Chemical equation and reaction 2
1. Why is combustion of Liquified Petroleum Gas (LPG) a chemical change?
Ans: Combustion of Liquified Petroleum Gas (LPG) is a chemical change because after its combustion, a new substance is formed and cannot be turned back into LPG.
2. What is wrong with the following equation?
Mg + O → MgO
Identify the mistake and balance the equation.
Ans: In this equation, oxygen should be in molecular form (O2).
2Mg + O2 → 2MgO
3. What is meant by skeletal equation?
The equation where the number of atoms of each element on both the sides of a chemical equation are not equal is called skeletal equation.
4. Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. Write a balanced equation for this reaction.
2KClO3(s) → 2KCl(s) + 3O2(g)
5. What do you observe when magnesium ribbon is burnt?
When magnesium ribbon is burnt, formation of white powder is observed alongwith white dazzling flame.
6. On heating blue coloured powder of copper(II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas ‘X’ is formed. Identify the brown gas ‘X’.
2Cu(NO3)2(s)Copper(II)nitrate→Heat2CuO(s)Copperoxide+O2(g)+4NO2(g)
‘X’ is nitrogen dioxide gas.
7 Convey the following information in the form of a balanced chemical equation:
“An aqueous solution of ferrous sulphate reacts with an aqueous solution of sodium hydroxide to form a precipitate of ferrous hydroxide and sodium sulphate remains in solution.”
FeSO4(aq) + 2NaOH (aq) → Fe(OH)2(s) + Na2SO4(aq)
8. Balance the following chemical equation: Pb(NO3)2(s)→HeatPbO(s)+NO2(g)+O2(g)
2Pb(NO3)2(s)→Heat2PbO(s)+4NO2(g)+O2(g)
9. Give one example of a combination reaction which is also exothermic.
When quicklime or calcium oxide (CaO) reacts with water, slaked lime [Ca(OH)2] is formed. During this reaction a large amount of heat is released. So, this reaction is an exothermic reaction.
CaO(s)Calciumoxide(Quicklime)+H2O(l)Water→Ca(OH)2(aq)Calciumhydroxide(Slakedlime)+Heat
10. Why will the colour of heated copper powder become black when air is passed over it?
When copper powder is heated in the presence of air, black copper oxide is formed.
2Cu(s)Copperpowder+O2(g)Oxygen(Fromtheair)→Heat2CuO(s)Copperoxide(Black)
11. What is the difference between the following two types of reactions?
AgNO3 + HCl → AgCl + HNO3
Mg + 2HCl → MgCl2 + H2
The first reaction is a double displacement reaction whereas second reaction is a single displacement reaction.
12. Why is hydrogen peroxide kept in coloured bottles?
This is done in order to cut off light because hydrogen peroxide decomposes into water and oxygen in the presence of light.
2H2O2→Light2H2O+O2
13. Consider the following reactions:
Fe + CuSO4 → FeSO4 + Cu
Cu + FeSO4 → CuSO4 + Fe
Reaction (i) will take place. This is because Fe (iron) is more reactive than Cu (copper) and so it can displace Cu from its compound (CuSO4). On the other hand, copper (Cu) cannot displace iron (Fe), so reaction (ii) will not take place.
14. Give one example of a reaction which is a double displacement reaction as well as a precipitation reaction.
AgNO3(aq)Silvernitrate+NaCl(aq)Sοdiumchloride→AgCl(s)⏐↓Silverchloride(ppt.)+NaNO3(aq)Sodiumnitrate
15. Why is photosynthesis considered an endothermic reaction?
Photosynthesis is an endothermic reaction because energy, in the form of sunlight is absorbed during the process of photosynthesis by green plants.
16. What type of reaction is represented by the digestion of food in our body?
Decomposition reaction.
17. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?
When an active metal like Zn reacts with HCl, the gas produced burns with a pop sound which indicates that it is hydrogen gas.
Zn(s) + 2HCl(aq) → ZnCl2(s) + H2(g)
18. Can a double displacement reaction take place when the products are highly soluble or highly ionised?
No, double displacement reaction takes place when there is a formation of a slightly soluble salt.
19. What changes in the colour of iron nails and copper sulphate solution do you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes?
Iron nails become brownish in colour and the blue colour of copper sulphate solution fades. This is because iron displaces copper from copper sulphate solution and forms ferrous sulphate which is light green in colour.
20. Name the oxidising and reducing agent in the following reaction:
2H2S + SO2 → 2H2O + 3S ↓
H2S is the reducing agent while SO2 is the oxidising agent.
21. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified potassium permanganate solution. The light purple colour of the solution fades and finally disappears. Write the correct explanation for this observation.
Potassium permanganate solution (KMnO4) is an oxidising agent. It oxidises ferrous sulphate (FeSO4) to ferric sulphate [Fe2(SO4)3].
22. Define rancidity.
The oxidation of oils or fats in food resulting in a bad taste and smell is called rancidity.
23. Name one synthetic antioxidant.
Butylated hydroxyanisole (BHA).
