Ideal Gas Laws
1. The Ideal Gas Equation
PV=nRT

P = pressure (in Pa)

V = volume (in m³)

n = number of moles

R = gas constant = 8.31 J mol⁻¹ K⁻¹

T = temperature (in K)
5. Assumptions of the Ideal Gas Model

Gas particles have negligible volume.

No intermolecular forces.

Collisions between particles are elastic.

Particles move in random, constant motion.
6. Real vs Ideal Gases

Ideal gas law works best at high temperature and low pressure.

Real gases deviate due to intermolecular forces and particle volume.
GROUP-17
� Group 17 – The Halogens (9701 AS-Level Summary)
Group 17 Elements:
Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At)
🔬 Physical Properties
Element State at r.t.p
Colour
Volatility (boiling point ↓
down group)
F₂
Pale yellow gas Very pale yellow
Highest
Cl₂
Green gas
Greenish-yellow
↓
Br₂
Brown liquid
Reddish-brown
↓
I₂
Grey solid
Sublimes to purple vapour
↓
➡ Trend:

Boiling point increases down the group due to increasing Van der Waals’ forces (larger molecules
→ more electrons → stronger intermolecular forces).

Volatility decreases down the group.
⚛️ Electronegativity Trend

Decreases down the group.

Due to increasing atomic radius and shielding → less effective nuclear attraction for bonding
electrons.
� Reactivity & Displacement Reactions
Trend: Reactivity decreases down the group.
A more reactive halogen displaces a less reactive halide ion from solution:
� Halide Ion Tests – Silver Nitrate
Add dilute nitric acid, then AgNO₃(aq):
Halide Ion Precipitate with AgNO₃ Colour Solubility in NH₃
⚗️ Reactions of Halide Ions with Sulfuric Acid
🔥 Trend: Reducing ability of halide ions increases down the group.
I⁻ is strong enough to reduce H₂SO₄ to H₂S.
🔥 Disproportionation Reactions
A reaction where a species is both oxidised and reduced.
1. Chlorine with Cold Dilute NaOH:
(Cl⁰ → Cl⁻ and Cl⁺1)
→ NaClO = bleach (oxidising agent)
2. Chlorine in Water:

HClO is a weak acid and kills bacteria (used in water treatment).
⚗️ Uses of Halogens & Halides

Chlorine: Water sterilisation, bleach manufacture.

Fluoride ions (F⁻): Added to toothpaste/water to prevent tooth decay.

Halide salts: Used in photographic film (AgBr).
� Common Exam Points (with Model Answers)
Q: Explain why the boiling point of iodine is higher than that of chlorine.
A:
Iodine has more electrons than chlorine, so it has stronger van der Waals’ forces between molecules.
Therefore, more energy is needed to separate the molecules, giving it a higher boiling point.
🔬 Group 2 Elements: The Alkaline Earth Metals (Be → Ba)
🔹 General Trends in Physical Properties (Top to Bottom):
Property
Trend
Explanation
Atomic radius
Increases
More shells added
First ionisation energy Decreases
Outer electrons further from nucleus; more shielding
Melting point
Generally decreases Weaker metallic bonding (except Mg which is anomalous)
Density
Increases
More mass per volume (with some irregularities)
🔹 Reactivity Trends

Increases down the group
E.g. Ba reacts more vigorously with water than Mg.
🔹 Reaction with Water
General equation:

Mg reacts slowly with cold water but rapidly with steam.

Ca → Ba react increasingly vigorously with cold water.
🔹 Hydroxide Solubility (M(OH)₂)

Increases down the group
o
Mg(OH)₂: sparingly soluble (used in milk of magnesia)
o
Ba(OH)₂: very soluble → strongly alkaline
🔹 Sulfate Solubility (MSO₄)

Decreases down the group
o
BaSO₄: insoluble, used in medical "barium meals" (visible in X-rays)
o
Used in test for sulfate ions:
🔹 Thermal Decomposition of Nitrates and Carbonates
✅ Carbonates (MCO₃):
More heat needed down the group (more stable).

Smaller cations polarise CO₃²⁻ more → easier to decompose.
✅ Nitrates (M(NO₃)₂):

Brown gas (NO₂) forms.

Same trend: more heat needed down group.
🔹 Flame Tests

Ca: Brick red

Sr: Red

Ba: Apple green

(Mg and Be: no colour – electrons too tightly held)
🔍 Common Exam Questions:
1. Compare reactions of Mg and Ba with water (speed, alkalinity).
2. Describe thermal decomposition of Group 2 carbonates.
3. Explain solubility trends for hydroxides and sulfates.
4. Write ionic equations for testing sulfate with Ba²⁺.
5. Identify unknowns using flame test + solubility properties.