Elements are classified by the subshell into which the last (differentiating) electron enters: s-block (last electron in s), p-block (in p), d-block (in d) and f-block (in f). d-block elements are called transition elements because they lie between the s-block (metals) and p-block (non-metals) and show a gradual transition in properties; their atoms or common ions have partially filled d orbitals.

BF3: central B has 3 bond pairs and no lone pair -> trigonal planar, bond angle 120 deg. NH3: central N has 3 bond pairs and 1 lone pair -> pyramidal (trigonal pyramidal), bond angle about 107 deg. H2O: central O has 2 bond pairs and 2 lone pairs -> bent/angular, bond angle about 104.5 deg. Lone pairs repel more than bond pairs, reducing the angle from the ideal 109.5 deg in NH3 and H2O.

In ethyne each carbon is sp hybridised. The two sp hybrid orbitals on each carbon form sigma bonds: one C-C sigma bond and one C-H sigma bond, giving a linear molecule with a bond angle of 180 deg. The remaining two unhybridised p orbitals on each carbon overlap sideways to form two pi bonds, so there is a triple bond (1 sigma + 2 pi) between the carbon atoms.

H-C(triple bond)C-H

O2 has 16 electrons. Configuration: sigma(1s)2 sigma*(1s)2 sigma(2s)2 sigma*(2s)2 sigma(2pz)2 pi(2px)2 pi(2py)2 pi*(2px)1 pi*(2py)1. Bonding electrons Nb = 10, antibonding Na = 6. Bond order = (Nb - Na)/2 = (10 - 6)/2 = 2. The two unpaired electrons in the pi* antibonding orbitals make O2 paramagnetic.

Bond order = (Nb - Na)/2

Fajans' rules: covalent character in an ionic compound increases when (1) the cation is small, (2) the anion is large, and (3) the cation has a high charge (all three increase the polarising power/polarisability). Also, cations with a pseudo-noble gas (18-electron) configuration cause greater polarisation than those with a noble gas configuration. Greater polarisation of the anion by the cation increases covalent character.

Dipole moment (mu) is the product of the magnitude of charge and the distance between the centres of positive and negative charge; it is a vector quantity (unit: debye). CO2 is linear (O=C=O) and its two C=O bond dipoles are equal and opposite, so they cancel and the net dipole moment is zero. H2O is bent/angular, so its two O-H bond dipoles do not cancel and add up to give a net dipole moment (1.85 D). Hence CO2 is non-polar but H2O is polar.

mu = q x d

Convert temperatures to Kelvin: T1 = 27 + 273 = 300 K, T2 = 127 + 273 = 400 K. By Charles's law V1/T1 = V2/T2, so V2 = V1 x T2/T1 = 300 x 400/300 = 400 mL.

V1/T1 = V2/T2

Using PV = nRT, V = nRT/P = (2 x 0.0821 x 300) / 2 = 49.26/2 = 24.63 L.

V = nRT / P

1) A gas consists of a large number of tiny particles (molecules) whose actual volume is negligible compared with the volume of the container. 2) There are no forces of attraction or repulsion between the molecules. 3) The molecules are in constant, rapid, random motion and collide with one another and with the walls of the container. 4) The collisions are perfectly elastic (no loss of kinetic energy). 5) The average kinetic energy of the molecules is directly proportional to the absolute temperature.

KE(avg) proportional to T

Heat supplied to the system q = +1 kJ = +1000 J. Work is done BY the gas, so work done on the system w = -200 J. By the first law, delta U = q + w = 1000 + (-200) = 800 J. The internal energy increases by 800 J.

delta U = q + w

We want C(s) + 1/2 O2(g) -> CO(g). By Hess's law, subtract the second equation from the first: delta H(reqd) = delta H1 - delta H2 = (-393.5) - (-283.0) = -110.5 kJ. So the enthalpy of formation of CO is -110.5 kJ/mol.

delta H(reqd) = delta H1 - delta H2

The Gibbs energy change is given by delta G = delta H - T delta S. Criteria of spontaneity: if delta G < 0 (negative), the process is spontaneous; if delta G = 0, the system is at equilibrium; if delta G > 0 (positive), the process is non-spontaneous (the reverse is spontaneous). A reaction is always spontaneous when delta H is negative and delta S is positive.

delta G = delta H - T delta S

Convert delta S to kJ: -198 J/K = -0.198 kJ/K. delta G = delta H - T delta S = -92.4 - (298 x -0.198) = -92.4 - (-59.0) = -92.4 + 59.0 = -33.4 kJ. Since delta G is negative, the reaction is spontaneous at 298 K.

delta G = delta H - T delta S

Kc = [NH3]^2 / ([N2][H2]^3). The relation between Kp and Kc is Kp = Kc (RT)^(delta ng), where delta ng = (moles of gaseous products) - (moles of gaseous reactants). Here delta ng = 2 - (1 + 3) = -2, so Kp = Kc (RT)^-2.

Kp = Kc (RT)^(delta ng)

Kc = [NO2]^2 / [N2O4] = (0.04)^2 / 0.02 = 0.0016 / 0.02 = 0.08 mol/L. The units are mol/L because delta ng = 1 for this reaction.

Kc = [products]^coeff / [reactants]^coeff

Temperature: increasing temperature favours the endothermic direction and decreasing temperature favours the exothermic direction (as per Le Chatelier's principle); temperature also changes the value of the equilibrium constant K. Catalyst: a catalyst speeds up both the forward and backward reactions equally, so it helps the system reach equilibrium faster but does not shift the position of equilibrium or change the value of K.

NaOH is a strong base and dissociates completely, so [OH-] = 0.01 M = 10^-2 M. pOH = -log[OH-] = -log(10^-2) = 2. Since pH + pOH = 14, pH = 14 - 2 = 12. The solution is basic (pH > 7).

pH + pOH = 14

A buffer solution is one that resists a change in its pH on the addition of a small amount of acid or base. An acidic buffer is made from a weak acid and its salt with a strong base, e.g. acetic acid + sodium acetate (CH3COOH + CH3COONa). The Henderson-Hasselbalch equation is pH = pKa + log([salt]/[acid]).

pH = pKa + log([salt]/[acid])

AgCl dissociates as AgCl <=> Ag+ + Cl-. If solubility = s mol/L, then [Ag+] = [Cl-] = s. Ksp = [Ag+][Cl-] = s x s = s^2. So s = sqrt(Ksp) = sqrt(1.8 x 10^-10) = 1.34 x 10^-5 mol/L.

Ksp = s^2 (for AB type salt)

Reduction half: MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O. Oxidation half: Fe2+ -> Fe3+ + e-. To balance electrons, multiply the oxidation half by 5: 5Fe2+ -> 5Fe3+ + 5e-. Add the two halves: MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+. This is the balanced equation.