, take 3 mL barium chloride solution. Mix the two solutions.
A white substance (BaSO4) is formed by the reaction of SO42– and Ba2+. This water insoluble substance is called precipitate. Any reaction that produces a precipitate is called precipitation reaction.
The other product (sodium chloride) remains in solution.
Reaction between lead nitrate & potassium iodide to form a yellow precipitate of lead iodide.
Pb(NO3)2(aq)+ 2KI(aq) → PbI2(s) + 2KNO3(aq)
If a substance gains oxygen during a reaction, it is said to be oxidised. Such reaction
is called oxidation.
2Cu + O2
If hydrogen gas is passed over the heated CuO, the black coating turns brown as the reverse reaction takes place and copper is obtained.
CuO + H2
Other examples of redox reactions:
Carbon is oxidised to CO and ZnO is reduced to Zn.
ZnO + C → + Zn + CO
HCl is oxidised to Cl2 whereas MnO2 is reduced to MnCl2.
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
If a substance gains oxygen or loses hydrogen during a reaction, it is oxidised. If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.
A magnesium ribbon burns in air (oxygen) to form magnesium oxide. Here magnesium is oxidised.
It is a process by which a metal is attacked by substances such as moisture, acids, etc. E.g. black coating on silver, green coating on copper.
- Adding antioxidants (substances which prevent oxidation) to foods containing fats and oil.
- Keeping food in air tight containers.
- Flushing bags of chips with gas such as nitrogen.