Unit 1: Chemical Equations

                  CHEMICAL EQUATIONS

Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.

Reactant and Product:

Reactant: Substances which take part in a chemical reaction are called reactants.

Product: New substance formed after a chemical reaction is called product.

Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.

Example: A + B ⇨ C + D

In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.

When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.

Hydrogen + Oxygen ⇨ Water

H2 + O2 ⇨ H2O

In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations.

Chemical Equation is a way to represent the chemical reaction in concise and informative way.

Chemical equation can be divided into two types –

Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.

Example: Zn + H2SO4 ⇨ ZnSO4 + H2

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.

Law of Conservation of Mass;- 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.

Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.

Example: Fe + H2O ⇨ Fe3O4 + H2

Balancing a chemical equation:

To balance the given or any chemical equation, follow these steps:

Fe + H2O ⇨ Fe3O4 + H2

Write the number of atoms of elements present in reactants and in products in a table; as shown here.

Balance the atom which is the maximum in number; on either side of chemical equation.

In this equation, the number of oxygen atom is the maximum on the RHS.

To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.

Fe + 4 x H2O ⇨ Fe3O4 + H2

Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.

Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2

After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.

3 x Fe + 4 x H2O ⇨ Fe3O4 + 4 x H2

Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation. After balancing, the above equation can be written as follows.

3Fe + 4H2O ⇨ Fe3O4 + 4H2

TYPES OF CHEMICAL REACTIONS

Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

AB + CD ⇨ AC + BD

Example: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.

BaCl2 + Na2SO4 ⇨ BaSO4 + 2NaCl

Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.

Exothermic and Endothermic Reaction; Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.

Oxidation and Reduction Reaction:

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.

Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.

Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.

Reducing agent: Compounds or elements which can cause reduction are called reducing agents.

In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.

Example: When iron reacts with air, it forms iron oxide (rust)

4Fe + 3O2 ⇨ 2Fe2O3

In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent.

When cupric oxide reacts with hydrogen, it gives copper and water.

CuO + H2 ⇨ Cu + H2O

In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.

When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.

NaOH + HCl ⇨ NaCl + H2O

In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent.

In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.

Significance of Oxidation Reduction in Everyday Life:

1.Respiration is oxidation reaction in which food is oxidized to produce energy.

2.Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.

3.Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.

4.Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.

5.Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.

6.We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.

                             TEXTUAL QUESTIONS

1. Why should a magnesium ribbon be cleaned before it is burnt in air?
Answer-Magnesium is a 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.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer-(i) H2 (g) + Cl2 (g) → 2HCl (g)
(ii) 3BaCl2 (s) + Al2(SO4)3 (s) → 3BaSO4 (s) + 2AlCl3 (s)
(iii) 2Na(s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
3. Write a balanced chemical equation 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 solution and water.
Answer-(i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCL (aq) + H2O (l)
1. 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.
Answer-(i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

(ii) 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.

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

Answer-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.

Answer-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.
(i) 4Na (s) + O2 (g) → 2Na2O (s)
(ii) CuO (s) + H2 (g) → Cu (s) + H2O (l)
Answer-(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).

EXERCISE

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

Answer-(a) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) 2H2S (g) + 3O2 (g) → 2H2O (l) + 2SO2 (g)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 (s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)
6. Balance the following chemical equations.
(i) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(ii) NaOH + H2SO4 → Na2SO4 + H2O
(iii) NaCl + AgNO3 → AgCl + NaNO3
(iv) BaCl2 + H2SO4 → BaSO4 + HCl
Answer-(i) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(ii) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(iii) NaCl + AgNO3 → AgCl + NaNO3
(iv) BaCl2 + H2SO4 → BaSO4 + 2HCl
7. 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
Answer-(a) Ca(OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2 → 2AlCl3 + 3Cu
(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)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Answer-(a) 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (s): Double displacement reaction

(b) ZnCO3 (s) →  ZnO (s) + CO2 (g): Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl (g): Combination reaction
(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) : Displacement Reaction
9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer-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.

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

Answer

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

Answer-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.
Answer-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.
Answer-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
Answer-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.
Answer-‘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?

Answer-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?
Answer-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

Answer-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.nH2O
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.

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.
(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
► (i) (a) and (b)
2. 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.
► (d) displacement reaction

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.
(c) No reaction takes place.
(d) Iron salt and water are produced.

► (a) Hydrogen gas and iron chloride are produced.