This MCQ module is based on: Reactivity Series and Ionic Compounds
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Reactivity Series and Ionic Compounds
🎓 Class 10
Science
CBSE
Theory
Ch 3 — Metals and Non-metals
⏱ ~19 min
🌐 Language: [gtranslate]
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3.2.5 The Reactivity (Activity) Series
Based on the observed reactions of metals with water, acids and salt solutions (Part 1), chemists have arranged metals in a vertical list called the reactivity (activity) series. The metal at the top loses electrons most easily; the one at the bottom the least.
| Metal(s) | Reacts with cold H2O? | Reacts with dilute HCl? |
|---|---|---|
| K, Na, Ca | Yes — vigorously | Yes — explosive/dangerous |
| Mg | Only hot water | Yes |
| Al, Zn, Fe | Only steam | Yes |
| Cu, Hg, Ag, Au, Pt | No | No |
3.3 How Do Metals and Non-metals React?
The noble gases (Ne, Ar, Kr...) are famously unreactive because their outermost shells are already full (8 electrons — the octet). All other atoms attempt to achieve such a stable noble gas configuration. They can do this either by losing or gaining electrons.
Metals Lose Electrons — Cations
Metals have 1, 2 or 3 electrons in their outermost shell. It is energetically easier to lose them:
\(\text{Na} \rightarrow \text{Na}^+ + e^-\) (2,8,1 → 2,8)
\(\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-\) (2,8,2 → 2,8)
\(\text{Al} \rightarrow \text{Al}^{3+} + 3e^-\) (2,8,3 → 2,8)
Non-metals Gain Electrons — Anions
Non-metals have 5, 6 or 7 electrons in their outermost shell. It is easier to gain a few to complete the octet:
\(\text{Cl} + e^- \rightarrow \text{Cl}^-\) (2,8,7 → 2,8,8)
\(\text{O} + 2e^- \rightarrow \text{O}^{2-}\) (2,6 → 2,8)
Ionic (electrovalent) bond: The strong electrostatic force of attraction between oppositely charged ions formed by complete transfer of one or more electrons from a metal atom to a non-metal atom.
More Examples
MgO: Mg → Mg2+ + 2e−; O + 2e− → O2−. One Mg gives 2 electrons; one O accepts 2.
MgCl2: Mg → Mg2+ + 2e−; 2Cl + 2e− → 2Cl−. Mg needs 2 chlorines to dispose of its 2 electrons.
CaCl2: Ca2+ + 2Cl−; Na2O: 2Na+ + O2−; AlCl3: Al3+ + 3Cl−
The NaCl Crystal Lattice
In solid NaCl, each Na+ ion is surrounded by six Cl− ions and each Cl− by six Na+ ions in a giant, orderly cubic lattice. There are no discrete "NaCl molecules" — the formula merely gives the smallest ratio.
3.3.1 Properties of Ionic Compounds
| Property | Reason |
|---|---|
| Hard crystalline solids, brittle | Strong electrostatic forces arrange ions in a rigid lattice. When layers slide, like charges repel, cracking the crystal. |
| High melting and boiling points | Large amount of energy needed to overcome strong interionic forces. |
| Soluble in water, insoluble in kerosene/petrol | Polar water molecules can separate and surround the ions; non-polar solvents cannot. |
| Conduct electricity when molten or dissolved, NOT as solid | Ions are locked in the lattice in the solid state. On melting or dissolution they become free to move, carrying charge. |
Quick MP data: NaCl = 801°C; MgO = 2852°C — ionic compounds all have sharply higher melting points than simple covalent substances like ice (0°C) or sugar (186°C).
Activity — Testing Properties of Common SaltL3 Apply
Predict: Will a small NaCl crystal conduct electricity in a dry cell? What about in a glass of water with the same circuit?
- Take a few NaCl crystals and attempt to light a bulb with them between two electrodes.
- Now dissolve the NaCl in water and repeat.
- Try placing the crystals between the electrodes again after melting at about 800°C.
Observations: The solid crystals do NOT conduct — bulb stays off. The salt solution and molten salt DO conduct — the bulb glows. Conclusion: Ionic conduction requires freely mobile ions, obtained only in the molten or aqueous state.
Worked Examples
Example 1 — Draw the Lewis (electron dot) structure for NaCl.
Solution: Na has configuration 2,8,1 and Cl has 2,8,7. Na donates its single outer electron to Cl:
Na+ [ :Cl̤̇: ]− (Cl now has 8 dots)
Example 2 — Predict the formula of the ionic compound between Al and O.
Solution: Al forms Al3+, O forms O2−. Cross the charges: 2 Al3+ and 3 O2−. Formula: Al2O3.
Example 3 — Show the formation of CaO with electron transfer.
Solution:
Ca (2,8,8,2) → Ca2+ (2,8,8) + 2e−
O (2,6) + 2e− → O2− (2,8)
Ca2+ + O2− → CaO
Example 4 — Why is NaCl a hard but brittle solid?
Solution: The strong electrostatic attraction between Na+ and Cl− holds the lattice together tightly (hardness). If a force shifts one layer by a half-unit, similar charges now face each other and repel strongly — the crystal cleaves (brittleness).
Example 5 — Why does molten KCl conduct electricity but solid KCl does not?
Solution: In the solid lattice, K+ and Cl− are held in fixed positions. In the molten state the lattice breaks down and ions become mobile; they drift towards the oppositely charged electrodes, carrying current.
Example 6 — Compare ionic compounds and covalent compounds in MP and solubility.
Solution: Ionic compounds (e.g., NaCl, 801°C) have much higher MPs than simple covalent compounds (e.g., naphthalene, 80°C) because more energy is needed to break the strong ionic lattice. Ionic compounds dissolve readily in polar solvents (water) and are insoluble in non-polar solvents (kerosene); covalent compounds usually show the reverse.
Example 7 — Predict the formula of magnesium nitride.
Solution: Mg forms Mg2+; N forms N3−. To balance charges, take 3 Mg2+ (+6) and 2 N3− (−6). Formula: Mg3N2.
Interactive: Ionic Compound Builder L3 Apply
Pick a metal cation and a non-metal anion. The tool will balance the charges and print the formula.
Competency-Based Questions
Four metals W, X, Y, Z are tested against water and dilute HCl. W reacts explosively with cold water. X reacts with hot water only. Y reacts only with dilute HCl. Z shows no reaction with either.
Q1. L4 Analyse Arrange W, X, Y, Z in decreasing order of reactivity. (1 mark)
W > X > Y > Z.
Q2. L2 Understand Which metal(s) could Z plausibly be? (1 mark)
D. Copper, silver, gold — metals below hydrogen in the activity series do not react with water or dilute HCl.
Q3. L3 Apply Show with electron dot structures the formation of MgCl2. (3 marks)
Mg (2,8,2) loses 2e− to become Mg2+. Each of two Cl atoms (2,8,7) gains 1e− to become Cl− with octet. Dot structure: Mg2+[:Cl:]−2. Balanced equation: Mg + Cl2 → MgCl2.
Q4. L4 Analyse Ionic solids do not conduct electricity but ionic solutions do. Explain. (2 marks)
Electrical conduction in an ionic compound is due to the movement of ions. In the solid state, ions are held in fixed positions in the lattice and cannot migrate; in the molten state or in aqueous solution the lattice breaks up, the ions become free to drift towards the electrodes, and current flows.
Q5. L5 Evaluate Which would have a higher melting point, NaCl or MgO? Justify. (2 marks)
MgO has the higher melting point. Mg2+ and O2− carry double the charge of Na+ and Cl−; the electrostatic force is proportional to the product of charges, so the lattice energy of MgO is much greater and more heat is required to break it. (NaCl ~801°C; MgO ~2852°C.)
Assertion-Reason Questions
Assertion (A): Ionic compounds are brittle.
Reason (R): When a layer slides, like charges come adjacent and repel, cracking the crystal.
A.
Assertion (A): Sodium chloride is soluble in kerosene.
Reason (R): Ionic compounds are soluble only in polar solvents like water.
D. A is false (NaCl is NOT soluble in kerosene). R is true.
Assertion (A): Copper lies below hydrogen in the reactivity series.
Reason (R): Copper cannot displace H2 from dilute HCl.
A. Cu being below H is the reason it cannot displace H2 from acid.
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