IB Chemistry Practice Test 20

Correct: (D)

Amount of atoms: H2O: 0.50 × 3 = 1.5 mol; He: 1.0 mol; CH4: 0.40 × 5 = 2.0 mol; NH3: 0.60 × 4 = 2.4 mol, which is the greatest.

Correct: (A)

Copper is an exception to the expected filling order: a completely filled 3d subshell with a single 4s electron is more stable than 4s^2 3d^9.

Correct: (C)

Mass of oxygen in one mole = 4 × 16 = 64 g; percentage = (64 / 98) × 100 = 65.3%.

Correct: (B)

P = nRT / V = (0.500 × 8.31 × 300) / 12.0 = 104 kPa (using V in dm^3 gives P in kPa).

Correct: (D)

n = V / molar volume = 48 / 24 = 2.0 mol.

Correct: (A)

The amount of solute does not change on dilution: c2 = 2.00 × (25.0 / 250) = 0.200 mol dm^-3.

Correct: (C)

n = 0.250 × 0.400 = 0.100 mol; mass = 0.100 × 40.0 = 4.00 g.

Correct: (B)

Ar = (0.20 × 10) + (0.80 × 11) = 2.0 + 8.8 = 10.8.

Correct: (D)

During a change of state at constant temperature, the energy absorbed increases the potential energy (separation) of the particles; their average kinetic energy, and hence the temperature, stays constant.

Correct: (A)

N: 1.4 / 14 = 0.10 mol; H: 0.30 / 1 = 0.30 mol. The ratio 0.10 : 0.30 simplifies to 1 : 3, giving NH3.

Correct: (C)

Carbon dioxide has two electron domains around the central carbon atom and no lone pairs on it, so the molecule is linear with a bond angle of 180°.

Correct: (B)

Oxygen has 6 valence electrons; two are used in the bonds to the hydrogen atoms, leaving four electrons as two lone pairs.

Correct: (D)

Bond polarity increases with the difference in electronegativity between the atoms, and this difference is greatest for H-F because fluorine is the most electronegative element.

Correct: (A)

In SiO2 each silicon atom is covalently bonded to four oxygen atoms in one continuous network; I2, CO2 and S8 consist of discrete molecules.

Correct: (C)

In solid sodium chloride the ions are locked in the lattice and cannot move; when molten, the ions are free to move and carry the charge. Copper and graphite conduct in the solid state as well.

Correct: (B)

Diamond is a giant covalent structure in which every carbon is bonded to four others; melting requires breaking many strong covalent bonds.

Correct: (D)

In SF6 the sulfur atom is surrounded by 12 electrons (6 bonding pairs), which is more than an octet. BF3 has only 6 electrons around boron, and N and C in the others have 8.

Correct: (A)

HCl is a polar molecule, but hydrogen is not bonded to N, O or F, so true hydrogen bonding does not occur; permanent dipole-dipole attractions are the strongest intermolecular forces (stronger here than London dispersion forces).

Correct: (C)

Nitrogen molecules are held together by a triple bond, consisting of one sigma bond and two pi bonds.

Correct: (B)

Aluminium atoms contribute three electrons each to the delocalized electron sea, giving stronger metallic bonding than magnesium (two electrons) or sodium (one electron), so aluminium has the highest melting point.

Correct: (D)

Iron is a transition metal: it has a partially filled d subshell, forms coloured compounds and shows variable oxidation states. Calcium, aluminium and lead are main-group metals.

Correct: (A)

Across the period the elements change from metallic to non-metallic, so metallic character decreases; electronegativity, first ionization energy and atomic number all increase.

Correct: (C)

Ligands split the d orbitals into groups of different energies; electrons absorb visible light as they are promoted between these groups, and the colour seen is complementary to the colour absorbed.

Correct: (B)

Alkanes are saturated hydrocarbons with general formula CnH2n+2. (CnH2n applies to alkenes and CnH2n-2 to alkynes.)

Correct: (D)

2-methylpropane has the same molecular formula, C4H10, but a different structural arrangement from butane.

Correct: (A)

The longest chain containing the -OH group has 4 carbons, and the hydroxyl group is on carbon 2, giving butan-2-ol.

Correct: (C)

CH3COOCH3 (methyl ethanoate) contains the ester functional group, -COO-. CH3COOH is a carboxylic acid, CH3CH2OH an alcohol and CH3CHO an aldehyde.

Correct: (B)

First ionization energy generally increases across the period, and phosphorus, the furthest right of these elements, also benefits from a stable half-filled 3p subshell, so it has the highest value.

Correct: (D)

q = 100 × 4.18 × 6.8 = 2842 J released; n(H2O formed) = 0.0500 × 1.00 = 0.0500 mol; ΔH = -2842 / 0.0500 = -56.8 kJ mol^-1.

Correct: (A)

Thermal decomposition requires a continuous input of heat, so it is endothermic; combustion, neutralization and condensation all release energy to the surroundings.

Correct: (B)

ΔH = [2 × (-394)] – [2 × (-110) + 0] = -788 + 220 = -568 kJ.

Correct: (C)

Bonds broken: C=C + H-H = 614 + 436 = 1050 kJ; bonds formed: C-C + 2(C-H) = 347 + 826 = 1173 kJ; ΔH = 1050 – 1173 = -123 kJ mol^-1.

Correct: (A)

The enthalpy change of formation is the enthalpy change when 1 mol of a compound is formed from its elements in their standard states, which only option (A) shows.

Correct: (D)

Gases have a far greater degree of disorder than liquids or solids, so steam has the greatest entropy of the samples listed.

Correct: (B)

ΔG = ΔH – TΔS. With ΔS negative, the -TΔS term is positive; it becomes small enough for ΔG to be negative only at low temperatures.

Correct: (C)

Lattice enthalpy is the enthalpy change when 1 mol of an ionic solid is broken apart into its separate gaseous ions.

Correct: (A)

Rate constants with units dm^3 mol^-1 s^-1 are characteristic of second-order reactions (first order: s^-1; zero order: mol dm^-3 s^-1).

Correct: (D)

A catalyst lowers Ea without changing the distribution of particle energies, so the fraction of particles able to react successfully increases.

Correct: (B)

Doubling [A] (experiments 1 to 2) doubles the rate, so the reaction is first order in A; doubling [B] (experiments 1 to 3) leaves the rate unchanged, so it is zero order in B.

Correct: (C)

Although collision frequency rises slightly, the main effect is the large increase in the fraction of collisions with E ≥ Ea.

Correct: (A)

Pure solids are omitted from equilibrium constant expressions because their concentrations are constant, leaving Kc = [CO2].

Correct: (D)

There are 3 mol of gas on the left and 2 mol on the right, so increasing the pressure shifts the equilibrium towards the products. A catalyst speeds up both reactions equally and does not change the position of equilibrium.

Correct: (B)

[PCl3] = [Cl2] = 0.500 – 0.300 = 0.200 mol dm^-3; Kc = (0.200 × 0.200) / 0.300 = 0.133.

Correct: (C)

Increasing the temperature favours the endothermic (reverse) reaction, so less product is present at equilibrium and Kc decreases.

Correct: (A)

Four moles of reactant gas form two moles of product gas; by Le Chatelier's principle, increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, increasing the yield.

Correct: (D)

In dynamic equilibrium both reactions continue at equal rates, so there is no net change in concentrations (although the concentrations themselves need not be equal).

Correct: (B)

For a first-order reaction the half-life is independent of the starting concentration, so it stays constant throughout the reaction. (A straight-line concentration-time graph is characteristic of zero order.)

Correct: (C)

Q = [B] / [A] = 0.10 / 0.40 = 0.25, which is less than Kc = 0.50, so the forward reaction occurs until Q = Kc.

Correct: (A)

KOH is a strong base, so [OH^-] = 0.0010 mol dm^-3, pOH = -log(0.0010) = 3.00, and pH = 14.00 – 3.00 = 11.00.

Correct: (D)

[OH^-] = sqrt(Kb × c) = sqrt(1.8 × 10^-5 × 0.100) = 1.34 × 10^-3 mol dm^-3; pOH = 2.87, so pH = 14.00 – 2.87 = 11.13.

Correct: (B)

A buffer consists of a weak acid (or weak base) together with its conjugate salt. Mixtures based on the strong acids HCl and HNO3 cannot act as buffers.

Correct: (C)

At the equivalence point the solution contains sodium ethanoate; the ethanoate ions hydrolyse (react with water) to produce OH^- ions, so the solution is alkaline.

Correct: (A)

Hydrochloric acid is a strong acid and is fully ionized in aqueous solution, whereas ethanoic acid, carbonic acid and the ammonium ion are all weak acids.

Correct: (D)

Taking K as +1 and each O as -2: (+1) + Mn + 4(-2) = 0, so Mn = +7.

Correct: (B)

Fe^3+ gains an electron to form Fe^2+ (its oxidation number falls from +3 to +2), so it is reduced; I^- is oxidized.

Correct: (C)

At the anode (positive electrode), bromide ions lose electrons and are discharged as bromine. Lead is formed at the cathode.

Correct: (A)

The strongest oxidizing agent is the species that is most easily reduced, which is the one whose couple has the most positive E°. Ag^+ has the most positive value, +0.80 V.

Correct: (D)

The pi bond of the alkene is attacked by the electrophile H^+ from HBr, and Br^- then adds to the carbocation formed: an electrophilic addition reaction.

Correct: (B)

Two molecules join together with the elimination of a small molecule (water), which is a condensation reaction (esterification).

Correct: (C)

The tertiary halogenoalkane ionizes in the rate-determining step to give a stable tertiary carbocation (SN1), so its hydrolysis is fast; bromoethane, a primary halogenoalkane, reacts much more slowly by SN2.