National MDCAT Chemistry Syllabus

What are the two possible values of the spin quantum number (s) and what do they represent?

+1/2 and -1/2, representing the two opposite directions of electron spin (clockwise and anticlockwise).

Describe the shapes of s, p, and d orbitals.

s orbitals are spherical; p orbitals are dumbbell-shaped (three orientations: px, py, pz); most d orbitals are double-dumbbell (cloverleaf) shaped (five orientations).

How many electrons can be accommodated in the s, p, d, and f subshells?

s holds 2, p holds 6, d holds 10, and f holds 14 electrons.

State Boyle's law and give its mathematical form.

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure. PV = constant, or P1V1 = P2V2.

State Charles's law and give its mathematical form.

At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature. V/T = constant, or V1/T1 = V2/T2.

State Avogadro's law.

Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules; i.e., volume is directly proportional to the number of moles (V proportional to n).

Write the ideal gas equation and identify each term.

PV = nRT, where P = pressure, V = volume, n = number of moles, R = universal gas constant (0.0821 L atm mol^-1 K^-1 or 8.314 J mol^-1 K^-1), T = absolute temperature in Kelvin.

State Dalton's law of partial pressures.

The total pressure of a mixture of non-reacting gases equals the sum of the partial pressures of the individual gases: P_total = P1 + P2 + P3 + ...

State Graham's law of diffusion/effusion.

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

List the main postulates of the kinetic molecular theory of gases.

Gases consist of tiny particles in constant random motion; the volume of the particles is negligible compared to the container; there are no intermolecular forces; collisions are perfectly elastic; and the average kinetic energy is directly proportional to absolute temperature.

According to kinetic molecular theory, how is temperature related to molecular motion?

The absolute (Kelvin) temperature is directly proportional to the average kinetic energy of the gas molecules: KE_avg = (3/2) k T.

Under what conditions does a real gas behave most ideally?

At high temperature and low pressure, because intermolecular forces and the actual volume of molecules become negligible under these conditions.

Why do real gases deviate from ideal behaviour?

Because real gas molecules have a finite volume and exert intermolecular attractive forces, both of which are assumed to be zero/negligible in the ideal gas model. Deviations are largest at high pressure and low temperature.

Write van der Waals' equation for n moles of a real gas and explain the correction terms.

(P + an^2/V^2)(V - nb) = nRT. The term an^2/V^2 corrects for intermolecular attractions (pressure correction); nb corrects for the finite volume of molecules (volume correction).

What is surface tension and why does it arise?

Surface tension is the force per unit length acting along the surface of a liquid that minimizes its surface area. It arises because surface molecules experience a net inward pull due to unbalanced intermolecular forces.

Define vapour pressure of a liquid. How does it change with temperature?

Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid at a given temperature. It increases with increasing temperature.

Define the boiling point of a liquid in terms of vapour pressure.

The boiling point is the temperature at which the vapour pressure of the liquid becomes equal to the external (atmospheric) pressure.

What is viscosity, and how does it generally change with temperature for liquids?

Viscosity is the resistance of a liquid to flow, caused by internal friction between layers. For liquids, viscosity decreases as temperature increases.

What is the difference between crystalline and amorphous solids?

Crystalline solids have a regular, long-range ordered arrangement of particles, sharp melting points, and are anisotropic. Amorphous solids have a disordered (short-range) arrangement, melt over a range, and are isotropic (e.g., glass).

List the four main types of crystalline solids based on the particles and bonding.

Ionic solids (ions, electrostatic bonds), covalent/network solids (atoms, covalent bonds), molecular solids (molecules, van der Waals/H-bonds), and metallic solids (cations in a sea of electrons).

Compare the melting points and electrical conductivity of ionic, covalent, molecular, and metallic solids.

Ionic: high m.p., conduct only when molten/dissolved. Covalent network: very high m.p., usually non-conducting (except graphite). Molecular: low m.p., non-conducting. Metallic: variable m.p., conduct in solid and molten states.

What is a crystal lattice and a unit cell?

A crystal lattice is the regular, repeating three-dimensional arrangement of particles in a crystal. The unit cell is the smallest repeating unit that, when repeated in all directions, generates the entire lattice.

Define isomorphism and polymorphism with respect to crystals.

Isomorphism: different substances having the same crystalline form/shape (e.g., due to similar formulae). Polymorphism: the same substance existing in two or more distinct crystalline forms (e.g., diamond and graphite for carbon).

Write the expression for the equilibrium constant Kc for the reaction aA + bB <-> cC + dD.

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

What does a very large value of K and a very small value of K tell you about a reaction at equilibrium?

A large K means products are favoured (reaction goes nearly to completion); a small K means reactants are favoured (very little product forms at equilibrium).

State Le Chatelier's principle.

If a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium shifts in the direction that tends to counteract (oppose) that change.

How does increasing pressure affect a gaseous equilibrium according to Le Chatelier's principle?

Increasing pressure (decreasing volume) shifts the equilibrium toward the side with the fewer moles of gas; if both sides have equal moles of gas, pressure has no effect on the position of equilibrium.

In the Haber process (N2 + 3H2 <-> 2NH3, exothermic), what conditions favour maximum yield of ammonia, and why are compromise conditions used?

High pressure and low temperature favour ammonia yield (fewer gas moles and exothermic). In practice moderate temperature (about 450 C), high pressure (about 200 atm), and an iron catalyst are used as a compromise to achieve a reasonable rate and yield.

Define the rate of a chemical reaction.

The rate of a reaction is the change in concentration of a reactant or product per unit time (e.g., mol dm^-3 s^-1).

What is the difference between average rate and instantaneous rate of reaction?

Average rate is the change in concentration over a measurable time interval; instantaneous rate is the rate at a particular instant, obtained from the slope of the tangent to the concentration-time curve at that point.

Write the general form of a rate law and define the order of reaction.

Rate = k[A]^m[B]^n, where k is the rate constant. The order with respect to a reactant is its exponent (m or n); the overall order is the sum (m + n). Orders are determined experimentally, not from the balanced equation.

State the Arrhenius equation and explain how activation energy and temperature affect the rate constant.

k = A e^(-Ea/RT), where A is the frequency factor and Ea is the activation energy. A higher temperature or a lower Ea gives a larger rate constant k and thus a faster reaction.

List the main factors that affect the rate of a chemical reaction.

Nature of reactants, concentration (or pressure for gases), temperature, surface area of solids, presence of a catalyst, and (for some reactions) light/radiation.

How does a catalyst increase the rate of a reaction?

A catalyst provides an alternative reaction pathway with a lower activation energy, allowing more molecules to react successfully, without being consumed and without changing the position of equilibrium.

Define internal energy (E or U) of a system.

Internal energy is the total energy (kinetic + potential) of all the particles in a system. Its absolute value cannot be measured; only the change (delta E) is determined.

Define enthalpy (H) and write the relationship between enthalpy change and internal energy change.

Enthalpy H = E + PV is the heat content of a system at constant pressure. The change is delta H = delta E + P delta V; delta H equals the heat absorbed or released at constant pressure (q_p).

What is the sign convention of delta H for exothermic and endothermic reactions?

Exothermic reactions release heat and have delta H negative; endothermic reactions absorb heat and have delta H positive.

State the first law of thermodynamics and its mathematical form.

Energy can neither be created nor destroyed, only converted from one form to another. delta E = q + w, where q is heat added to the system and w is work done on the system.

State Hess's law of constant heat summation.

The total enthalpy change for a reaction is the same whether it occurs in one step or several steps, depending only on the initial and final states and not on the pathway taken.

How is Hess's law used to calculate an unknown enthalpy change?

By adding or subtracting the enthalpy changes of known reactions (manipulating equations and their delta H values, reversing signs when an equation is reversed) so they sum to the target reaction, since delta H is a state function.