10 CHEMICAL REACTIONS

CLASS 10 CHEM CHAPTER 1

Ques. Why should magnesium ribbon be cleaned before burning in air?

Ans. Magnesium is a reactive metal. When kept exposed in air for a long time a layer of magnesium oxide is normally formed on the surface of the metal. The oxygen present in air slowly combines with the metal to form magnesium oxide (MgO). The oxide coating or layer does not burn when flame is brought in contact with the metal. The surface should be properly cleaned preferably with a sand paper before burning the ribbon in air.

Ques. Write the balanced equations for the following chemical reactions :

 (i) Hydrogen + Chlorine → Hydrogen chloride

 (ii) Sodium +Water → Sodium hydroxide + Hydrogen

(iii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride.

Ans. 

Ques. Write the balanced equations with state symbols for the following reactions:

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

 (ii) Sodium hydroxide solution (in water)  reacts with hydrochloric acid solution (in water) to produce sodium chloride (in water) and  water.

Ans. The symbol equations in balanced form for the reactions are

Q- A solution of the substance 'X' is used for white washing.

(i) Name the substance ‘X’ and write its formula.

(ii) Write the reaction of the substance 'X‘ with water.

Ans. (i) The substanee ‘X’ is calcium oxide (also called quick lime). Its formula is CaO

(ii) CaO(s) + H₂O→ Ca(OH)₂(s)

2. In the electrolysis of acidulated water. The volume of one gas is twice the volume of the other gas. Name the gas.

Ans. The gas is hydrogen and its volume is twice that of oxygen.

2H₂O (aq) →2H₂(g) + O₂(g)

Ques. Why does the colour of copper sulphate  change when an iron nail is dipped in it?

Ans. Iron nail acquires a brown coating of copper as a result of the displacement reaction,

Fe(s) + CuSO₄(aq) → Cu(s) + FeSO₄(aq)

Give one example of the double displacement reaction.

Ans. BaCl₂(aq) + CuSO₄(aq) →BaSO₄↓(s)+ CuCl₂

What are combination  reactions? Give examples.

Ans  When two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.

Ques. What are exothermic reactions? Give examples.
Ans. Reactions in which heat (energy)  is released along with the formation of products are called exothermic chemical reactions.

Ques. How is respiration an exothermic process?
Ans. During digestion, food is broken down into simpler substances. For example, rice, potatoes and  bread contains carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. Thus respiration is an exothermic reaction. The reaction involved is

Ques. A house wife wanted her house to be white washed. She bought 10 kg of quicklime from the market and dissolved in 30 litres of water. On adding lime to water, she notices that the water started boiling even when it was not being heated. Give reasons for the observation. Write the corresponding equation and name the product formed.
Ans. The reaction between quicklime and water is exothermic as shown:

The product formed is slaked lime- Ca(OH)₂
Ques: What are decomposition reaction? Give examples.
Ans. In this reaction a single reactant breaks down to give two or more simpler products. e.g

Ques: What are thermal decomposition  reactions (pyrolysis )? Give examples.
Ans. Reactions in which a reactant decomposes to give simpler products on heating are called thermal decomposition.

Ques: What happens when iron sulphate  is heated in test tube?
Ans. When green coloured hydrated iron(II) sulphate is heated , it decomposes to give iron (III) oxide which is brown in colour. Also along with iron (III) oxide, two acidic gases sulfur dioxide and sulfur trioxide are evolved.

Ques: What happens when marble pieces are heated?
Ans. When marble pieces i.e calcium carbonate decomposes to give calcium oxide i.e quicklime and carbon dioxide. Quicklime produced is used in manufacture of cement.

Ques: What happens when lead nitrate is heated in a test tube?
Ans. When lead nitrate is heated in test tube yellow coloured lead oxide is formed along with liberation of brown fumes of NO₂ and oxygen gas.

Ques. What is electrolysis?
Ans. [Electo-electricity, Lysis- break]. When decomposition reaction takes place by passing electricity, it is referred as electrolysis, e.g. electrolysis of acidulated water gives
H₂O (l) → 2H₂ (g) + O₂

Ques. Why acid is added to water for electrolysis?
Ans. To make water conducting.

Ques. What is photolysis?
Ans. [photo- light, lysis- break down] When decomposition reaction takes place by light, it is referred as photolysis, e.g.

 or

Ques- Why compounds of silver are kept in coloured bottles?
Ans. To prevent photolysis.

Ques. Write the reaction which is used in the process of black and white photography.

Ans

Ques. What are endothermic reactions? Give examples.
Ans Reactions in which energy is absorbed are known as endothermic reactions, e.g-
Ba(OH)₂ (aq) + NH₄Cl (aq) → BaCl₂ (s) + NH₄OH (aq)

Ques. What are displacement reactions?
Reactions in which more reactive atom or group of atoms displaces another atom or group of atoms from their solutions is called displacement/ substitution reactions, e.g

Ques. Write down reactivity series of metals.
Potassium >sodium >calcium> magnesium >aluminium> zinc> lead> hydrogen> copper >mercury> silver> gold> platinum

Ques. What are double displacement reactions?
Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

Ques. What are precipitation reactions?
Reactions which results in the formation of insoluble compounds (precipitates) are called precipitation reactions.

Pb(NO₃ )₂ + KI→ PbI₂↓+ KNO₃
                                   (yellow)
Ques. What are redox reactions?

Ans. A chemical reaction in which one substance is oxidised and the other is reduced is called a redox reaction. All oxidation-reduction reactions are redox reactions. In a chemical reaction, a substance gets oxidised only when another substance is present, which gets reduced.

Ques. What is corrosion? What are its effects ?

Ans. Due the effect of moisture and atmospheric gases, metals get corroded. This effect is called corrosion.

Corrosion causes damages to metal articles like car bodies, bridges, iron railings, ships and other substances of daily use.

Ques. What is rusting of iron?

Ans. The corrosion of iron due to moisture is called rusting of iron. Shiny iron articles on exposure to moisture get coated with a brownish powder which peels off easily. In this way enormous amount of iron is damaged.

3Fe      +          2O₂      +          xH₂O        →   Fe₃O₄.xH₂O

Ques. Explain the following terms with examples in each case : (i) Oxidation and (ii) Reduction.

Ans. (i) Oxidation- It is a process (a) in which oxygen or any electronegative element is added up and (b) hydrogen or any electropositive element is removed. Examples are:

(a) Sulphur burns in air with a blue flame to form sulphur dioxide. We see that oxygen is added up to sulphur.

S(s)                  +                     O2(g)   →                    SO2(g)                                               (Addition of oxygen to sulphur)

sulphur oxygen                                                     sulphur dioxide

(b) Hydrogen sulphide combines with iodine to give hydrogen iodide and sulphur

H₂S                              +                      I₂        →                    2HI                 +                     S        (Removal of hydrogen)

hydrogen                                             iodine              hydrogen iodide                                  sulphur

sulphide

(ii) Reduction. It is a process in which (a) hydrogen or an electropositive element is added up. (b) Oxygen or electronegative element is removed. Examples are :

(a) Hydrogen reacts with chlorine to form hydrogen chloride ‘

Cl₂(g)               +          H₂(g)              →                    2HCl(g)

Chlorine                      hydrogen                     hydrogen chloride

(Addition of hydrogen to chlorine)

(b) Copper oxide is reduced with hydrogen.

CuO(s)                        +         H₂(g) → Cu(s) + H₂O(l)

copper oxide               hydrogen                                                                                                         (Removal of oxygen)

Ques.  Define the terms : (i) Oxidising agent and (ii) Reducing agent?

Ans- (i) Oxidising agent it in that substance which in a reaction (a) gives Oxygren or any electronegatlve element, (b) accepts hydrogen or any electropositive element or (c)accepts electrons.

(ii) Reducing agent. It is that substance which  in reaction (a) gives up hydrogen or any electropositive element, (b) accept: oxygen or any electronegative element or (c) releases electrons,

In the following reaction :

Oxidizing agent= CuO

Reducing agent= H₂

Ques. Name the substance oxidized, reduced, oxidizing agent, Reducing agent in following reactions.

Ans. Substance oxidized- C, HCl

Substance Reduced – ZnO, MnO₂

Oxidizing agent  -  ZnO, MnO₂

 Reducing agent-  C, HCl

Ques. What is Rancidity?

When fats and oils are oxidised, they become rancid and their smell and taste change. This is called rancidity. Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. To prevent rancidity antioxidants (which prevents oxidation) are added to foods containing fats and oils. Rancidity can also be prevented by flushing out oxygen with an inert gas like nitrogen. For example, packets of food items like chips are flushed with nitrogen so that these can be used even after long duration.