THE MAGIC OF CHEMICAL REACTIONS


Ch. No. 2. THE MAGIC OF CHEMICAL REACTIONS
Q1. Fill in the blanks:.
1.      The chemical formula of POP is 2CaSO­4.H2O.
2.      The chemical reaction during which H2(g) is lost is termed as oxidation.
3.      Corrosion can be prevented by using anti-rust solution.
4.      When acids and alkalis react together, salt and water are formed.
5.      The chemical reactions is which heat is liberated are called exothermicreactions.

Q2. Match the following
Reactants
Products
Types of reactions.
Fe + S
CuSO­4 + Zn
2Cu+O2
HCl + NaOH
NaCl + H2O
2CuO
ZnSO4 + Cu
FeS
Oxidation
Neutralization
Displacement
Combination
Ans
Reactants
Products
Types of reactions.
Fe + S
CuSO­4 + Zn
2Cu+O2
HCl + NaOH
FeS
ZnSO4 + Cu
2CuO
NaCl + H2O
Combination
Displacement
Oxidation
Neutralization

Q3. Answer the following.
1.      What is the importance of a chemical equation?
Ans.  A chemical equation can convey the following information.
i.                    Reactants and products involved in the reaction.
ii.                  Symbols and formulae of all substances involved in the reaction.
iii.                Relative number of reactants and product participating in the reaction.
iv.                Relative masses of reactants and products.
v.                  Physical states of reactants and products.  

2.      What do you observe when H2S gas is passed through Cadmium Chloride solution? Name the type of reaction.
Ans.
i.                    When H2S is passed through cadmium chloride solution, yellow precipitate of cadmium sulphide and hydrochloric acid solution are obtained.
ii.                  The reaction is a double displacement reaction.

3.      What do you understand by the term “Redox Reactions”? Explain with one example.
Ans.
i.                    When oxidation and reduction take place simultaneously in a given chemical reaction, it is called redox reaction or oxidation-reduction.
ii.                  No oxidation can take place without reduction and there is no reduction without oxidation at the same time.
iii.                When carbon monoxide is passed over heated ferric oxide, carbon monoxide gains oxygen (i.e. gets oxidized) to form carbon dioxide. At the same time, iron oxide gets reduced to metallic iron by the loss of oxygen.
iv.                Fe2O3 + CO → 2Fe +2CO2

4.      What is corrosion? Do gold ornaments corrode? Justify.
Ans.
i.                    The slow process of decay or destructions of a metal due to the effect of air, moisture, acids, alkalies, or other chemicals on it is called corrosion or rusting of iron. Its chemical formula is Fe2O3. H2O.
ii.                  Ordinarily, gold ornaments do not get corroded.
iii.                Gold is one of the least reactive metals. It does not react with air, moisture, sweat, or ordinary pollutants in air or water. Hence, it does not get corroded.

Q4. Give scientific reasons.
1.      Grills of doors and windows are always painted before they are used.
Ans.
i.                    Usually grills of doors and windows are made of iron.
ii.                  Iron objects get rusted (Fe2O3. H2O)on exposure to moist air.  
iii.                Since air always contains moisture, iron grills get rusted.
iv.                Painting iron objects prevents rusting, hence iron grills of doors and windows are always painted before they are used.

2.      Physical states of reactants and products are mentioned while writing a chemical equation.
Ans.
i.                    To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae.
ii.                  The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s) respectively.
iii.                The word aqueous (aq) is written if the reactant or product is present as a solution in water.  

3.      Potassium Ferrocyanide is stored in dark coloured bottles and kept away from sunlight.
Ans.
i.                    Potassium Ferrocyanide gets decomposed when exposed to bright light.
ii.                  The dark colours of the bottles absorb a lot of energy from the light passing through it.
iii.                If stored in dark – coloured bottles, it does not get enough light energy to decompose.
iv.                Hence, potassium Ferrocyanide is stored in dark – coloured bottles and kept away from sunlight to prevent its decomposition.

4.      Iron articles rust readily whereas steel which is also mainly made of iron will not undergo corrosion.
Ans.
i.                    One of the properties of iron is that it reacts with oxygen in the presence of moisture and gets corroded by forming rust(Fe2O3. H2O).
ii.                  Air contains oxygen and moisture. Hence, iron readily rust when exposed to air.
iii.                Steel (stainless steel, not mild steel) is an alloy of iron having the property to resist corrosion including rusting.
iv.                Hence, stainless steel, though it contains iron, does not undergo corrosion.

5.      Edible oil is not allowed to stand for a long time in an iron or tin container.
Ans.
i.                    If edible oil is allowed to stand for a long time in an iron or tin container the fatty acids in the oil react with rust flakes or powder to form salts.
ii.                  These salts contaminate the oil and hence the oil becomes rancid.
iii.                Rancid oils have a foul odour and unpleasant taste.
iv.                Thus, rancid oil is of no use. Hence, Edible oil is not allowed to stand for a long time in an iron or tin container to avoid rancidity.

6.      Edible oil is not allowed to stand for a long time in air.  
Ans.
i.                    When edible oils are left exposed to air for long period of time, they become rancid (i.e. the oils get oxidized.)
ii.                  Rancid oils have a foul odour and unpleasant taste.
iii.                Thus, rancid oil is of no use.
iv.                Edible oil is not allowed to stand exposed to air for long to avoid rancidity.

Q5. We feel fresh while on a morning walk in natural surroundings. Why don’t we feel the same after 10 a.m. and during rush hours? Write a chemical reaction to support you statement?
Ans.
i.                    Early in the morning the air is fresh, has less pollutants due to low vehicular traffic. Breathing less polluted air gives us the feeling of freshness.
ii.                  After 10 a.m. vehicular traffic increases. Fast moving vehicles raise dust in the air, and the partially burnt fuels set free carbon particles and carbon monoxide. The air also gets polluted with oxides of nitrogen and sulphur dioxide (diesel, petrol contains certain amount of sulphur.
iii.                Inhaling polluted air makes us feel tired.
iv.                (a) sulphur burns in the air to produce sulphur dioxide: S + O2 → SO2↑.
v.                  Partial burning of carbon (petrol, diesel, being hydrocarbon contain carbon) which produced carbon monoxide: .

Q6. Explain the following terms:
Ans.
1.      Rancidity: Rancidity is a chemical process in which edible oils or fats get oxidized over a period of time and become inedible (unfit for human consumption) is called rancidity.
2.      Endothermic reactions: The reactions in which heat is absorbed are called endothermic reactions. The reactants absorb heat for form products.
3.      Redox reaction: The chemical reactions in which oxidation and reduction takes place simultaneously are called redox reactions. ‘Red’ stands for reduction and ‘ox’ stands for oxidation.
4.      Neutralization: The chemical reaction in which an acid and alkali (base) react together to form salt and water is called neutralization. The resulting solution is neither acidic nor basic but is neutral. This reaction is known as neutralization reaction.

Q7. Take finely powdered Zinc and allow it to react with CuSO4 solution. Then take Zinc granules and carry out the same reaction. Which reaction will take place faster and why?
Ans.
i.                    The reaction between powdered zinc and copper sulphate solution is faster than that between zinc granules and copper sulphate solution.
ii.                  This is because one of the factors that control the rate of reaction is the size of the particles. The smaller the particles, the faster s the reaction.
iii.                Zinc granules are larger than powdered zinc. Hence, the rate of reaction is faster when powdered zinc is used.