NTS NAT-IM Chemistry Syllabus

Define a covalent bond.

A covalent bond is formed by the mutual sharing of one or more pairs of electrons between two atoms (typically non-metals) to achieve a stable octet.

Distinguish between a sigma (pi) bond and a pi bond.

A sigma bond forms by head-on (axial) overlap of orbitals and is stronger; a pi bond forms by sidewise (lateral) overlap of p-orbitals and is weaker. Single bond = 1 sigma; double = 1 sigma + 1 pi; triple = 1 sigma + 2 pi.

What is a coordinate (dative) covalent bond? Give an example.

A covalent bond in which both shared electrons are donated by only one atom. Example: the bond between N and H in the ammonium ion (NH4+), or in H3O+.

Compare ionic and covalent compounds in terms of melting point and electrical conductivity.

Ionic compounds: high melting points, conduct electricity when molten or in aqueous solution. Covalent compounds: low melting points, generally do not conduct electricity (poor conductors).

Define electronegativity.

Electronegativity is the tendency (relative ability) of an atom to attract a shared (bonding) pair of electrons toward itself in a covalent bond.

How does electronegativity difference determine bond type?

Difference of 0 = nonpolar covalent; small difference (roughly 0 to 1.7) = polar covalent; large difference (greater than ~1.7) = ionic bond.

What is a polar covalent bond and what is a dipole moment?

A polar covalent bond has unequal electron sharing, creating partial charges (delta+ and delta-). Dipole moment (mu) = charge x distance, a vector measuring bond/molecule polarity (units: Debye).

According to VSEPR theory, what shape and bond angle does a molecule with 4 bonding pairs and no lone pairs have? Example?

Tetrahedral shape with bond angle 109.5 degrees. Example: methane (CH4).

Why is water (H2O) bent (angular) with a bond angle of 104.5 degrees rather than 109.5?

The oxygen has two lone pairs; lone pair-lone pair and lone pair-bond pair repulsions are greater than bond pair-bond pair repulsions, compressing the H-O-H angle from 109.5 to 104.5 degrees.

What is the shape and bond angle of ammonia (NH3)?

Pyramidal (trigonal pyramidal) shape with a bond angle of 107 degrees, due to one lone pair on nitrogen.

Define hybridization.

Hybridization is the mixing of atomic orbitals of similar energy (e.g., s and p) to form an equal number of new, identical hybrid orbitals with specific geometry.

What is the hybridization, geometry, and bond angle of an sp3 hybridized carbon? Example?

sp3: tetrahedral geometry, 109.5 degree bond angle, 4 sigma bonds. Example: methane (CH4), ethane.

What is the hybridization, geometry, and bond angle in ethene (C2H4)?

sp2 hybridization, trigonal planar geometry, 120 degree bond angle. Each carbon forms 3 sigma bonds and 1 pi bond (the C=C double bond).

What is the hybridization, geometry, and bond angle in ethyne/acetylene (C2H2)?

sp hybridization, linear geometry, 180 degree bond angle. Each carbon forms 2 sigma bonds and 2 pi bonds (the C triple-bond C).

How do you determine the type of hybridization from the number of sigma bonds and lone pairs?

Add the number of sigma bonds and lone pairs (steric number): 2 = sp, 3 = sp2, 4 = sp3, 5 = sp3d, 6 = sp3d2.

State Boyle's Law (with the formula).

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

State Charles's Law (with the formula).

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 an equal number of molecules. At STP, 1 mole of any gas occupies 22.4 liters.

Write the Ideal Gas Equation and define each term.

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

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 + ...

What are the main postulates that distinguish an ideal gas from a real gas?

An ideal gas assumes: gas molecules have negligible volume and no intermolecular forces. Real gases deviate from this, especially at high pressure and low temperature, where these assumptions fail.

Define evaporation and explain why it causes cooling.

Evaporation is the escape of high-energy molecules from a liquid surface into vapor below the boiling point. It causes cooling because the most energetic molecules leave, lowering the average kinetic energy (temperature) of the remaining liquid.

What are the three main types of intermolecular forces (van der Waals/related), from weakest to strongest?

London dispersion forces (weakest), dipole-dipole interactions, and hydrogen bonding (strongest).

What is hydrogen bonding and what conditions are required for it?

Hydrogen bonding is a strong dipole-dipole attraction occurring when H is bonded to a highly electronegative atom (F, O, or N) and is attracted to a lone pair on F, O, or N of a neighboring molecule. It explains water's high boiling point.

Define boiling point in terms of vapor pressure.

The boiling point is the temperature at which the vapor pressure of a liquid equals the external (atmospheric) pressure, allowing bubbles of vapor to form throughout the liquid.

Compare the four types of crystalline solids (ionic, covalent/network, molecular, metallic) by their constituent particles.

Ionic: positive and negative ions; Covalent network: atoms joined by covalent bonds (e.g., diamond); Molecular: molecules held by intermolecular forces (e.g., ice); Metallic: metal cations in a sea of delocalized electrons.

What is the difference between crystalline and amorphous solids?

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

What is a reversible reaction and what defines chemical equilibrium?

A reversible reaction proceeds in both forward and backward directions. Chemical equilibrium is the state where the rates of the forward and reverse reactions are equal, so the concentrations of reactants and products remain constant.

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

Kc = ([C]^c [D]^d) / ([A]^a [B]^b), where concentrations are at equilibrium. Pure solids and liquids are omitted.

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 (reduce) the effect of that change.

Define exothermic and endothermic reactions in terms of enthalpy change (delta H).

Exothermic reaction: releases heat, delta H is negative (products have lower enthalpy than reactants). Endothermic reaction: absorbs heat, delta H is positive (products have higher enthalpy than reactants).