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1. Chemical Reactions and Equations

​Chapter Review:

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• Chemical changes are also known as chemical reaction. 
• There are two parts of chemical reaction, (i) reactants (ii) products.
• The substances that undergo chemical change in the reaction are the reactants.
• The new substance, formed during the reaction, is the product.
• A word-equation shows change of reactants to products through an arrow placed between them.
• The arrowhead points towards the products, and shows the direction of the reaction.
• The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them.
• Chemical equations can be made more concise and useful if we use chemical formulae instead of words.
•  A chemical equation represents a chemical reaction.
• The number of atom of each element on both sides of arrow is equal.
• Unbalance chemical equation is called skeletal equation.
• Chemical equation is balanced to satisfy the law of conservation of mass.
• The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
• The number of atoms of each element remains the same, before and after a chemical reaction.
• Skeletal equation can be balanced by using Hit and trial method or inspecting method.
• Combination reaction, Decomposition reaction, Displacement reaction, Double displacement reaction, oxidation and reduction reaction are the types of chemical reaction.
• Corrosion and rancidity are the effects of oxidation.
•  A complete chemical equation represents the reactants, products and their physical states symbolically.
• In a combination reaction two or more substances combine to form a new single substance.
• Reactions in which energy is absorbed are known as endothermic reactions.
• Precipitation reactions produce insoluble salts.
• Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
• Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen.
• When an element displaces another element from its compound, a displacement reaction occurs.

1. Chemical Reactions and Equations

1. Chemical Reactions and Equations

Exercise (chap-1, NCERT Text Book)

Q1. Which of the statements about the reaction below are incorrect?

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

Ans:  (ii) (a) and (c)

Q2.  Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(c) decomposition reaction.

(d) displacement reaction.

Ans:  (d) displacement reaction.

Q3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(c) No reaction takes place.

(d) Iron salt and water are produced.

Ans: (a) Hydrogen gas and iron chloride are produced.

Q4. What is a balanced chemical equation? Why should chemical equations be balanced?

Ans: The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. Such a chemical equation is called balanced chemical equation.  A chemical equation is  balanced to satisfy the law of conservation of mass.

Q5. Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans:     

(a) N₂(g) + ____  H₃(g) → ____ NH₃ (g)

Sol: N₂(g) +  3H₃(g) → 2 NH₃ (g)

(b) ____ H₂S(g) + ___ O₃ (g) → ____ H₂O(i) + ____ SO₂ (g)

Sol: 2H₂S(g) + 3O₃ (g) → 2H₂O(i) + 2SO₂ (g)

(c) ____ BaCl₂ + Al₂(SO₄)₃ → _____ BaSO₄ + _____ AlCl₃

Sol: 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 3AlCl₃

(d) ____ K + ____ H₂O → _____ KOH + H₂(g)

Sol: 2K +  2H₂O → 2KOH + H₂(g)

Q6. Balance the following chemical equations.

(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

(b) NaOH + H₂SO₄ → Na₂SO₄ +H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO4 → BaSO₄ + HCl

Ans :

(a) ___ HNO₃ + ____ Ca(OH)₂ → ____ Ca(NO₃)₂ + ____ H₂O

Sol: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(b) ___NaOH + ____ H₂SO₄ → ____ Na₂SO₄ + ___ H₂O

Sol: 2NaOH +  H₂SO₄ → Na₂SO₄ + 2H₂O

(c) ___NaCl + ____ AgNO₃ → ____ AgCl + ____ NaNO₃

Sol: NaCl + AgNO₃ →  AgCl +  NaNO₃

(d) ___BaCl₂ + ____ H₂SO4 → ____ BaSO₄ + ___ HCl

Sol: BaCl₂ +  H₂SO4 → BaSO₄ + 2HCl

Q7. Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Ans:     

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Sol: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

    Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (Already Balanced)

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

Sol: Zn +  AgNO₃  →  Zn (NO₃)₂ + Ag

   Zn + 2AgNO₃  →  Zn (NO₃)₂ +  2Ag balanced)

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

Sol:  Al + CuCl₂ → AlCl₃ + Cu₂

       Al + 3CuCl₂ → 2AlCl₃ + 3Cu (balanced)

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Sol: BaCl₂ + K₂SO₄ → BaSO₄  + KCl

    BaCl₂ + K₂SO₄ → BaSO₄  + 2KCl     (balanced)

Q8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Ans:  

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

Sol: KBr(aq) + BaI₂(aq) → KI (aq) + BaBr₂ (aq)

2KBr(aq) + BaI₂(aq) → 2KI (aq) + BaBr₂ (aq)  (balanced)

Type of Reaction is double displacement.

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

Sol: ZnCO₃(s) → ZnO(s) + CO₂(g)

     ZnCO₃(s) → ZnO(s) + CO₂(g)   (Already balanced)

Type of Reaction is decomposition reaction.

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Sol: H₂(g) + Cl₂(g) →   HCl (g)

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

Type of Reaction is combination reaction.

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Sol: Mg (s) + HCl (aq) → MgCl₂(aq) + H₂(g)

    Mg (s) + 2HCl (aq) → MgCl₂(aq) + H₂(g) (balanced)

Q9. What does one mean by exothermic and endothermic reactions? Give examples.

Ans:Exothermic Reaction: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

Like;(i) Burning of coal

C(s) + O2(g) → CO2(g) (energy)

Other examples of exothermic reactions are –

(i) Burning of natural gas

CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g) (energy)

Endothermic Reaction: Reactions in which energy is absorbed are known as endothermic reactions.

Q10. Why is respiration considered an exothermic reaction? Explain.

Ans: Our body requires energy to support life. Our body obtains energy from food that we eat. During digestion, body mechanisms breaks down complex energy molecules of food into simpler substances such as glucose. This glucose is used with oxygen during the cellular respiration and gives energy. The releasing of energy carries on during the whole process. That is why the respiration is considered as an exothermic reaction. 

Q11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Ans: In decomposition reactions a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions. In the combination reaction, two or more substances combine to give a new substance with the releasing of energy.

Example of decomposition reaction: 

2H₂O(l)  Electrolysis → 2H₂(g) + O₂(g)

Example of combination reaction : 

2H₂(g) + O₂(g) → 2H₂O(l) + Energy 

Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Ans: 

Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans: 

In a displacement reaction a more reactive element replace a less reactive element from its compound. 

General equation for displacement reaction: 

A + Bx  → Ax + B 

Here, A is more reactive element and B is less reactive element. 

For example: 

CuSO₄(aq) + Zn  →  ZnSO₄(aq) + Cu 

In double displacement reaction, the exchanging of ions are taken place. 

General equation for this reaction: 

ABx + CDx → CBx + ADx 

For exampe: 

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

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

(silver nitrate)           (copper)       (copper nitrate)        (Silver) 

Q15. What do you mean by a precipitation reaction? Explain by giving examples.

Ans: A reaction in which precipitate (an insoluble substance) is formed is called a precipitation reaction.

Example (1).

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

The white precipitate of BaSO₄ is formed by this reaction. 

Example (2). 

Na₂CO₃(aq) + CaCl₂(aq)  →  CaCO₂(S) + 2NaCl(aq) 

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Q16. Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Ans:

(a) Oxidation:  if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised.�€‹ Such reaction is known as oxidation. Here copper gains oxygen and gets oxidised.

2Cu + O₂ heat → 2CuO

(b) Reduction

CuO  + H₂ heat → Cu + H₂O

ZnO + C →  Zn + CO

Ans: 

(a) Oxidation: if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised.�€‹ Such reaction is known as oxidation.

Example1:  Copper gains oxygen and gets oxidised. 

Example2: Hydrogen gains oxygen and gets oxidised. 

(b) Reduction: If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.�€‹ Such reaction is called reduction.

Example1:

Here Copper loses oxygen and gets reduced. 

Example2: Here zinc loses oxygen and it gets reduced. 

ZnO + C → Zn + CO

Q17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour.

Name the element ‘X’ and the black coloured compound formed.

Ans: X is copper and black coloured compound is copper oxide. 

Q18. Why do we apply paint on iron articles?

Ans: We apply paint on iron articles to prevent corrosion. Paints or oil keeps away iron surfaces by contacting with air or air moisture.

Q19. Oil and fat containing food items are flushed with nitrogen. Why?

Ans: Nitrogen is an anti-oxydising agent (antioxidant). It reduces the possibility of getting oxidised of food items. So Oil and fat containing food items are flushed with nitrogen. 

Q20. Explain the following terms with one example each.

(a) Corrosion :

(b) Rancidity

Ans: 

(a) Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. Corrosion is a process of a term corrode in which metal loses in metal surface. 

Example: Rusting of iron surfaces and loses of copper's rod surface when it comes in contact of air and forms black compound copper oxide.

(b) Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change. The process of oxidising such a food material is known as Rancidity.

Example : Any oily food items which are left for a long time they get rancid. 

1. Chemical Reactions and Equations

Important Question Answers: 

Q1. Write the general Name of CaO (s).

Ans: ​ The general Name of Calcium oxide is Quick lime. 

Q2. By which physical states, reactant and product are shown in a chemical reaction.

Ans: 

 (s) – For Solid

 (g) – For gas

 (l) – For liquid

 (aq) – For Aqueous (Solution) 

Q3. What happens when a magnesium ribbon is burnt in presence of Oxygen?

Ans:  when a magnesium ribbon is burnt in presence of Oxygen, it gets converted to magnesium oxide.

Q4. Why is respiration considered an exothermic reaction? Explain.

Ans:  Respiration takes place in our body cells regularly. This is an exothermic reaction. After breaking down of obtained carbohydrate from food converts into glucose, which provides energy to our body combine with oxygen. Hence the exothermic reaction gives energy in very large quantity so the respiration is also called an exothermic reaction.

Q5. Name the reaction in which two or more than two reactants form one product to combine each other. Write a balance reaction of this reaction.

Ans: Combination reaction.

           CaO(S)    +     H₂O(l)   →   Ca(OH)₂(aq)

           (Quick lime)                         (Slaked lime)

Q6. A solution of a substance ‘X’ is used for white washing.

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

(ii)Write the reaction of the substance ‘X’ named in (i) above with water.

(iii)Write it’s an industrial use.

(iv)Name the substance which chemical formula is same as ‘X’.

Ans:

(i)‘X’ is Calcium oxide Or Quick lime. Its formula is CaO.

(ii) CaO(S)    +     H₂O(l) →  Ca(OH)₂(aq)

    (Quick lime)                     (Slaked lime)

(iii)In cement industry.

(iv)Marble.   

Q7.Write two reactions name which are the effects of oxidation reaction in everyday life.

Ans: 

(i)    Corrosion.

(ii)    Rancidity 

Q8.A white substance, which is insoluble in water, is formed by reaction between sodium sulphate and barium chloride.

(i)What is called such insoluble substance?

(ii)Name this white substance.

(iii)Show the products of this reaction with writing a balance equation.

(iv)What is called this type of reaction?

Ans: 

(i)Precipitate.

(ii)Barium sulphate (BaSO₄ )

(iii)

(iv)Double displacement reaction.

Q9.  What is rancidity?

Ans: Some fat or oil containing food materials are left for a long time, they are oxidized and become rancid. Their smell and taste change. This change is called rancidity.

Q10.With which gas, Oily or fat containing food like chips are packed to kept for a long time. Why?

Ans: Nitrogen, preventing from getting oxidized.

Q11.What do you mean by a precipitation reaction? Explain by giving examples.

Ans:  Such a reaction which produces insoluble substance. This formed insoluble substance is known as a precipitate. Any reaction that produces a precipitate is called precipitation reaction.

For example: A white substance of Barium sulphate, which is insoluble in water, is formed by reaction between sodium sulphate and barium chloride.

Q12.A shiny brown coloured element ‘X’ on heating in air becomes black in colour.

(i)Name the element ‘X’.

(ii)Name the black coloured compound formed.

 Ans: 

(i)‘X’ is Aluminium.

(ii)Al₂ O₃

Q13. Write the methods to prevent iron articles from corrosion.

Ans:

(i)By galvanization.

(ii)Painting and greasing.

(iii)Oiling and by plastic coating. 

Q14.A reddish brown element X on heating converted into a black coloured compound Y, which turns phenolphthalein pink.

(i) Identify compound ‘Y’.

(ii) Identify the element ‘X’.

(iii) Give inter conversion reaction of X and Y.

Ans: 

(i)   compound ‘Y’ is copper oxide.

(ii)   Element ‘X’ is copper.

(iii)   2 Cu   +   O₂    → Heat     2Cuo

Q15.What is rancidity? Give four methods to slow down it.

Ans: Some fat or oil containing food materials are left for a long time, they are oxidized and become rancid their smell and taste change .this change is called rancidity.

1. Chemical Reactions and Equations