A-Level Chemistry Practice Test 8

Correct: (D)

Ar = (24 × 79.0 + 25 × 10.0 + 26 × 11.0) / 100 = (1896 + 250 + 286) / 100 = 24.32, i.e. 24.3.

Correct: (A)

Magnesium loses a 3s electron whereas aluminium loses a 3p electron; the 3p electron is at a higher energy level and slightly further from the nucleus, so it is removed more easily.

Correct: (C)

Each of the five 3d orbitals contains one unpaired electron and the 4s orbital contains one more, giving 5 + 1 = 6 unpaired electrons.

Correct: (B)

Sodium has one outer electron; the second electron must be removed from a full inner shell (n = 2) that is much closer to the nucleus, so the second ionisation energy is very much larger.

Correct: (D)

All ions are given the same kinetic energy, so the ion with the largest mass (largest m/z) travels most slowly and has the longest flight time.

Correct: (A)

480 cm^3 = 0.480 dm^3; amount = 0.480 / 24.0 = 0.0200 mol.

Correct: (B)

Moles NaOH = 0.125 × 0.0200 = 0.00250 mol = moles HCl; concentration = 0.00250 / 0.0250 = 0.100 mol dm^-3.

Correct: (C)

NH4NO3 contains two nitrogen atoms: mass of N = 28; percentage = (28 / 80) × 100 = 35.0%.

Correct: (D)

p = nRT / V = (0.500 × 8.31 × 300) / 0.0100 = 124 650 Pa, which is approximately 125 kPa.

Correct: (A)

0.200 mol of Mg would need 0.400 mol of HCl, so HCl is the limiting reagent; moles of H2 = 0.300 / 2 = 0.150 mol.

Correct: (C)

Moles = 5.85 / 58.5 = 0.100 mol; concentration = 0.100 / 0.500 = 0.200 mol dm^-3.

Correct: (B)

Nitrogen has three bonding pairs and one lone pair; the lone pair repels more strongly, compressing the bond angle to 107° in a trigonal pyramidal shape.

Correct: (D)

Sodium chloride has a giant ionic lattice with strong electrostatic attractions in all directions; the others are simple molecular substances with weak intermolecular forces.

Correct: (A)

Lattice enthalpy is more exothermic for small ions with high charges; Mg^2+ and O^2- are both small and doubly charged, so MgO has the most exothermic lattice enthalpy.

Correct: (C)

Iodine molecules are non-polar, so the only attractions between them are temporary (van der Waals / London dispersion) forces.

Correct: (B)

Sulfur in SF6 has six bonding pairs and no lone pairs, which repel to give an octahedral shape with 90° bond angles.

Correct: (D)

In ice each water molecule is hydrogen-bonded into an open, tetrahedral lattice with large spaces, so ice has a lower density than liquid water and floats.

Correct: (C)

q = 200 × 4.18 × 11.0 = 9196 J = 9.20 kJ. Moles of ethanol = 0.500 / 46.0 = 0.0109 mol. ΔH = −9.20 / 0.0109 = −846 kJ mol^-1.

Correct: (A)

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

Correct: (B)

Gases are far more disordered than liquids or solids, so 1 mol of steam has the greatest entropy.

Correct: (D)

ΔG = ΔH − TΔS becomes negative when TΔS exceeds ΔH, i.e. when T = 50 000 / 200 = 250 K is exceeded, so the reaction is feasible above 250 K.

Correct: (C)

Moles of water formed = 2.00 × 0.0250 = 0.0500 mol; q = 0.0500 × 57.0 = 2.85 kJ = 2850 J. Mass of mixture = 50.0 g, so ΔT = 2850 / (50.0 × 4.18) = 13.7 K.

Correct: (A)

Standard enthalpy of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions (100 kPa, 298 K).

Correct: (B)

3^2 = 9, so the rate is proportional to [A]^2 and the reaction is second order with respect to A.

Correct: (D)

A larger activation energy means a smaller fraction of molecules have enough energy to react, so k decreases.

Correct: (C)

For a first-order reaction, t(1/2) = ln 2 / k = 0.693 / 0.0231 = 30 s.

Correct: (A)

For a zero-order reaction the rate constant has the same units as rate itself, mol dm^-3 s^-1.

Correct: (B)

At a higher temperature the distribution flattens and shifts to higher energies; the area stays the same because the number of molecules is unchanged. The activation energy does not change.

Correct: (D)

Kc = [Q]^2 / [P] = (1.00)^2 / 0.50 = 2.00 mol dm^-3.

Correct: (C)

Raising the temperature shifts the equilibrium in the endothermic (reverse) direction, so the equilibrium concentration of NH3 falls and Kc decreases.

Correct: (B)

Partial pressure = mole fraction × total pressure = 0.40 × 200 = 80 kPa.

Correct: (A)

A lower temperature gives a better yield but an unacceptably slow rate; 450 °C is a compromise between the rate of reaction and the equilibrium yield.

Correct: (D)

Kp = p(SO3)^2 / [ p(SO2)^2 × p(O2) ]. The top has pressure^2 and the bottom pressure^3, so the units are pressure^-1, i.e. kPa^-1.

Correct: (A)

2(+1) + x + 4(−2) = 0, so x = +6.

Correct: (C)

The strongest oxidising agent is the species in the half-equation with the most positive electrode potential, which is Cl2 at +1.36 V.

Correct: (B)

EMF = +0.34 − (−2.37) = 2.71 V.

Correct: (D)

Sodium is too reactive to be discharged from aqueous solution, so H+ ions are discharged at the cathode to form hydrogen gas.

Correct: (A)

Chlorine starts with oxidation state 0 and is simultaneously reduced to −1 (in NaCl) and oxidised to +1 (in NaClO), which is disproportionation.

Correct: (C)

[H+] = 0.200 mol dm^-3, so pH = −log(0.200) = 0.70.

Correct: (B)

[H+] = sqrt(Ka × c) = sqrt(1.3 × 10^-5 × 0.0500) = sqrt(6.5 × 10^-7) = 8.1 × 10^-4 mol dm^-3; pH = −log(8.1 × 10^-4) = 3.09.

Correct: (D)

[H+] = Ka × [acid] / [salt] = 1.8 × 10^-5 × 0.300 / 0.100 = 5.4 × 10^-5 mol dm^-3; pH = 4.26 (equivalently, pH = pKa + log([salt]/[acid]) = 4.74 + log(1/3) = 4.74 − 0.48).

Correct: (A)

In neutral water [H+] = sqrt(Kw) = sqrt(5.5 × 10^-14) = 2.3 × 10^-7 mol dm^-3; pH = −log(2.3 × 10^-7) = 6.63. Neutral water has [H+] = [OH-] even though the pH is below 7.

Correct: (C)

At the half-neutralisation point [HA] = [A-], so [H+] = Ka and pH = pKa = 5.00.

Correct: (B)

Solubility of Group 2 hydroxides increases down the group (the opposite trend to the sulfates), so Ba(OH)2 is the most soluble.

Correct: (D)

Cl2 + 2NaOH → NaCl + NaClO + H2O; the chlorine is disproportionated to chloride and chlorate(I) ions.

Correct: (A)

Zn^2+ has a full 3d subshell (3d10), so no d−d electron transitions can occur and its compounds are colourless (white).

Correct: (C)

Ag+ ions give a white precipitate of AgCl with chloride ions; the solution is acidified with dilute nitric acid to remove carbonate ions first.

Correct: (B)

Al(OH)3 is amphoteric; it first precipitates as a white solid and then dissolves in excess NaOH to form the colourless [Al(OH)4]- complex.

Correct: (D)

Cracking splits a long-chain alkane into a shorter alkane and an alkene: C10H22 → C8H18 + C2H4, and the atoms balance (C: 8 + 2 = 10; H: 18 + 4 = 22).

Correct: (A)

The pi electrons in the C=C bond attack the electrophile Br^δ+, forming a carbocation that then reacts with Br-, giving dibromoethane overall by electrophilic addition.

Correct: (C)

Butan-2-ol is a secondary alcohol, so it is oxidised to a ketone, butanone; ketones are not oxidised further by dichromate(VI).

Correct: (B)

2-Bromo-2-methylpropane is a tertiary halogenoalkane; it hydrolyses fastest because the tertiary carbocation formed in the SN1 mechanism is stabilised by three alkyl groups.

Correct: (D)

In propagation, a radical reacts with a molecule to give a product and a new radical, so the chain continues; the other steps are initiation (A) and termination (B and C).

Correct: (A)

The acid catalyst causes an elimination (dehydration) reaction, removing H2O from the alcohol to form propene.

Correct: (C)

Propanal is an aldehyde and reduces Fehling's solution to give a brick-red precipitate of Cu2O on warming; propanone, a ketone, does not react. 2,4-DNP reacts with both compounds.

Correct: (B)

Acid hydrolysis of an ester gives the parent carboxylic acid and alcohol: CH3COOCH2CH3 + H2O → CH3COOH + CH3CH2OH.

Correct: (D)

The −OH group activates the ring so strongly that bromine substitutes at the 2, 4 and 6 positions without a catalyst, forming white 2,4,6-tribromophenol.

Correct: (A)

Nylon-6,6 forms when a diamine and a dicarboxylic acid react together with the elimination of water molecules, which is condensation polymerisation.

Correct: (B)

All six hydrogen atoms are in the same chemical environment (two equivalent CH3 groups), so there is only one peak (a singlet).

Correct: (C)

Rf = distance moved by the spot / distance moved by the solvent front = 4.8 / 8.0 = 0.60.