JEE Chemistry recall cards

Gallium — it melts at ~303 K yet boils near 2700 K (2400 °C), the widest liquid range in the group; that is why it fills high-temperature thermometers. (At 256 K it is solid, so it cannot read a brine freezing point. Br boils at 332 K; Li melts at 453 K; Cs reacts with water.)

Asked in 4 past papers

Au — the lanthanoid contraction (4f fills before 5d) shrinks the period-6 elements, leaving Ag and Au almost identical in size.

Asked in 1 past paper

$\mathrm{Cr}^{3+}$ — trace chromium(III) replacing aluminium in the corundum ($\mathrm{Al}_2\mathrm{O}_3$) lattice gives ruby its red colour, not $\mathrm{Co}^{3+}$.

Asked in 1 past paper

$\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}$. F's compact 2p subshell repels the incoming electron, so Cl releases the most energy. Values: Cl $-349$, F $-328$, Br $-325$, I $-295$ kJ mol$^{-1}$.

Asked in 3 past papers

$\mathrm{NF}_3$ — a strong N–F bond between two small atoms. $\mathrm{NCl}_3$ is explosive; $\mathrm{NBr}_3$ and $\mathrm{NI}_3$ exist only as unstable adducts.

Asked in 1 past paper

KCN gives the alkyl cyanide, AgCN gives the isocyanide. KCN is ionic, so free $\mathrm{CN^-}$ attacks through its carbon (the better nucleophilic site) to give the nitrile. AgCN is predominantly covalent — $\mathrm{Ag^+}$ polarises the bond — so only the nitrogen lone pair is available and the isocyanide forms. The 2024 paper paired this true assertion with the reason "KCN and AgCN both are highly ionic", which is FALSE: that is the whole trap.

Asked in 1 past paper

Lindlar delivers both hydrogens to the SAME face of the alkyne held on the metal surface — syn addition — so you get the cis (Z) alkene. Sodium in liquid ammonia goes through a radical anion whose trans vinyl anion is the lower-energy intermediate, so the two hydrogens end up anti and you get the trans (E) alkene. 2-Butyne gives cis-2-butene with the first and trans-2-butene with the second.

Asked in 2 past papers

DIBAL-H (diisobutylaluminium hydride) at low temperature delivers exactly one hydride and stops. $\mathrm{LiAlH_4}$ is the trap: it is strong enough to take the ester all the way to the primary alcohol. $\mathrm{NaBH_4}$ is at the other extreme and will not touch an ester at all.

Asked in 2 past papers

The choice between the two is decided entirely by what ELSE the molecule cannot survive. Clemmensen's concentrated HCl dehydrates a tertiary alcohol, so an acid-sensitive substrate goes to Wolff–Kishner. Conversely a base-sensitive group sends you to Clemmensen — a Wolff–Kishner on an $\alpha$-halo ketone loses the halide to the KOH.

Asked in 2 past papers

Molisch-positive says carbohydrate; biuret-negative rules out protein; and Barfoed is the one that splits the sugars, being positive for MONOsaccharides and negative for disaccharides. Positive, negative, negative therefore reads as a disaccharide such as lactose. Change Barfoed to positive and it would be glucose.

Asked in 1 past paper

Molarity, because it is moles per litre of **solution** and volume expands or contracts with temperature. The other three are all ratios of masses or of mole counts, and mass is temperature-independent — which is exactly why molality is preferred whenever an experiment runs across a temperature range.

Asked in 1 past paper

$\Delta H < 0$ with $\Delta S > 0$: both terms of $\Delta G = \Delta H - T\Delta S$ push negative, so $T$ cannot rescue or spoil it. The mirror case ($+,-$) is non-spontaneous at every temperature. The two mixed cases are the ones temperature decides: $(+,+)$ turns spontaneous only at **high** $T$, and $(-,-)$ only at **low** $T$.

Asked in 1 past paper

An absorption spectrum is the photographic negative of the emission spectrum: the atom absorbs at exactly the wavelengths it can emit, so dark gaps appear on a bright continuum precisely where the emission lines sit. No wavelength shift is involved — the same transitions are being run in the opposite direction.

Asked in 1 past paper

Adsorption. A gas losing its freedom to a surface is a large entropy loss — which is why adsorption is always exothermic. The general test is degrees of freedom: freezing, and any reaction that consumes gas moles such as $\mathrm{N}_2 + 3\mathrm{H}_2 \rightarrow 2\mathrm{NH}_3$ ($4 \rightarrow 2$), also lower $S$. Dissolving, melting and subliming all raise it.

Asked in 1 past paper

$\mathrm{AgCl}$. $\mathrm{Ag}^{+}$ has a pseudo-noble-gas core ($4d^{10}$), and $d$ electrons shield the nuclear charge poorly, so it polarises $\mathrm{Cl}^{-}$ far more strongly than a noble-gas-core cation of similar size. Size alone would not predict this — it is why $\mathrm{CuCl}$ is more covalent than $\mathrm{NaCl}$ too.

Asked in 2 past papers