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HSC Chemistry & Laboratory Principles

Fundamental quantum principles, bonding theories, equilibrium laws, and organic reaction mechanisms for Bangladesh HSC Chemistry.

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Anomalous Ground-State Electronic Configurations of Chromium and Copper

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Anomalous Ground-State Electronic Configurations of Chromium and Copper

Cr (

Z = 24

): [Ar] 3d^5 4s^1 (half-filled stability); Cu (

Z = 29

): [Ar] 3d^{10} 4s^1 (fully-filled stability).

Rydberg Formula for Hydrogen Spectral Lines

\frac{1}{λ} = R_{H} (\frac{1}{n _{1}^{2}} - \frac{1}{n _{2}^{2}})

. L y man (

n_1 = 1

, U V); B a l m er (

n_1 = 2

, v i s ib l e); P a sc h e n (

n_1 = 3$, IR).

Pauli Exclusion Principle vs Hund's Rule

Pauli: No two electrons in an atom have identical sets of all 4 quantum numbers. Hund: Orbitals of equal energy are occupied singly with parallel spins before pairing.

VSEPR Shapes and Bond Angles of CH4, NH3, and H2O

CH4: Tetrahedral (109.5 deg); NH3: Trigonal pyramidal (107.0 deg); H2O: Bent (104.5 deg) (all sp^3 hybridized).

Hybridization and Geometry of PCl5 and SF6

PCl5: sp^3d, trigonal bipyramidal (axial bonds longer than equatorial); SF6: sp^3d^2, regular octahedral (all 90 deg).

Element with Highest Electronegativity vs Highest Electron Affinity

Highest electronegativity: Fluorine (F, 3.98). Highest first electron affinity: Chlorine (Cl, -349 kJ/mol) due to less inter-electronic repulsion in 3p than 2p.

Why Liquid Water Has Higher Boiling Point Than H2S

Water molecules form extensive intermolecular hydrogen bonds due to small, highly electronegative oxygen atoms; H2S has only weak dipole/dispersion forces.

Relation Between Equilibrium Constants Kp and Kc

K_{p} = K_{c} (R T)^{Δ n}

w h er e

\Delta n = \Sigma n_{\text{gas products}} - \Sigma n_{\text{gas reactants}}$.

Le Chatelier's Effect of Pressure on Haber Ammonia Synthesis

N_{2} (g) + 3 H_{2} (g) ⇌ 2 N H_{3} (g)

$: Reactants have 4 moles of gas, products have 2 moles. Increasing pressure shifts equilibrium forward to produce more NH3.

Henderson-Hasselbalch Equation for Acidic Buffers

pH = p K_{a} + lo g (\frac{[ conjugate base / salt ]}{[ weak acid ]})

Solubility Product Relationship for an AB2 Salt (e.g. PbCl2)

K_{sp} = [A^{2 +}] [B^{-}]^{2} = (s) (2 s)^{2} = 4 s^{3}

w h er e

s$ is the molar solubility in mol/L.

Arrhenius Equation for Temperature Dependence of Reaction Rate

k = A e^{- E_{a} / (R T)}

or

ln k = ln A - \frac{E _{a}}{R T}

. A pl o t o f

\ln k

v s

1/T$ has slope $

- E_{a} / R

Definition and Threshold of Acid Rain

Precipitation with pH < 5.6 caused primarily by sulfur dioxide (

S O_{2}

) and nitrogen oxides (

N O_{x}

) forming

H_{2} S O_{4}

and

H N O_{3}

Biochemical Oxygen Demand (BOD) vs Chemical Oxygen Demand (COD)

BOD: Dissolved oxygen needed by aerobic microbes to degrade organic matter over 5 days at 20 deg C. COD: Oxygen required to chemically oxidize all organic matter using strong

K_{2} C r_{2} O_{7}

. COD >= BOD always.

Bangladesh Drinking Water Standard for Arsenic vs WHO Guideline

Bangladesh Department of Environment (DoE) standard: 0.05 mg/L (50 ppb); WHO international guideline: 0.01 mg/L (10 ppb).

Role of Chlorofluorocarbons (CFCs) in Stratospheric Ozone Depletion

UV light photolyzes CFCs releasing free **chlorine radicals (

C l^{∙}

)**, which catalytically destroy

O_{3}

molecules in self-propagating chain reactions.

Markovnikov's Rule vs Peroxide Effect (Kharasch Addition)

Markovnikov: Electrophilic

H^{+}

adds to double-bonded carbon with more hydrogens (carbocation intermediate). Peroxide Effect: Free-radical addition of HBr yields anti-Markovnikov 1-bromoalkane.

Ozonolysis Products of 2-Butene (

C H_{3} - C H = C H - C H_{3}

)

Reductive ozonolysis (

O_{3}

then

Z n / H_{2} O

) cleaves the

C = C

bond yielding **two molecules of Acetaldehyde (Ethanal,

C H_{3} C H O

)**.

Distinction Between Terminal and Non-Terminal Alkynes

Terminal alkynes (

R - C \equiv C H

) react with **Tollens' reagent (ammoniacal

A g N O_{3}

)** to give white silver acetylide precipitate; non-terminal alkynes give no precipitate.

Active Electrophile in Benzene Nitration

**Nitronium ion (

N O_{2}^{+}

)** generated by protonation and dehydration of

H N O_{3}

by concentrated

H_{2} S O_{4}

S_N1 vs S_N2 Nucleophilic Substitution Mechanisms

S_N1: Two steps, unimolecular, carbocation intermediate, racemization; favored by tertiary halides. S_N2: One step, bimolecular, backside attack, 100% Walden inversion; favored by primary halides.

Lucas Test Observations for 1st, 2nd, and 3rd Degree Alcohols

Reagent: **anhydrous

Z n C l_{2}

+ conc.

H C l

. 3rd degree: immediate turbidity; 2nd degree: turbidity in ~5 minutes; 1st degree: no turbidity at room temperature**.

Iodoform Test Specificity

Positive (yellow

C H I_{3}

precipitate with

I_{2} + N a O H

) for compounds with **

C H_{3} - C = O

(methyl ketones, acetaldehyde) or

C H_{3} - C H (O H) -

** (ethanol, 2-propanol).

Faraday's First Law of Electrolysis Formula

m = Z I t = \frac{M}{n F} I t

w h er e

Z$ is electrochemical equivalent and $F \approx 96,500 \text{ C/mol}$.

Standard Daniell Cell Potential (

E_{cell}^{\circ}

)

E_{cell}^{\circ} = E^{\circ} (C u^{2 +} / C u) - E^{\circ} (Z n^{2 +} / Z n) = + 0.34 - (- 0.76) = 1.10 Volts

Standard Hydrogen Electrode (SHE) Reference Potential

E^{\circ} = 0.00 V

$ assigned by international convention at all temperatures for the reaction $

2 H^{+} + 2 e^{-} ⇌ H_{2} (g)

(1 a t m, 1 M

H^+$).

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