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Wednesday, September 7, 2011

chemical reaction and equation: Why do fire flies glow at night?


1. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?
Answer:  KCl (aq) + AgNO3 (aq) AgCl (s) + KNO3 (aq)
It is a double displacement and precipitation reaction.
2. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.
Answer:  2FeSO4(s) --H---e---at Fe2O3(s) + SO2(g) + SO3(g)
It is a thermal decomposition reaction
3. Why do fire flies glow at night?
Answer: Fire flies have a protein which in the presence of an enzyme undergoes aerial oxidation. This is a chemical reaction which involves emission of visible light. Therefore, fire flies glow at night.
4. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?
Answer: Grapes when attached to the plants are living and therefore their own immune system prevents fermentation. The microbes can grow in the plucked grapes and under anaerobic conditions these can be fermented. This is a chemical change.
5. Which among the following are physical or chemical changes?
(a) Evaporation of petrol 
(b) Burning of Liquefied Petroleum Gas (LPG)
(c) Heating of an iron rod to red hot.
(d) Curdling of milk
(e) Sublimation of solid ammonium chloride
Answer:  (a), (c) and (e) — are physical changes. 
(b) and (d) are chemical changes
6. During the reaction of some metals with dilute hydrochloric acid, following observations were made.
(a) Silver metal does not show any change
(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
(c) The reaction of sodium metal is found to be highly explosive
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.  
Explain these observations giving suitable reasons.
Answer:(a) Silver metal does not react with dilute HCl
(b) The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.
(c) Reaction of sodium metal is found to be highly explosive because it is an exothermic reaction
(d) When lead is treated with hydrochloric acid, bubbles of hydrogen gas are evolved
                  Pb + 2HCl PbCl2 + H2
6. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.
Answer: Calcium oxide CaO(s) + H2O(l) Ca(OH)2(aq)
7. Write a balanced chemical equation for each of the following reactions and also classify them.
(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.
(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.
(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.
(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.
Answer:(a) Pb(CH3COO)2 + 2HCl PbCl2 + CH3COOH
 Double displacement reaction
(b) 2Na + 2C2H5OH 2C2H5ONa + H2
 Displacement reaction
(c) Fe2O3 + 3CO 2Fe + 3CO2
 Redox reaction
(d) 2H2S + O2 2S + 2H2O
 Redox reaction
8. Why do we store silver chloride in dark coloured bottles?
Answer:  Silver chloride on exposure to sunlight may decompose as per the following reaction.
2AgCl 2Ag + Cl2 Therefore, it is stored in dark coloured bottles.
9.Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?
Answer:  Ba(OH)2 +2NH4 Cl ----------------à BaCl2 + 2NH4(OH)      Endothermic reaction
10. What is the difference between the displacement and double displacement reactions? Write equations for these reactions.  
Answer: In displacement reaction, one element from its salt is displaced by a more reactive element e.g. in following example, Cu is displaced by Zn from CuSO4 because Zn is more reactive.
CuSO4 + Zn ---> ZnSO4 + Cu  
Also, Zn(s) + CuSO4 (aq) ZnSO4 (aq) + Cu(s) ;   Pb(s) + CuCl2(aq) PbCl2(aq) + Cu(s)
Zinc and lead are more reactive elements than copper. They displace copper from its compounds
In double displacement reaction, exchange of ions takes place between two reactants to form new products.
Na2SO4 + BaCl2 ---> BaSO4 + 2NaCl
Here, the white precipitate of BaSO4 is formed by the reaction of SO4-2 and Ba2+               

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