LANGUAGE OF CHEMISTRY - Questions & Answers

EXERCISE

Introduction

1. Explain the term 'symbol'. State a reason why - the symbol of calcium is 'Ca' & of copper is 'Cu'.
A symbol is the short form or abbreviated name of an element. It represents a specific element or one atom of an element. The symbol for Calcium is 'Ca' because it uses the first two letters of its English name to distinguish it from Carbon (C). The symbol for Copper is 'Cu' because it is derived from its Latin name 'Cuprum'.

Valency

2. Define the term 'valency'. With reference to water & ammonia as compounds respectively, state the valency of oxygen & nitrogen. Magnesium [2, 8, 2] has valency 2⁺. Give reasons.
Valency is the number of hydrogen atoms which can combine with [or displace] one atom of the element [or radical] to form a compound. In water (H₂O), two atoms of hydrogen combine with one atom of oxygen, so the valency of oxygen is 2. In ammonia (NH₃), three atoms of hydrogen combine with one atom of nitrogen, so the valency of nitrogen is 3. Magnesium [2, 8, 2] has a valency of 2⁺ because it has 2 electrons in its outer shell which it loses during a chemical reaction to achieve a stable electronic configuration.

3. Explain the term 'variable valency'. Copper having electronic configuration 2, 8, 18, 1 exhibits variable valency. Give a reason for the same & name the compound CuCl & CuCl₂.
Certain elements exhibit more than one valency; this property is called variable valency. It occurs because sometimes electrons from the penultimate shell (the shell before the outermost shell) also jump to the outermost shell and participate in chemical reactions. In Copper, the outermost shell has 1 electron and the penultimate shell has 18 electrons. Since the penultimate shell is not stable, one electron can jump to the outer shell, increasing the valency from 1 to 2.
CuCl: Cuprous chloride [Copper (I) chloride]
CuCl₂: Cupric chloride [Copper (II) chloride]

4. State the valencies of the following metallic elements - a] Potassium b] Sodium c] Calcium d] Magnesium e] Zinc f] Aluminium g] Chromium [write each symbol with the valency]
a] Potassium: K¹⁺
b] Sodium: Na¹⁺
c] Calcium: Ca²⁺
d] Magnesium: Mg²⁺
e] Zinc: Zn²⁺
f] Aluminium: Al³⁺
g] Chromium: Cr³⁺

5. Certain metals exhibit variable valencies which include valencies: 1⁺, 2⁺, 3⁺ & 4⁺. State the variable valency of the following metals - a] Copper b] Silver c] Mercury d] Iron e] Tin f] Lead - [write each symbol with the variable valency]
a] Copper: Cu^{1+, 2+}
b] Silver: Ag^{1+, 2+}
c] Mercury: Hg^{1+, 2+}
d] Iron: Fe^{2+, 3+}
e] Tin: Sn^{2+, 4+}
f] Lead: Pb^{2+, 4+}

6. State which of the following ions or radicals given below of non-metallic elements exhibit valency: 1^-, 2^- & 3^- - a] Chloride b] Bromide c] Iodide d] Nitrate e] Hydroxide f] Bicarbonate g] Bisulphite h] Bisulphate i] Aluminate j] Permanganate k] Oxide l] Sulphide m] Sulphite n] Sulphate o] Carbonate p] Dichromate q] Zincate r] Plumbite s] Phosphate t] Nitride - [write each ion or radical with the correct valency]
Valency 1^-: Chloride (Cl^1-), Bromide (Br^1-), Iodide (I^1-), Nitrate (NO₃^1-), Hydroxide (OH^1-), Bicarbonate (HCO₃^1-), Bisulphite (HSO₃^1-), Bisulphate (HSO₄^1-), Aluminate (AlO₂^1-), Permanganate (MnO₄^1-).
Valency 2^-: Oxide (O^2-), Sulphide (S^2-), Sulphite (SO₃^2-), Sulphate (SO₄^2-), Carbonate (CO₃^2-), Dichromate (Cr₂O₇^2-), Zincate (ZnO₂^2-), Plumbite (PbO₂^2-).
Valency 3^-: Phosphate (PO₄^3-), Nitride (N^3-).

7. Differentiate between the terms - 'Ion' & 'radical' with suitable examples.
An Ion is an atom or a group of atoms carrying a positive or negative charge due to the loss or gain of electrons (e.g., Na¹⁺, Cl^1-). A Radical is a group of atoms of different elements that behave like a single unit and show a specific valency (e.g., Ammonium radical NH₄¹⁺, Carbonate radical CO₃^2-).

Chemical formula

8. Write the chemical formula of the following compounds in a step-by-step manner - a] Potassium chloride b] Sodium bromide c] Potassium nitrate d] Calcium hydroxide e] Calcium bicarbonate f] Sodium bisulphate g] Potassium sulphate h] Zinc hydroxide i] Potassium permanganate j] Potassium dichromate k] Aluminium hydroxide l] Magnesium nitride m] Sodium zincate n] Copper [II] oxide o] Copper [I] sulphide p] Iron [III] chloride q] Iron [II] hydroxide r] Iron [III] sulphide s] Iron [II] oxide.
a] Potassium chloride: KCl
b] Sodium bromide: NaBr
c] Potassium nitrate: KNO₃
d] Calcium hydroxide: Ca(OH)₂
e] Calcium bicarbonate: Ca(HCO₃)₂
f] Sodium bisulphate: NaHSO₄
g] Potassium sulphate: K₂SO₄
h] Zinc hydroxide: Zn(OH)₂
i] Potassium permanganate: KMnO₄
j] Potassium dichromate: K₂Cr₂O₇
k] Aluminium hydroxide: Al(OH)₃
l] Magnesium nitride: Mg₃N₂
m] Sodium zincate: Na₂ZnO₂
n] Copper [II] oxide: CuO
o] Copper [I] sulphide: Cu₂S
p] Iron [III] chloride: FeCl₃
q] Iron [II] hydroxide: Fe(OH)₂
r] Iron [III] sulphide: Fe₂S₃
s] Iron [II] oxide: FeO

Chemical equations

9. What is a chemical equation. How is it represented. Differentiate between a 'word equation' and a 'molecular equation' with a suitable example.
A chemical equation is a shorthand form representing the result of a chemical change using symbols and formulas. It is represented with reactants on the left-hand side and products on the right-hand side.
Word Equation: Uses the names of the substances (e.g., Zinc + Sulphuric acid → Zinc sulphate + Hydrogen).
Molecular Equation: Uses the chemical formulas and symbols (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂).

10. State the information provided by a chemical equation. Chemical equations suffer from a number of limitations. State the main limitations of a chemical equation.
Information provided: The formulas and symbols of reactants and products, the physical state (if specified), special conditions (heat, catalyst), and the ratio in which substances react.
Limitations: It does not naturally tell us the physical states, conditions (temperature/pressure), concentration, nature of reaction, speed of reaction, heat changes, or the completion of the reaction unless explicitly added as additional information.

11. State what is a balanced equation with a relevant example. Give a reason why an equation is balanced with reference to the law of conservation of matter.
A balanced equation is one in which the number of atoms of each element on the reactant side is equal to the number of atoms of each element on the product side.
Example: Mg + H₂SO₄ → MgSO₄ + H₂.
Equations are balanced to comply with the Law of Conservation of Matter, which states that matter is neither created nor destroyed during a chemical change; hence, the total mass of reactants must equal the total mass of products.

12. Write balanced molecular equations for the following word equations:
a] Calcium + oxygen → Calcium oxide
b] Calcium + water → Calcium hydroxide + hydrogen
c] Zinc + sulphuric acid → Zinc sulphate + hydrogen
d] Lead sulphate + ammonium hydroxide → Ammonium sulphate + lead hydroxide
e] Copper hydroxide + nitric acid → Copper nitrate + water
f] Lead nitrate + sodium chloride → Sodium nitrate + lead chloride
a] 2Ca + O₂ → 2CaO
b] Ca + 2H₂O → Ca(OH)₂ + H₂
c] Zn + H₂SO₄ → ZnSO₄ + H₂
d] PbSO₄ + 2NH₄OH → (NH₄)₂SO₄ + Pb(OH)₂
e] Cu(OH)₂ + 2HNO₃ → Cu(NO₃)₂ + 2H₂O
f] Pb(NO₃)₂ + 2NaCl → 2NaNO₃ + PbCl₂

13. Balance the following equations:
a] P + O₂ → P₂O₅
b] Na₂O + H₂O → NaOH
c] K + H₂O → KOH + H₂
d] Fe + H₂O → Fe₃O₄ + H₂
e] CaO + HCl → CaCl₂ + H₂O
f] Fe + Cl₂ → FeCl₃
g] Al + H₂O → Al₂O₃ + H₂
h] Al + H₂SO₄ → Al₂(SO₄)₃ + H₂
i] Fe₂O₃ + H₂ → Fe + H₂O
j] C + H₂SO₄ → CO₂ + H₂O + SO₂
k] Pb₃O₄ → PbO + O₂
l] Al + O₂ → Al₂O₃
m] NO + O₂ → NO₂
n] ZnS + O₂ → ZnO + SO₂
o] Pb₃O₄ + HCl → PbCl₂ + H₂O + Cl₂
p] ZnO + NaOH → Na₂ZnO₂ + H₂O
q] H₂S + Cl₂ → S + HCl
r] FeCl₃ + NaOH → NaCl + Fe(OH)₃
s] Fe₂O₃ + CO → Fe + CO₂
t] KHCO₃ → K₂CO₃ + H₂O + CO₂
u] CuO + NH₃ → Cu + H₂O + N₂
a] 4P + 5O₂ → 2P₂O₅
b] Na₂O + H₂O → 2NaOH
c] 2K + 2H₂O → 2KOH + H₂
d] 3Fe + 4H₂O → Fe₃O₄ + 4H₂
e] CaO + 2HCl → CaCl₂ + H₂O
f] 2Fe + 3Cl₂ → 2FeCl₃
g] 2Al + 3H₂O → Al₂O₃ + 3H₂
h] 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
i] Fe₂O₃ + 3H₂ → 2Fe + 3H₂O
j] C + 2H₂SO₄ → CO₂ + 2H₂O + 2SO₂
k] 2Pb₃O₄ → 6PbO + O₂
l] 4Al + 3O₂ → 2Al₂O₃
m] 2NO + O₂ → 2NO₂
n] 2ZnS + 3O₂ → 2ZnO + 2SO₂
o] Pb₃O₄ + 8HCl → 3PbCl₂ + 4H₂O + Cl₂
p] ZnO + 2NaOH → Na₂ZnO₂ + H₂O
q] H₂S + Cl₂ → S + 2HCl
r] FeCl₃ + 3NaOH → 3NaCl + Fe(OH)₃
s] Fe₂O₃ + 3CO → 2Fe + 3CO₂
t] 2KHCO₃ → K₂CO₃ + H₂O + CO₂
u] 3CuO + 2NH₃ → 3Cu + 3H₂O + N₂

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Q.1 Complete the statements given below by filling in the blank with the correct word/s.
1. The formula of silver chloride is AgCl - [AgCl/ AgCl₂].
2. The basic unit of an element is a/an atom [molecule / atom / ion]
3. Atom contains nucleus [neutron/ nucleus], with positively charged protons [electrons/ protons].
4. Element lead [calcium/lead/ carbon] has the symbol derived from its Latin name 'plumbum'.
5. From the elements - He, Br, Pt & O; the element which forms a polyatomic molecule is Oxygen & which is liquid at room temperature is Bromine.
6. The valency of iron in FeO is 2⁺ [ 2⁺ / 3⁺], of chlorine [chloride] in CaCl₂ is 1^- [1^-/2^-] and of dichromate in K₂Cr₂O₇ is 2^- [2⁺/2^-].

Q.2 Match the statements - 1 to 10 below with their correct answers from - A to J.
1. Elements having valency of two: B: Divalent
2. An anion: A: Br^1-
3. A gaseous non-metal: F: Nitrogen
4. A cation: J: K¹⁺
5. The term used for the substances which take part in the chemical reaction: C: Reactants
6. The meaning of the symbol 'Δ' over the arrow in a chemical equation: I: Heat required
7. The chemical name for nitrogen monoxide: E: Nitric oxide
8. A radical containing nitrogen & hydrogen only: D: Ammonium
9. The chemical name for dinitrogen oxide: H: Nitrous oxide
10. The valency of noble gases: G: Zero

Q.3 Match the compounds in List I - 1 to 20 with their correct formulas in List II - A to T.
1. Copper [I] sulphide: M: Cu₂S
2. Potassium permanganate: A: KMnO₄
3. Phosphoric acid: L: H₃PO₄
4. Copper [II] oxide: S: CuO
5. Carbonic acid: G: H₂CO₃
6. Aluminium sulphide: H: Al₂S₃
7. Iron [II] oxide: P: FeO
8. Iron [III] sulphide: K: Fe₂S₃
9. Iron [II] sulphate: Q: FeSO₄
10. Sodium zincate: E: Na₂ZnO₂
11. Nitrous oxide: F: N₂O
12. Aluminium sulphate: D: Al₂(SO₄)₃
13. Magnesium nitride: B: Mg₃N₂
14. Iron [III] sulphate: R: Fe₂(SO₄)₃
15. Copper [I] oxide: T: Cu₂O
16. Iron [III] oxide: O: Fe₂O₃
17. Nitric oxide: I: NO
18. Copper [II] sulphide: N: CuS
19. Iron [II] sulphide: J: FeS
20. Magnesium nitrate: C: Mg(NO₃)₂

Q.4 Underline the incorrectly balanced compound in each equation & rewrite the correct equation.
1. 2Na + 3H₂O → 2NaOH + H₂
Correct Equation: 2Na + 2H₂O → 2NaOH + H₂

2. 4P + 4O₂ → 2P₂O₅
Correct Equation: 4P + 5O₂ → 2P₂O₅

3. Fe₂O₃ + 2H₂ → 2Fe + 3H₂O
Correct Equation: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O

4. 2Al + 2H₂SO₄ → Al₂(SO₄)₃ + 3H₂
Correct Equation: 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂

5. N₂ + 3H₂ ↔ NH₃
Correct Equation: N₂ + 3H₂ ↔ 2NH₃

6. ZnO + 3NaOH → Na₂ZnO₂ + H₂O
Correct Equation: ZnO + 2NaOH → Na₂ZnO₂ + H₂O

7. FeCl₃ + 3NH₄OH → 2NH₄Cl + Fe(OH)₃
Correct Equation: FeCl₃ + 3NH₄OH → 3NH₄Cl + Fe(OH)₃

8. FeS + 2HCl → 2FeCl₂ + H₂S
Correct Equation: FeS + 2HCl → FeCl₂ + H₂S

9. 3NH₃ + H₂SO₄ → (NH₄)₂SO₄
Correct Equation: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄

10. PbO₂ + 4HCl → PbCl₂ + H₂O + Cl₂
Correct Equation: PbO₂ + 4HCl → PbCl₂ + 2H₂O + Cl₂