A-Level Chemistry Practice Test 18

Correct: (D)

Ionisation energies always refer to gaseous atoms forming gaseous ions, with one mole of electrons removed from one mole of atoms.

Correct: (B)

Ar = (28 × 92.2 + 29 × 4.7 + 30 × 3.1) / 100 = (2581.6 + 136.3 + 93.0) / 100 = 28.1.

Correct: (A)

Across a period the proton number rises but shielding stays roughly constant, so the effective nuclear charge increases and the electrons are held more tightly.

Correct: (C)

Copper is an exception to the expected filling pattern: a full 3d subshell is especially stable, so the configuration is [Ar] 4s^1 3d^10 rather than [Ar] 4s^2 3d^9.

Correct: (D)

Flight time is proportional to sqrt(m/z): sqrt(25/100) = 0.5, so the time is 0.5 × 2.0 × 10^-5 = 1.0 × 10^-5 s.

Correct: (A)

n = V / molar volume = 0.480 / 24 = 0.0200 mol.

Correct: (C)

Moles of NaOH = 0.200 × 18.6/1000 = 0.00372, which equals the moles of HCl. Concentration = 0.00372 / 0.0250 = 0.149 mol dm^-3.

Correct: (B)

Percentage yield = (12.0 / 15.0) × 100 = 80%.

Correct: (D)

Moles of Fe = 3.36 / 56 = 0.0600; moles of O = 1.28 / 16 = 0.0800. The ratio 0.0600 : 0.0800 = 3 : 4, so the empirical formula is Fe3O4.

Correct: (A)

V = nRT/p = (0.200 × 8.31 × 300) / 100000 = 4.99 × 10^-3 m^3 = 4.99 dm^3.

Correct: (C)

In an addition reaction, all of the atoms in the reactants end up in the single product, so the atom economy is 100%.

Correct: (B)

The five electron pairs arrange in a trigonal bipyramidal geometry; the two lone pairs occupy equatorial positions, leaving a T-shaped molecule.

Correct: (D)

NH3 is trigonal pyramidal, so its bond dipoles do not cancel. BF3, CO2 and CH4 are symmetrical, so their bond dipoles cancel out.

Correct: (A)

Mg^2+ and O^2- are small ions carrying high charges, giving the strongest electrostatic attractions and the most exothermic lattice enthalpy.

Correct: (C)

SiO2 has a giant covalent structure, so melting requires breaking strong covalent bonds. Iodine is a simple molecular substance with only weak forces between its molecules.

Correct: (D)

The O−H group in ethanol allows hydrogen bonding between molecules; methoxymethane has no O−H bond and can form only weaker permanent dipole-dipole forces.

Correct: (B)

Larger molecules have bigger, more easily distorted electron clouds, giving stronger instantaneous dipole-induced dipole (London) forces between the molecules.

Correct: (A)

q = mcΔT = 100 × 4.18 × 6.8 = 2842 J. Moles of water formed = 0.0500. ΔH = −2842 / 0.0500 = −56.8 kJ mol^-1.

Correct: (C)

For 2C + 2H2 → C2H4: ΔHf = 2(−393) + 2(−286) − (−1411) = −786 − 572 + 1411 = +53 kJ mol^-1.

Correct: (D)

Bonds broken = 945 + 3(436) = 2253 kJ mol^-1. Bonds formed = 6(391) = 2346 kJ mol^-1. ΔH = 2253 − 2346 = −93 kJ mol^-1.

Correct: (B)

The number of moles of gas decreases from 3 to 2, so the disorder decreases and ΔS is negative. The other reactions produce gas or a liquid from a solid, increasing disorder.

Correct: (A)

The reaction becomes feasible when ΔG = ΔH − TΔS becomes negative, i.e. above T = ΔH/ΔS = 120000/300 = 400 K.

Correct: (C)

ΔG = −50000 − (298 × −100) = −50000 + 29800 = −20200 J mol^-1 = −20.2 kJ mol^-1, so the reaction is feasible.

Correct: (D)

The rate is proportional to [X]^2 and is independent of [Y].

Correct: (B)

k = rate / [A]^2 = (4.0 × 10^-3) / (0.20)^2 = (4.0 × 10^-3) / 0.040 = 0.10 mol^-1 dm^3 s^-1.

Correct: (A)

Ea and A are effectively constant; as T increases, −Ea/RT becomes less negative, so the exponential term, and hence k, increases.

Correct: (C)

In a zero-order reaction the rate is constant, so the concentration falls at a constant rate. A constant half-life and a straight ln plot are features of first-order reactions.

Correct: (D)

A catalyst provides an alternative reaction pathway with a lower activation energy; it does not change the energy distribution of the molecules.

Correct: (A)

Kc = [HI]^2 / ([H2][I2]) = (0.80)^2 / (0.10 × 0.10) = 0.64 / 0.010 = 64. Kc has no units here because there are equal numbers of moles on both sides.

Correct: (C)

Kp is the partial pressures of the products, each raised to its stoichiometric coefficient, divided by the same for the reactants.

Correct: (B)

0.80 mol of A has reacted, forming 0.80 mol each of B and C. Concentrations: [A] = 0.600, [B] = [C] = 0.400 mol dm^-3. Kc = (0.400 × 0.400) / 0.600 = 0.267 mol dm^-3.

Correct: (D)

Equilibrium constants depend only on temperature. A pressure change may shift the position of equilibrium, but it does not change K.

Correct: (A)

Changing the concentrations changes the position of equilibrium, but Kc remains the same at a given temperature.

Correct: (C)

2(+1) + 2S + 3(−2) = 0, so 2S = +4 and the average oxidation state of sulfur is +2.

Correct: (B)

EMF = 0.34 − (−0.44) = +0.78 V.

Correct: (D)

The oxidising agent must come from a system with an E° more positive than +0.77 V, so that E°cell is positive. Only acidified manganate(VII), at +1.51 V, qualifies.

Correct: (A)

Q = It = 2.00 × 4825 = 9650 C, which is 0.100 mol of electrons. Al^3+ + 3e^- → Al, so moles of Al = 0.100/3 = 0.0333 and mass = 0.0333 × 27.0 = 0.900 g.

Correct: (C)

At the negative electrode (anode), hydrogen is oxidised and releases electrons; oxygen is reduced at the positive electrode.

Correct: (D)

[H^+] = 2 × 0.00100 = 0.00200 mol dm^-3, so pH = −log(0.00200) = 2.70.

Correct: (A)

pKa = −log(4.0 × 10^-4) = 3.40.

Correct: (B)

The ethanoate ions (the conjugate base in the buffer) remove most of the added H^+, forming the weak acid, so the pH stays almost constant.

Correct: (C)

Moles of H^+ = 0.0100; moles of OH^- = 0.00500; excess H^+ = 0.00500 mol in 75.0 cm^3, so [H^+] = 0.0667 mol dm^-3. pH = −log(0.0667) = 1.18.

Correct: (D)

The ionisation of water is endothermic, so Kw increases with temperature and both [H^+] and [OH^-] rise equally. The water remains neutral ([H^+] = [OH^-]) but the pH falls below 7.

Correct: (A)

The solubility of the Group 2 hydroxides increases down the group, so Ba(OH)2 is the most soluble.

Correct: (B)

A more reactive halogen displaces a less reactive halogen from its halide. Bromine is above iodine in Group 7, so bromine displaces iodide ions.

Correct: (C)

Ligand substitution by chloride ions gives the tetrahedral [CoCl4]^2- complex, which is blue; the coordination number falls from 6 to 4.

Correct: (D)

Ba^2+ ions form a white precipitate of BaSO4 with sulfate ions. The hydrochloric acid is added to remove carbonate ions, which would also give a white precipitate with Ba^2+.

Correct: (A)

Transition metals generally have high melting and boiling points because of strong metallic bonding involving 3d and 4s electrons. The other statements are characteristic properties of transition metals.

Correct: (B)

The CN group replaces Br, and the nitrile carbon adds one carbon to the chain: CH3CH2Br → CH3CH2CN, propanenitrile.

Correct: (C)

Electrophilic addition of water follows Markovnikov's rule, so the OH group ends up on the middle carbon, giving propan-2-ol.

Correct: (D)

In 2-methylpropan-2-ol, the carbon atom bonded to the OH group is attached to three other carbon atoms.

Correct: (B)

The alcohol is dehydrated in an elimination reaction, giving the alkene ethene (and water).

Correct: (A)

In a termination step, two radicals combine to form a stable molecule, removing radicals from the chain reaction. (B) and (C) are propagation steps and (D) is initiation.

Correct: (B)

In a limited supply of oxygen, methane burns incompletely to give toxic carbon monoxide and water. Equation (A) is complete combustion.

Correct: (D)

Acyl chlorides react vigorously with alcohols to give an ester and HCl: CH3COCl + C2H5OH → CH3COOC2H5 + HCl.

Correct: (B)

LiAlH4 is a powerful reducing agent that reduces carboxylic acids to primary alcohols. NaBH4 is not strong enough to reduce a carboxylic acid, and the other two reagents are oxidising agents.

Correct: (C)

Ninhydrin reacts with amino acids to form purple compounds, making the otherwise invisible spots visible.

Correct: (B)

The diamine and the diacyl chloride condense together, eliminating HCl, to form the polyamide. (The pair in option C forms the polyester Terylene.)

Correct: (A)

The CH3 protons, the CH2 protons and the OH proton are in three different environments, giving three peaks with an integration ratio of 3 : 2 : 1.

Correct: (C)

Rf = distance moved by the spot / distance moved by the solvent = 4.5 / 9.0 = 0.50.