IB Chemistry Practice Test 19

Correct: (B)

Aluminium has 13 protons and 27 – 13 = 14 neutrons. Al^3+ has lost 3 electrons, so it has 13 – 3 = 10 electrons.

Correct: (C)

Fe is [Ar] 4s^2 3d^6. When iron forms Fe^2+, the 4s electrons are removed first, leaving [Ar] 3d^6.

Correct: (A)

Ar = (0.692 × 63) + (0.308 × 65) = 43.6 + 20.0 = 63.6.

Correct: (D)

n = m / Mr = 8.80 / 44.0 = 0.200 mol.

Correct: (B)

V = nRT / P = (0.250 × 8.31 × 273) / 100 = 5.67 dm^3 (using P in kPa gives V in dm^3).

Correct: (C)

n = 5.85 / 58.5 = 0.100 mol; c = 0.100 / 0.500 = 0.200 mol dm^-3.

Correct: (A)

C: 40.0 / 12 = 3.33; H: 6.7 / 1 = 6.7; O: 53.3 / 16 = 3.33. The ratio 3.33 : 6.7 : 3.33 simplifies to 1 : 2 : 1, giving CH2O.

Correct: (D)

Each CH4 molecule has 4 H atoms, so n(H) = 0.250 × 4 = 1.00 mol; number of atoms = 1.00 × 6.02 × 10^23 = 6.02 × 10^23.

Correct: (B)

Sublimation is the change of state from solid directly to gas, without passing through the liquid state.

Correct: (C)

n(HCl) = 0.0250 × 0.100 = 0.00250 mol = n(NaOH); V = 0.00250 / 0.200 = 0.0125 dm^3 = 12.5 cm^3.

Correct: (A)

In NH4Cl the atoms inside the NH4^+ ion are covalently bonded, and the NH4^+ and Cl^- ions are held together by ionic bonding.

Correct: (D)

Sulfur has 5 electron domains (4 bonding pairs and 1 lone pair), giving a seesaw molecular geometry.

Correct: (B)

Methane has four bonding pairs arranged tetrahedrally, so the bond angle is 109.5°.

Correct: (C)

Water is bent and has two lone pairs on oxygen, so its bond dipoles do not cancel and the molecule is polar. The other molecules are symmetrical and nonpolar.

Correct: (A)

HF molecules form hydrogen bonds between molecules, which are much stronger than the dipole-dipole forces in the other hydrogen halides, giving HF the highest boiling point.

Correct: (D)

In graphite each carbon forms three covalent bonds; the fourth electron of each carbon is delocalized and can carry charge along the layers.

Correct: (B)

MgO has doubly charged ions (Mg^2+ and O^2-) and small ionic radii, so the electrostatic attraction between the ions is the strongest.

Correct: (C)

Each carbon in ethene has three electron domains in a trigonal planar arrangement, which corresponds to sp^2 hybridization.

Correct: (A)

Ethyne has 3 sigma bonds (2 C-H and 1 C-C), and the carbon-carbon triple bond contains 2 pi bonds in addition to its sigma bond.

Correct: (D)

Metallic bonding is non-directional: when layers of cations slide, the delocalized electron sea still holds them together, so the metal changes shape without breaking.

Correct: (C)

Atomic radius decreases across Period 3 because nuclear charge increases while shielding stays similar, so sodium, on the far left, has the largest radius.

Correct: (B)

In aluminium the outer electron is in the 3p subshell, which is higher in energy and more shielded than the 3s electron removed from magnesium, so it is lost more easily despite the extra proton.

Correct: (D)

Each oxide ion has an oxidation state of -2, so two oxide ions give -4 in total; titanium must be +4 for the compound to be neutral.

Correct: (A)

The -COOH group is the carboxyl functional group, characteristic of carboxylic acids.

Correct: (C)

Ethene is an alkene with general formula CnH2n; C4H8 also fits CnH2n, whereas the others are alkanes (CnH2n+2).

Correct: (B)

The longest chain has 3 carbons with a methyl branch on the central carbon, so the name is 2-methylpropane.

Correct: (D)

Butane and 2-methylpropane are the two structural isomers of C4H10.

Correct: (A)

Non-metal oxides such as SO3 are acidic; Na2O, MgO and CaO are basic metal oxides.

Correct: (C)

q = mcΔT = 50.0 × 4.18 × 10.0 = 2090 J = 2.09 kJ.

Correct: (B)

q = 200 × 4.18 × 15.0 = 12 540 J = 12.54 kJ released; ΔH = -12.54 / 0.0200 = -627 kJ mol^-1. The negative sign shows that combustion is exothermic.

Correct: (D)

By Hess's law: ΔHf(CH4) = (-394) + 2(-286) – (-890) = -966 + 890 = -76 kJ mol^-1.

Correct: (A)

Bonds broken: 436 + 242 = 678 kJ; bonds formed: 2 × 431 = 862 kJ; ΔH = 678 – 862 = -184 kJ mol^-1.

Correct: (C)

Elements in their standard states have ΔHf = 0, so ΔH = 2 × (-46) – 0 = -92 kJ.

Correct: (B)

A gas is produced from a solid reactant, so the number of freely moving particles increases and the entropy of the system increases.

Correct: (D)

ΔG = ΔH – TΔS = -100 – (298 × -0.200) = -100 + 59.6 = -40.4 kJ mol^-1.

Correct: (A)

Ea = energy of the transition state – energy of the reactants = 150 – 40 = 110 kJ mol^-1.

Correct: (C)

For rate = k[A], k = rate / [A], so the units are (mol dm^-3 s^-1) / (mol dm^-3) = s^-1.

Correct: (B)

At higher temperature the particles have a wider range of energies; the maximum of the curve shifts to higher energy and the peak height falls, while the total area (total number of particles) stays the same.

Correct: (D)

A catalyst offers a different reaction route with a lower Ea, so a greater fraction of collisions are successful. It does not change ΔH.

Correct: (A)

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

Correct: (C)

60 s corresponds to 3 half-lives, so the fraction remaining is (1/2)^3 = 1/8.

Correct: (B)

Kc is the product concentrations divided by the reactant concentrations, each raised to the power of its stoichiometric coefficient.

Correct: (D)

Kc = (0.80)^2 / (0.10 × 0.10) = 0.64 / 0.010 = 64.

Correct: (A)

The forward reaction is endothermic, so increasing the temperature shifts the equilibrium towards the products. Increasing the pressure would shift it towards fewer gas molecules, i.e. towards N2O4.

Correct: (C)

Q > Kc means the mixture contains too much product relative to equilibrium, so the reverse reaction occurs until Q = Kc.

Correct: (B)

The value of Kc depends only on temperature; catalysts, pressure changes and concentration changes affect the position of equilibrium or the rates, not Kc itself.

Correct: (D)

The amount of A that reacted is 0.60 mol dm^-3, so [B] = [C] = 0.60 mol dm^-3; Kc = (0.60 × 0.60) / 0.40 = 0.90.

Correct: (A)

At higher pressure the particles are closer together, so successful collisions occur more frequently and the rate increases.

Correct: (C)

HNO3 is a strong acid and is fully ionized, so [H^+] = 0.010 mol dm^-3 and pH = -log(0.010) = 2.00.

Correct: (B)

[OH^-] = 0.050 mol dm^-3, so pOH = -log(0.050) = 1.30 and pH = 14.00 – 1.30 = 12.70.

Correct: (D)

[H^+] = sqrt(Ka × c) = sqrt(1.8 × 10^-5 × 0.100) = 1.34 × 10^-3 mol dm^-3; pH = -log(1.34 × 10^-3) = 2.87.

Correct: (A)

pH = pKa + log([salt] / [acid]) = 4.74 + log(0.20 / 0.30) = 4.74 – 0.18 = 4.56.

Correct: (C)

A conjugate acid is formed when a species gains a proton: HCO3^- + H^+ gives H2CO3.

Correct: (B)

Taking H as +1 and O as -2: 2(+1) + S + 4(-2) = 0, so S = +6.

Correct: (D)

Zn is oxidized (its oxidation number rises from 0 to +2); the species that is oxidized is the reducing agent.

Correct: (A)

E°cell = E°(cathode, reduction) – E°(anode, oxidation) = (+0.34) – (-0.76) = +1.10 V.

Correct: (C)

At the cathode, hydrogen ions from water are discharged in preference to sodium ions, so hydrogen gas is produced. Chlorine is produced at the anode.

Correct: (B)

Initiation is the homolytic fission of the Cl-Cl bond by UV light, forming two chlorine radicals. Options (A) and (C) are propagation steps and (D) is a termination step.

Correct: (D)

The hydroxide ion acts as a nucleophile and substitutes the bromine atom in the halogenoalkane.

Correct: (A)

Under reflux with excess oxidizing agent, a primary alcohol is oxidized fully to the carboxylic acid, ethanoic acid. (Ethanal is obtained only if it is distilled off as it forms.)