NUMS Entry Test Chemistry Syllabus

Write the electronic configuration of chromium (Z=24) and explain its anomaly.

[Ar] 3d5 4s1. Chromium adopts this instead of 3d4 4s2 because a half-filled 3d subshell gives extra stability.

What are the main postulates of Bohr's atomic model?

Electrons revolve in fixed circular orbits (stationary states) of definite energy without radiating; energy is absorbed or emitted only when an electron jumps between orbits; angular momentum is quantized (mvr = nh/2pi).

How does an atomic emission spectrum arise?

When excited electrons fall from higher to lower energy levels, they emit photons of specific energies, producing bright lines at characteristic wavelengths (a line spectrum).

Name the series in the hydrogen spectrum that lies in the visible region.

The Balmer series (transitions ending at n=2).

What is the energy of a photon in terms of frequency?

E = hv, where h is Planck's constant (6.626 x 10^-34 J s) and v is the frequency.

Define ionic bond and covalent bond.

An ionic bond forms by complete transfer of electrons from a metal to a non-metal, producing oppositely charged ions held by electrostatic attraction. A covalent bond forms by the mutual sharing of electron pairs between non-metal atoms.

What is a coordinate (dative) covalent bond?

A covalent bond in which both shared electrons are donated by one atom (the donor) to an atom or ion that has an empty orbital (the acceptor). Example: NH4+.

Define hybridization.

The mixing of atomic orbitals of similar energy to form an equal number of new equivalent hybrid orbitals used in bonding.

Give the hybridization, shape and bond angle for sp, sp2 and sp3 carbon.

sp: linear, 180 degrees; sp2: trigonal planar, 120 degrees; sp3: tetrahedral, 109.5 degrees.

What are the shapes of water and ammonia molecules and why do their angles differ from 109.5?

Water is bent/angular (104.5 degrees) and ammonia is pyramidal (107 degrees). Lone pairs repel more strongly than bonding pairs, compressing the bond angles below the tetrahedral 109.5 degrees.

Define bond energy.

The average energy required to break one mole of a particular bond in the gaseous state, breaking it into gaseous atoms. It is always positive (endothermic).

What determines whether a covalent bond is polar?

A difference in electronegativity between the bonded atoms. Unequal electron sharing creates partial charges and a bond dipole; larger electronegativity difference means greater polarity.

Why is CO2 nonpolar despite having polar bonds?

CO2 is linear and symmetric, so the two C=O bond dipoles are equal and opposite and cancel out, giving zero net dipole moment.

List the three main types of intermolecular forces in increasing strength.

London dispersion (induced dipole) forces < dipole-dipole forces < hydrogen bonding.

What is hydrogen bonding and which atoms enable it?

A strong dipole-dipole attraction between a hydrogen atom bonded to a highly electronegative atom (F, O or N) and a lone pair on F, O or N of a neighboring molecule.

State Boyle's law.

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure (PV = constant).

State Charles's law.

At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature (V/T = constant).

Write the ideal gas equation and define its terms.

PV = nRT, where P = pressure, V = volume, n = moles, R = universal gas constant (8.314 J/mol K or 0.0821 atm dm3/mol K), T = absolute temperature in Kelvin.

State Avogadro's law.

Equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.

State Graham's law of diffusion.

At constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density (or molar mass): rate proportional to 1/sqrt(M).

Define vapor pressure and how it relates to boiling point.

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. A liquid boils when its vapor pressure equals the external (atmospheric) pressure.

How do surface tension and viscosity of a liquid change with increasing temperature?

Both surface tension and viscosity decrease as temperature increases, because higher kinetic energy weakens intermolecular attractions.

What is the difference between crystalline and amorphous solids?

Crystalline solids have a regular, long-range ordered arrangement of particles and sharp melting points. Amorphous solids lack long-range order, behave like supercooled liquids, and soften over a range of temperatures (e.g., glass).

What is a unit cell?

The smallest repeating structural unit of a crystal lattice that, when repeated in three dimensions, generates the entire crystal.

How many atoms are contained in a body-centered cubic (BCC) unit cell?

2 atoms: 8 corner atoms x 1/8 each (= 1) plus 1 atom fully inside the body center.

What characterizes a reversible reaction and dynamic equilibrium?

A reversible reaction proceeds in both forward and backward directions. At dynamic equilibrium the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant.

Write the equilibrium constant expression (Kc) for aA + bB <=> cC + dD.

Kc = ([C]^c [D]^d) / ([A]^a [B]^b), where concentrations are equilibrium molar concentrations.

State 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 opposes (counteracts) the change.

According to Le Chatelier's principle, how does increasing pressure affect a gaseous equilibrium?

The equilibrium shifts toward the side with the fewer number of gas molecules (lower total moles of gas).

Define solubility product (Ksp) and write it for a salt AB that dissociates into A+ and B-.

Ksp is the equilibrium constant for the dissolution of a sparingly soluble salt in water. For AB <=> A+ + B-, Ksp = [A+][B-].

What is the common ion effect?

The decrease in solubility of a sparingly soluble salt when a soluble compound providing a common ion is added, shifting the dissolution equilibrium backward (Le Chatelier's principle).