CHSE Odisha Elective Chemistry common mistakes
CHSE Odisha Elective Chemistry · Understand the traps and how to avoid them.
Calculates unit-cell density as d = ZM/a^3N_A without converting a from pm to cm and without checking whether Z is 1, 2 or 4 for simple cubic, bcc or fcc.
The formula is memorized as symbols, but a must be in cm to give g cm^-3; Z is the number of formula units per unit cell, not the number of atoms in the formula.
What to do instead
Write the conversion 1 pm = 10^-10 cm, cube it, then substitute; state Z from the lattice type; check that the density lies roughly between 1 and 20 g cm^-3 for common ionic or metallic solids.
Writes the Nernst equation with Q inverted or omits n, and assigns electrode signs by position in cell notation without checking oxidation and reduction.
Cell notation and reduction potentials are memorized separately; for the cell reaction Q = products/reactants and n is the number of electrons transferred after balancing the half-reactions.
What to do instead
Write separate oxidation and reduction half-reactions, balance electrons to obtain n, then use Ecell = E°cell - (0.059 V/n) log Q at 298 K; for concentration cells use the ion concentrations on the two sides; confirm spontaneous direction by Ecell > 0.
Treats stoichiometric coefficients in a balanced equation as reaction orders, or gives the units of k as mol L^-1 s^-1 for every order.
Rate law must be experimental; coefficients give molecularity only for an elementary step. Units of k depend on overall order n: mol^(1-n) L^(n-1) s^-1.
What to do instead
Derive order from rate data by comparing experiments; use coefficients as exponents only for an elementary step; calculate k units by dimensional analysis from rate = k[A]^m[B]^n; use Arrhenius with T in kelvin and Ea in J mol^-1.
Treats Freundlich and Langmuir isotherms as the same graph, or says physisorption is stronger because it has higher enthalpy, and forgets how temperature and pressure affect adsorption.
The log(x/m) versus log p straight line and the saturation plateau are memorized visually; physisorption is weak van der Waals adsorption, while chemisorption involves chemical bonds and higher enthalpy.
What to do instead
For Freundlich x/m = k p^(1/n), plot log(x/m) against log p; slope = 1/n and intercept = log k. For Langmuir the plot saturates at monolayer coverage. State physisorption: low ΔH, reversible, multilayer; chemisorption: high ΔH, irreversible, monolayer; adsorption generally decreases with temperature.
Chooses roasting for carbonate ores and calcination for sulphide ores, or applies froth flotation directly to oxide ores without sulphidisation.
Both processes involve heating, so the names are easily confused; froth flotation depends on the surface wettability of sulphide particles, not oxide particles.
What to do instead
Use calcination for carbonates and hydrated oxides in limited air; use roasting for sulphides in excess air; for oxide ores first sulphidise with Na2S or use another separation method; for bauxite use the Bayer process, and for zone refining place the impurity-soluble end at the moving molten zone.
Applies group trends without exceptions: calls H3PO4 stronger than HClO4, predicts NH3 is a stronger base than PH3 in water, or ignores the inert-pair effect for Tl, Pb and Bi.
Trends are learned as monotonic rules; oxoacid strength depends on the number of terminal oxygen atoms and inductive effects, and the inert-pair effect stabilises lower oxidation states down groups 13-15.
What to do instead
Rank oxoacids by (HO)mZOn: more terminal O and a more electronegative central atom give a stronger acid; for hydrides, basicity in water decreases down the group (NH3 > PH3 > AsH3); remember Tl(I), Pb(II) and Bi(III) stability.
Gives the oxidation state of the metal by adding ligand charges without accounting for overall complex charge, or writes the magnetic moment using total d electrons rather than unpaired electrons in the actual geometry.
Ligand names and charges are applied separately; spin-only formula μ = sqrt(n(n+2)) BM needs n unpaired electrons after crystal-field splitting, and strong-field ligands cause pairing.
What to do instead
Set charge balance: metal oxidation state + sum of ligand charges = complex charge; draw the d-electron configuration for the given geometry and ligand field; count unpaired electrons, then use the spin-only formula; name ligands alphabetically and use bis/tris for repeated polydentate ligands.
Predicts SN1 for a primary alkyl chloride in a polar protic solvent, or says chlorobenzene undergoes SN2 readily because the C-Cl bond is polar.
SN1 versus SN2 choice depends on substrate, nucleophile, solvent and leaving group; aryl C-Cl has partial double-bond character and backside attack is blocked.
What to do instead
Use primary + strong nucleophile + polar aprotic for SN2; tertiary + weak nucleophile + polar protic for SN1; treat allylic and benzylic halides separately. For haloarenes, nucleophilic substitution needs electron-withdrawing groups at ortho/para or extreme conditions; otherwise expect no SN2.
Uses Lucas test to distinguish primary, secondary and tertiary alcohols without controlling temperature and ZnCl2, or ranks phenol acidity by counting substituents without considering electron-withdrawing groups.
Lucas reagent reacts by an SN1 pathway, so tertiary alcohols respond fastest and primary alcohols slowly; phenol acidity depends on stabilisation of the phenoxide ion by electron-withdrawing groups, especially at ortho and para positions.
What to do instead
Lucas: tertiary gives immediate turbidity, secondary on heating, primary no reaction at room temperature; use Victor Meyer for primary/secondary/tertiary; phenol does not liberate CO2 from NaHCO3, unlike carboxylic acids; nitro groups increase phenol acidity, especially ortho/para.
Applies Tollens' or Fehling's test to every carbonyl, uses Cannizzaro for acetaldehyde, or forgets that Fehling's solution is not reduced by aromatic aldehydes.
Tests are remembered as aldehyde positive, but differentiation depends on aliphatic versus aromatic character and on the presence of alpha-hydrogens; Cannizzaro requires an aldehyde with no alpha-hydrogen.
What to do instead
Tollens' is positive for both aliphatic and aromatic aldehydes; Fehling's is positive for aliphatic aldehydes, not benzaldehyde; iodoform is for CH3CO- or CH3CH(OH)- groups; Cannizzaro only for HCHO and PhCHO; aldol requires alpha-H. For amides, Hoffmann bromamide removes CO to give an amine with one fewer carbon.