NEET AIIMS BIOLOGY : Chemical Reactions and Equations

Notes on Chemical Reactions and Equations

1. Chemical reactions- The transformation of chemical substance into a new chemical substance by making and breaking of bonds between different atoms is known as Chemical Reaction.

2. Signs of a chemical reaction- These factors denote that a chemical reaction has taken place- change of state of substance, change of color of substance, evolution of heat, absorption of heat, evolution of gas and evolution of light.

3. Chemical Equation: The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. - H2 + O2 ⇒ H2O

4. Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction.

*Note- According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

5. Types of Chemical Reactions-

I. Combination- When two elements or one element and one compound or two compounds combines to give one single product.

II. Decomposition- Splitting of a compound into two or more simple products.

III. Displacement- It takes place when a more reactive metal displaces a less reactive metal.

IV. Double displacement-Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

V. Precipitation- The insoluble compound called precipitate forms in this reaction.

VI. Exothermic- Reactions which produce energy are called exothermic reaction. Most of the decomposition reactions are exothermic.

VII. Endothermic- Reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic.

VIII. Oxidation: Gain of oxygen or removal of hydrogen or metallic element from a compound is known as oxidation.

IX. Reduction: Addition of hydrogen or removal of oxygen from a compound is called reduction.

X. Redox- A chemical reactions where oxidation and reduction both take place simultaneously are also known as redox reaction.

Eg - NaOH + HCl ⇒ NaCl + H2O

6. Rusting- When iron reacts with oxygen and moisture forms a red substance called rust.

7. Rancidity- Oils and fats when get oxidized on exposure to air show a change in taste and smell.

8. Corrosion- Metals when attacked by oxygen, water, acids, gases, present in air changes its surface which is called corrosion.

NCERT Solutions for Chemical Reactions and Equations

In Text Questions

Page No: 6

1. Why should a magnesium ribbon be cleaned before it is burnt in air?

Ans: Magnesium is an very reactive metal. When stored, it reacts with oxygen to form a layer of 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 for removing this layer so that the underlying metal can be exposed to air.

2. Write the balanced equation for the following chemical reactions.

(a) Hydrogen + Chlorine → Hydrogen chloride

(b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

Ans: (a) H2 (g) + Cl2 (g) → 2HCl (g)

(b) 3BaCl2 (s) + Al2(SO4)3 (s) → 3BaSO4(s) + 2AlCl3 (s)

3. Write a balanced chemical equation with state symbols for the following reactions.

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

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

Ans: (a) BaCl2 (aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl (aq)

(b) NaOH (aq) + HCl (aq) → NaCL (aq) + H2O (l)

Page No: 10

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

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

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

Ans: (a) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

(b) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

CaO (s) + H2O (l) → Ca(OH)2 (aq)

Calcium Oxide (Quick Lime) + Water → Calcium Hydroxide (Slaked Lime)

2. 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.

Ans: Water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water the amount of hydrogen gas collected in one of the test tubes is double than that of the oxygen produced and collected in the other test tube.

Page No: 13

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

Ans: When an iron nail dipped in the copper sulphate solution than 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 involved here is:

Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Ans: 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2(aq)

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(a) 4Na (s) + O2 (g) → 2Na2O (s)

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

Ans: (a) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(b) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).

Excercise

Page No: 14

1. 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.

(d) Lead oxide is getting reduced.

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Ans: (a) (a) and (b)

2. Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(d) displacement reaction.

Ans: (d) displacement reaction.

Page No: 15

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

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(d) Iron salt and water are produced.

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

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

Ans: A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.Chemical reaction should be balanced to follow law of conservation of mass.

5. 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 sulphur dioxide.

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

Ans: (a) 3H2 (g) + N2 (g) → 2NH3 (g)

(b) 2H2S (g) + 3O2 (g) → 2H2O (l) + 2SO2 (g)

(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

6. Balance the following chemical equations.

(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O

(b) NaOH + H2SO4 → Na2SO4 + H2O

(d) BaCl2 + H2SO4 → BaSO4 + HCl

Ans: (a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O

(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

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

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

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

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

Ans: (a) Ca(OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

(d) BaCl2+ K2SO4 → BaSO4 + 2KCl

8. 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)

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

Ans: (a) 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (s): Double displacement reaction

(b) ZnCO3 (s) → ZnO (s) + CO2 (g): Decomposition reaction

(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) : Displacement Reaction

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

Ans: Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example:

C (g) + O2 (g) → CO2 + Heat Energy

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

Example:

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

Ans: Respiration is considered as an exothermic reaction because in respiration oxidation of glucose takes place which produces large amount of heat energy.

C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy

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

Ans: Decomposition reactions are those in which 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 which two or more substances combine to give a new substance with the release of energy.

For Example: Decomposition Reaction:

Combination Reaction:

CaO (s) + H2O (l) → Ca(OH)2 (aq)

Page No: 16

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

Ans: [Akhilesh]

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

Ans: In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

For Example:

CuSo4 (aq) + Zn (s) → ZnSO4 (aq) + Cu (s)

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

For Example:

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

14. 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

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

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

Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)

Sodium Carbonate + Calcium Chloride → Calcium Carbonate + Sodium Chloride

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

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

(a) Oxidation

(b) Reduction

Ans: Oxidation Reaction: It is a chemical reaction in which gain of oxygen or loss of hydrogen takes place.

Reduction Reaction: It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place.

17. 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 (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

18. Why do we apply paint on iron articles?

Ans: Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented.

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

Ans: Oil and fat containing food items flushed with nitrogen because nitrogen acts as an antioxidant and it prevent them from being oxidised.

20. Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Ans: Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

4Fe + 3O2 + nH2O → 2Fe2O3.2H2O

Rancidity is the process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.

Multiple Choice Questions For Chemical Reactions and Equations

1. The chemical formula of lead sulphate is

(a) Pb2SO4

(b) Pb(SO4)2

(d) Pb2(SO4)3

2. Which information is not conveyed by a balanced chemical equation?

(a) Physical states of reactants and products

(b) Symbols and formulae of all the substances involved in a particular reaction

(d) Whether a particular reaction is actually feasible or not

3. Chemically rust is

(a) hydrated ferrous oxide

(b) only ferric oxide

(d) none of these

4. Both CO2 and H2 gases are

(a) heavier than air

(b) colourless

(d) soluble in water

5. Which of the following gases can be used for storage of fresh sampel of an oil for a long time?

(a) Carbon dioxide or oxygen

(b) Nitrogen or helium

(d) Nitrogen or oxygen

6. The electrolytic decomposition of water gives H2 and O2 in the ratio of

(a) 1 : 2 by volume

(b) 2 : 1 by volume

(d) 1 : 2 by mass

7. In the decomposition of lead (II) nitrate to give lead (II) oxide, nitrogen dioxide and oxygen gas, the coefficient of nitrogen dioxide (in the balanced equation) is

(a) 1

(b) 2

(d) 4

8. Fatty foods become rancid due to the process of

(a) oxidation

(b) corrosion

(d) hydrogenation

9. We store silver chloride in a dark coloured bottle because it is

(a) a white solid

(b) undergoes redoc reaction

(d) none of the above

10. Silver article turns black when kept in the open for a few days due to formation of

(a) H2S

(b) AgS

(d) Ag2S

11. When crystals of lead nitrate are heated strongly in a dry test tube

(a) crystals immediately melt

(b) a brown residue is left

(d) a yellow residue is left

12. Dilute hydrochloric acid is added to granulated zinc taken in a test tube. The following observations are recorded. Point out the correct observation.

(a) The surface of metal becomes shining

(b) The reaction mixture turns milky

(d) A colourless and odourless gas is evolved

13. When carbon dioxide is passed through lime water,

(a) calcium hydroxide is formed

(b) white precipitate of CaO is formed

(d) colour of lime water disappears.

14. When a magnesium ribbon is burnt in air, the ash formed is

(a) black

(b) white

(d) pink

15. In which of the following, heat energy will be evolved?

(a) Electrolysis of water

(b) Dissolution of NH4Cl in water

(d) Decomposition of AgBr in the presence of sunlight

16. Rancidity can be prevented by

(a) adding antioxidants

(b) storing food away from light

(d) all of these

17. The reaction of H2 gas with oxygen gas to form water is an example of

(a) combination reaction

(b) redox reaction

(d) all of these reactions

18. The reaction in which two compound exchange their ions to form two new compounds is called

(a) displacement reaction

(b) combination reaction

(d) redox reaction

19. On immersing an iron nail in CuSO4solution for few minutes, you will observe

(a) no reaction takes place

(b) the colour of solution fades away

(d) the colour of solution changes to green

20. An element X on exposure to moist air turns reddish-brown and a new compound Y is formed. The substance X and Y are

(a) X = Fe, Y = Fe2O3

(b) X = Ag, Y = Ag2S

(d) X = Al, Y = Al2O3

HOTS
Chemical Reactions and Equations

1. What is a redox reaction?

2. What is corrosion? Explain its advantage and disadvantage.

3. What is rancidity? How can we reduce the problem of rancidity?

4. How is corrosion different from rusting?

5. What is meant by endothermic and exothermic reactions? Give suitable example for each.

6. Define different types of chemical reaction and give examples for each.

7. Why is photosynthesis considered as an endothermic reaction?

8. In electrolysis of water, why is the volume of gas collected over one electrode double that of the other electrode?

9. What happens when water is added to solid calcium oxide taken in a container? Write a chemical formula for the same.

10. Give one use of quick lime.

11. Give three types of decomposition reaction.

12. Name the compound used for testing CO2 gas.

Sample Paper
Chemical Reactions and Equations

Q1. What happens chemically when quick lime is added to water?

Q2. How will you test for the gas which is liberated when HCL reacts with an active metal?

Q3. What is an oxidation reaction? Is it exothermic or endothermic? Give one example of oxidation Reaction.

Q4. Give an example of photochemical reaction.

Q5. Give an example of a decomposition reaction. Describe any activity to illustrate such a reaction by heating.

Q6. Why is respiration considered as exothermic process?

Q7. Balance the following chemical equation.

Fe(s) +H2O(g) = Fe3O4 + H2(g)

MnO2 + HCL = MnCl2 + Cl2 + H2O

HNO3 + Ca(OH)2 = Ca(NO3)2 + H2O

Q8. On what basis is a chemical equation balanced?

Q9. State any two observations in an activity suggesting the occurrence of a chemical reaction.

Q10. Name a reducing agent which may be used to obtain manganese from manganese dioxide.

Q11. What change in colour is observed when silver chloride is left exposed to sunlight? Also mention the type of chemical reaction.

Q12. Define a combination reaction. Give one example of an exothermic combination reaction.

Q13. What is observed when a solution of potassium iodide is added to lead nitrate solution?

What type of reaction is this? Write a balanced chemical equation for this reaction.

Q14. Distinguish between an exothermic and an endothermic reaction.

Q15. Distinguish between a displacement and a double displacement reaction.

Q16. Identify the type of reaction in the following:

Fe + CuSO4(aq) = FeSO4(aq) + Cu(s)

2H2 + O2 = 2H2O

Chemical Reaction and Equations Test

Maximum marks- 30

Maximum time- 45 minutes

1. Explain the process of corrosion and rusting. (2 marks)

2. How is exothermic reaction different from an endothermic reaction? (3 marks)

3. Explain four different types of chemical reaction with suitable examples. (8 marks)

4. How do we balance a chemical equation? (4 marks)

5. Why are oil and fat containing food items flushed with nitrogen? (3 marks)

6. What is the difference between displacement and double displacement reaction? Give suitable chemical equations for the same. (3 marks)

7. Give two uses of quick lime. (2 marks)

8. What is electrolytic decomposition? (2 marks)

9. Identify the following type of reactions:-

▪ Na2SO4 + BaCL2 = BaSO4 + 2NaCL

▪ CaCO3 = CaO + CO2

▪ Fe + CuSO4 = FeSO4 + Cu (3 marks)

NEET AIIMS BIOLOGY

Saturday, 6 July 2019

Chemical Reactions and Equations

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