Chapter 1 Chemical Reactions and Equations Solutions
Question - 1 : - Why should a magnesium ribbon be cleaned before burning in air?
Answer - 1 : - Magnesium ribbon should be cleaned before burning in air because Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer which is a very stable compound. In order to prevent further reactions with Oxygen, it is therefore necessary to clean the ribbon by to remove the layer of MgO.
Question - 2 : - Write a balanced equation for the following chemical reactions.
Answer - 2 : -
i) Hydrogen + Chloride —-> Hydrogen chloride
Answer (i) H2 + Cl2 → 2HCl
ii) Barium chloride + Aluminium sulphate —-> Barium sulphate + Aluminium chloride
Answer(ii) 3BaCl2 + Al2(SO4)3 →3BaSO4 + 2AlCl3
(iii) Sodium + Water —-> Sodium hydroxide + Hydrogen
Answer(iii) 2Na + 2H2O →2NaOH + H2
Question - 3 : - Write a balanced chemical equation with state symbols for the following reactions
Answer - 3 : - i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and solution of Sodium chloride.
Answer(i) BaCl2 + Na2SO4 →BaSO4 + 2NaCl
ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce Sodium chloride solution and water.
Answer(ii) NaOH + HCl → NaCl + H2O
Question - 4 : - A solution of a substance ‘X’ is used for whitewashing.
Answer - 4 : - (i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer
i) The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide and its formula is CaO.
ii) CaO + H2O → Ca(OH)2
Question - 5 : - Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas
Answer - 5 : - In activity 1.7, gas collected in one of the test tubes is double of the amount collected in the other because water gets hydrolysed to release H2 and O2 gas. Here, after electrolysis two molecules of Hydrogen and one molecule of oxygen gas is released, hence the amount of Hydrogen collected would be double than that of oxygen.
Question - 6 : - Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer - 6 : - When an iron nail dipped in the copper sulphate solution, iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes. The reaction is:
Question - 7 : - Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer - 7 : - Reaction Between silver nitrate (AgNO3) and Sodium chloride (NaCl) is an example of double displacement reaction. During the reaction negative and positive ions trade positions as a result in the formation of white silver chloride precipitate. The chemical reaction is given below.
Ag+ + NO3– + Na+ + Cl– → AgCl + Na+ + NO3–
Question - 8 : - Identify the substances that are oxidized and that are reduced in the following equation.
Answer - 8 : -
i) 4Na(s) + O2(g) → 2Na2O(s)
ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
The Sodium (Na) in the first equation is getting oxidized with the addition of Oxygen (O2) and the Copper (Cu) in the second equation is reduced due to the addition of Hydrogen (H2)
Question - 9 : - Which of the statements about the reaction below are incorrect?
Answer - 9 : -
2PbO(s) + C(s) →2Pb(s) + CO2(g)
(a) Lead is getting reduced
(b) Carbon Dioxide is getting oxidised
(c) Carbon is getting oxidised
(d) Lead oxide is getting reduced
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Answer
(i) (a) and (b)
Explanation: (a) because Oxygen is being removed and (b) because the removed oxygen from Lead is added to the elemental Carbon.
Question - 10 : - Fe2O3 +2Al → Al2O3 + 2FeThe above reaction is an example of a
Answer - 10 : -
1. Combination reaction.
2. Double displacement reaction.
3. Decomposition reaction.
4. Displacement reaction.
Answer is 4. Displacement reaction.
Explanation: The Oxygen from the Ferrous oxide is getting displaced to the Aluminium metal to form Aluminium Oxide. In this reaction Aluminum is more reactive metal than Fe. Therefore Al will displace Fe from its oxide. This type of chemical reactions in which one of the elements displace another is called displacement reaction. Here less reactive metal is displaced by more reactive metal. Since one-time displacement is occurring, therefore, it is called a single displacement reaction.