Solutions-Class-10-CBSE-NCERT-Science-Chapter-1-Chemical Reactions and Equations

Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be exposed into air.

(i) H₂ + Cl₂ → 2HCl

(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃ 

(iii) 2Na + 2H₂O → 2NaOH + H₂

(i) BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

(ii) NaOH + HCl → NaCl + H₂O

(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

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.

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:

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^–

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)

 (i) (a) and (b) is incorrect

[(c) and (d) is correct]

Answer is (d) Displacement reaction.

Answer is (a) Hydrogen gas and Iron chloride are produced.

A balanced equation is the one in which number of different atoms on both the reactant and product sides are equal. Balancing chemical equation is necessary for the reaction should obey The Law of Conservation of energy. Balancing the chemical equation has no defined method and is purely a trial and error attempt.

 (a) Unbalanced: H₂ + N₂ → NH₃

       Balanced: 3H₂ + N₂ → 2NH₃

(b) Unbalanced: H₂S + O₂ → H₂O + SO₂

      Balanced: 2H₂S + 3O₂ → 2H₂O + 2SO₂

(c)  Unbalanced: BaCl₂ + Al₂(SO₄)₃ → AlCl₃ + BaSO₄

      Balanced: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄

(d) Unbalanced: K + H₂O → KOH + H₂

     Balanced:  2K + 2H₂O → 2KOH + H₂

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

2Ca(OH)₂ + 2CO₂ → 2CaCO₃ + 2H₂O

Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

2Al + 3CuCl₃ → 2AlCl₃ + 3Cu

BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

2KBr + BaI₂ → 2KI + BaBr₂ (Double Displacement Reaction)

ZnCO₃ → ZnO + CO₂ (Decomposition Reaction)

H₂ + Cl → 2HCl (Combination Reaction)

Mg + 2HCl → MgCl₂ + H₂ (Displacement Reaction)

An endothermic reaction occurs when energy is absorbed from the surroundings in the form of heat.(Example: Photosynthesis, melting of ice, evaporation). Conversely, an exothermic reaction is one in which energy is released from the system into the surroundings. (Example: Explosions, concrete setting, nuclear fission and fusion).

The food taken by the living beings is ultimately broken down to glucose by the digestive system. The glucose so formed is slowly oxidised to carbon dioxide and water with the release of heat energy. Thus, respiration is an exothermic reaction.

The reaction taking place is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Combination reaction is said to be the reaction between two or more molecules to form a larger molecule; whereas the decomposition reaction is defined as the splitting of larger molecules into two or more smaller molecules. This essentially explains that the decomposition reaction is the opposite of the combination reaction.

(i) 2H₂ + O₂ → 2H₂O (Combination)

2H₂O \(\underrightarrow{electrolyte}\)   2H₂ + O₂ (Decomposition)

(ii) CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (Combination)

Ca(HCO₃)₂  \(\underrightarrow{Δ}\)   CaCO₃ + H₂O + CO₂ (Decomposition)

(i) Thermal decomposition reaction (Thermolysis)

When heated strongly, potassium chlorate decomposes into potassium chloride and oxygen. This reaction is used for the preparation of oxygen.

2KClO₃  \(\underrightarrow{heat}\)  2KCl + 3O₂

(ii) Electrolytic decomposition reaction (Electrolysis)

On passing electricity through molten silver chloride, it decomposes into silver and chlorine.

2AgCl(s) \(\underrightarrow{light}\)  2Ag(s) + Cl₂(g)

(iii) Photodecomposition reaction (Photolysis)

In the presence of light, hydrogen peroxide decomposes into water and oxygen.

H₂​O₂​ (l)  \(\underrightarrow{sunlight}\)     H₂​O (l) + O₂​ (g)

• A displacement reaction is the one when a more reactive substance displaces a less reactive one from its salt solution whereas a double displacement reaction is the one where a mutual exchange of ions happens between two compounds.
• In a displacement reaction, only a single displacement takes place whereas in the double displacement reaction, as the name suggests two displacement takes place between the molecules.

Example:

Displacement reaction :

Mg + 2HCl → MgCl₂ + H₂ (Here Mg displaces H from HCL)

Double displacement reaction :

2KBr + BaI₂ → 2KI + BaBr₂ (Here K and Ba displaces each other)

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

When two solutions containing soluble salts are combined, a double displacement reaction takes place in which the ions are exchanged between the compounds. When one of such compounds formed is in solid form (that is insoluble in aqua) then it settles down at the bottom of the container. This solid is known as the precipitate and the respective reaction is termed as the precipitation reaction.

Examples :

CdSO₄(aq) + K₂S(aq) → CdS(s) ↓ + K₂SO₄(aq)

2NaOH(aq) + MgCl₂(aq) → 2NaCl(aq) + Mg(OH)₂(s) ↓

(a) Oxidation : In a chemical reaction, when the oxygen is added to the element to form its respective oxide it is the element being oxidised.

Example:

4Na(s) + O₂(g) → 2Na₂O(s)

H₂S + O₂ → H₂O + SO₂

(b) Reduction : In a chemical reaction, when the oxygen is being removed from the compound then it is said to be reduced.

Example:

CuO(s) + H₂(g) → Cu(s) + H₂O(l)

2HgO → 2Hg + O₂

The shiny brown coloured element is the Copper metal (Cu). When the metal is heated in air, it reacts with atmospheric oxygen to form copper oxide. Hence, the black coloured compound is the copper oxide.

2Cu(s) + O₂(g) → 2CuO(s)

Iron articles are painted to prevent them from rusting. When left unpainted, the metal surface comes in contact with the atmospheric oxygen and in the presence of moisture it from Iron(III) oxide. But if painted the surface does not come in contact with moisture and air thus preventing Rusting.

The main purpose of flushing Nitrogen into food packets that contain oil and fat items is to prevent Rancidity which occurs when the oil or fat reacts with the oxygen letting out an unpleasant smell and taste. Therefore, by flushing Nitrogen, an unreactive surrounding is created thus preventing rancidity.

Corrosion : Corrosion is a process where a refined metal is oxidised by atmospheric oxygen to form a more stable compound such as oxides. The metal gradually degrades during the corrosion process.

Example : Rusting of iron is a good example of corrosion where the iron is converted to Iron oxide.

Rancidity : Rancidity is the condition produced by the aerial oxidation of the oil and fat present in the food material that produces an unpleasant taste and smell.

Example : Smell in old cooking oil is an example of rancidity.