Chemistry important questions

i) Rutherford atomic model: from the $\alpha$-scattering experiment, Rutherford proposed that an atom has a tiny, dense, positively charged nucleus carrying almost all its mass, with electrons revolving around it in the mostly empty space; the atom is electrically neutral. Its defect: it could not explain atomic stability or line spectra.

ii) Le-Chatelier's principle: if a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the equilibrium shifts in the direction that tends to undo the change. Applications: in Haber's process ($\ce{N2 + 3H2 <=> 2NH3}$) high pressure and low temperature increase $\ce{NH3}$ yield; in the contact process high pressure/excess $\ce{O2}$ increase $\ce{SO3}$.

iii) Characteristics of homologous series: same general formula and same functional group; successive members differ by $\ce{-CH2-}$ ($14\ \text{u}$); gradual gradation of physical properties with molar mass; similar chemical properties; all made by similar general methods.

iv) Lassaigne's test for nitrogen: the compound is fused with sodium to give $\ce{NaCN}$; the extract is boiled with $\ce{FeSO4}$ and acidified with $\ce{HCl}$; a Prussian-blue colour confirms nitrogen:

$$ \ce{6NaCN + FeSO4 -> Na4[Fe(CN)6] + Na2SO4} $$

;

$$ \ce{3Na4[Fe(CN)6] + 4FeCl3 -> Fe4[Fe(CN)6]3 + 12NaCl} $$

.

Conclusions of Rutherford's $\alpha$-scattering experiment: 1. Most of the atom is empty space (most $\alpha$ particles passed straight through). 2. There is a small, dense, positively charged nucleus at the centre (a few particles were ...

Observations (a thin gold foil bombarded with $\alpha$-particles):

1. Most of the $\alpha$-particles passed straight through the foil undeflected.
2. A few were deflected through small angles.
3. Very few (about 1 in 20000) were deflected through large angles, and some even bounced straight back.

Conclusions:

1. The atom is mostly empty space (most particles pass through).
2. There is a small, dense, positively charged nucleus at the centre (large-angle deflections and rebounds).
3. Almost the entire mass and all the positive charge are concentrated in the nucleus.
4. Electrons revolve around the nucleus, the atom as a whole being neutral.

Postulates of Bohr's model: 1. Electrons revolve around the nucleus only in certain fixed circular orbits (stationary states) of definite energy, without radiating energy. 2. Only those orbits are allowed in which the angular momentum is...

Conclusions:

1. Most of the space inside an atom is empty (most $\alpha$ particles passed undeflected).
2. There is a tiny, dense, positively charged nucleus at the centre (a few particles were strongly deflected or bounced back).
3. Nearly the whole mass and all the positive charge reside in the nucleus.
4. Electrons revolve around the nucleus, the atom being electrically neutral.

Limitation: it could not explain the stability of the atom (a revolving electron should radiate energy and collapse into the nucleus) nor the discrete line spectra of elements.

Hund's rule of maximum multiplicity: When electrons are filled into a set of orbitals of equal energy (degenerate orbitals, e.g. the three $p$ orbitals), every orbital is singly occupied with electrons of parallel spin first, and pairing...

Sodium ($Z=11$) has the configuration $\ce{1s^2 2s^2 2p^6 3s^1}$. The 11th electron is the $3s^1$ electron. - Principal, $n=3$ - Azimuthal, $l=0$ (for an $s$ orbital) - Magnetic, $m=0$ - Spin, $s=+\dfrac{1}{2}$

Nuclear fusion: the process in which two light nuclei combine (fuse) to form a heavier nucleus, with the release of a very large amount of energy. $$\ce{^21H + ^31H - ^42He + ^10n} + \text{energy}$$ Application: it is the source of energ...

Isotopes of hydrogen: protium $\ce{^11H}$, deuterium $\ce{^21H}$ and tritium $\ce{^31H}$. Least abundant: tritium ($\ce{^31H}$), which is radioactive and present only in traces. Application: tritium is used as fuel in nuclear fusion reac...

Magnetic quantum number ($m$): it gives the orientation (spatial direction) of an orbital in space relative to the applied magnetic field, and hence the number of orbitals in a subshell ($m$ ranges from $-l$ to $+l$).

For a 3p orbital: $l=1$, and $m$ can be $-1, 0, +1$. The $3p_x$ orbital corresponds to $l=1,\ m=+1$ (one of the $m=\pm1$ set).

Valence shell $4s^2$ means the full configuration is $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^2}$.

$$ \begin{aligned} \text{Total electrons} &= 2+2+6+2+6+2 \ &= 20 \end{aligned} $$

so atomic number $Z=20$ (calcium).

$$ \begin{aligned} p\text{-electrons} &= \underbrace{2p^6}{6}+\underbrace{3p^6}{6} \ &= \mathbf{12} \end{aligned} $$

Controlled nuclear fission: a fission chain reaction in which the number of neutrons causing further fission is regulated (e.g. by absorbing surplus neutrons with control rods) so that energy is released steadily rather than explosively.

Application: it is used in nuclear reactors to generate electricity (and to produce radioactive isotopes).

The least abundant isotope is tritium, $\ce{^3_1H}$ (radioactive, present only in traces).

Uses: (i) as a fuel in nuclear fusion (thermonuclear) reactions; (ii) as a radioactive tracer and in self-luminous (glow-in-the-dark) paints/dials.

Sodium ($Z=11$): $\ce{1s^2 2s^2 2p^6 3s^1}$. The outermost electron is $3s^1$. - Principal $n=3$ - Azimuthal $l=0$ (for an $s$ orbital) - Magnetic $m=0$

(i) $\ce{Al^{3+}}$: aluminium ($Z=13$) loses 3 electrons: $\ce{1s^2 2s^2 2p^6}$ (10 electrons, neon configuration). (ii) S ($Z=16$): $\ce{1s^2 2s^2 2p^6 3s^2 3p^4}$.

Nuclear fusion: the process in which two light nuclei combine to form a heavier nucleus, releasing a very large amount of energy (the mass lost appears as energy, $E=mc^2$).

Example: $$\ce{^2_1H + ^3_1H -> ^4_2He + ^1_0n} + \text{energy}$$ This is the source of the sun's energy.

$Z=19$ is potassium: $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 4s^1}$. The outermost electron is $4s^1$: - Principal quantum number $n=4$ - Azimuthal quantum number $l=0$ (an $s$ orbital).

Heisenberg's uncertainty principle goes against Bohr's idea of fixed, well-defined circular orbits. Statement: it is impossible to determine simultaneously and exactly both the position and the momentum (velocity) of a small moving parti...

Thermonuclear reaction: a nuclear fusion reaction that takes place only at extremely high temperatures (millions of degrees), where light nuclei combine to form a heavier nucleus with release of enormous energy.

Example (the reaction that powers the sun and the hydrogen bomb): $$\ce{^2_1H + ^3_1H -> ^4_2He + ^1_0n} + \text{energy}$$

$\ce{A}$ ($Z=29$) is copper: $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1}$. $\ce{A+}$ is formed by losing the $4s^1$ electron: $\ce{1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10}}$ (28 electrons). $$ \begin{aligned} s\text{-electrons} &= \underbrace{2}...

Pauli's exclusion principle: no two electrons in the same atom can have all four quantum numbers ($n, l, m, s$) identical. Consequently an orbital can hold at most two electrons and these must have opposite spins. Example: in the $1s$ or...

Nuclear reaction: a reaction that involves a change in the composition of the nucleus of an atom (its protons/neutrons), producing a different nucleus and releasing large energy, unlike ordinary chemical reactions which involve only elec...

Manufacture from carnallite ($\ce{KCl.MgCl2.6H2O}$, which carries bromides): after the carnallite is worked for potassium salts, the residual mother liquor (bittern) is rich in $\ce{MgBr2}$. It is acidified and warmed, and chlorine gas i...

Preparation (slaked lime + chlorine at about $40,^\circ\text{C}$): $$\ce{Ca(OH)2 + Cl2 - CaOCl2 + H2O}$$ (i) With excess dilute $\ce{H2SO4}$ (chlorine is liberated): $$\ce{CaOCl2 + H2SO4 - CaSO4 + H2O + Cl2(g)}$$ (ii) With ammonia (nitr...

Laboratory preparation of $\ce{SO2}$: warm copper turnings with hot concentrated sulphuric acid (or add dilute acid to a sulphite). $\ce{SO2}$ is a colourless, pungent gas collected by upward displacement of air: $$\ce{Cu + 2H2SO4(conc) ...