IB Chemistry Practice Test 8

Correct: (A)

A neutron has almost the same mass as a proton but carries no charge.

Correct: (C)

A magnesium atom loses its two 3s electrons when forming Mg^2+, leaving the stable configuration 1s^2 2s^2 2p^6.

Correct: (B)

n = m / Mr = 8.8 / 44.0 = 0.200 mol.

Correct: (D)

Concentration = 0.0500 mol / 0.250 dm^3 = 0.200 mol dm^-3.

Correct: (C)

At constant temperature, PV is constant for a fixed amount of gas, so doubling P halves V.

Correct: (A)

0.500 mol of H2 contains 1.00 mol of H atoms, so the number of atoms is 6.02 × 10^23.

Correct: (D)

Dividing each amount by 0.100 gives the simplest ratio C : H : O = 1 : 3 : 1.

Correct: (B)

The total number of electrons is 2 + 2 + 6 + 2 + 5 = 17, and a neutral atom has the same number of protons.

Correct: (D)

n = cV = 0.500 × 0.0200 = 0.0100 mol.

Correct: (B)

The 0.300 mol of H2 needs only 0.150 mol of O2, so hydrogen is limiting and forms 0.300 mol of H2O.

Correct: (A)

In the solid, ions are locked in the lattice. When molten, the ions can move and carry charge to the electrodes.

Correct: (C)

Carbon forms four bonding pairs and has no lone pairs, so the electron domains arrange tetrahedrally with 109.5° angles.

Correct: (B)

Methane is nonpolar and has no permanent dipole, so temporary induced dipoles produce London dispersion forces.

Correct: (D)

The two polar O-H bonds are arranged at an angle, giving one partially negative end near oxygen and a partially positive region near the hydrogen atoms.

Correct: (C)

Nitrogen uses three electron pairs to bond with hydrogen atoms and retains one lone pair, producing a trigonal pyramidal molecular shape.

Correct: (A)

The nondirectional attraction between metal ions and delocalised electrons continues when layers shift, so metals are malleable.

Correct: (C)

Silicon and oxygen atoms are connected by covalent bonds throughout a three-dimensional network, giving SiO2 a very high melting point.

Correct: (B)

Ethanol has an O-H group and forms intermolecular hydrogen bonds. Dimethyl ether has no O-H bond, so its molecules are held together by weaker forces and separate more easily.

Correct: (D)

Forming Na^+ removes the single 3s electron. The remaining ten electrons occupy fewer shells and are pulled closer by the nucleus.

Correct: (A)

Greater bond order means the nuclei are pulled closer together and more energy is needed to break the bond.

Correct: (A)

Down the group, the incoming electron enters a shell farther from the nucleus and is more shielded, so the halogens become weaker oxidising agents.

Correct: (D)

Silicon dioxide consists of silicon and oxygen atoms joined throughout a three-dimensional covalent network. The sodium, magnesium and aluminium oxides are ionic.

Correct: (C)

The -COOH group identifies the compound as propanoic acid, a carboxylic acid.

Correct: (B)

An acyclic monoalkene has two fewer hydrogen atoms than the corresponding alkane, giving CnH2n.

Correct: (B)

Zinc forms only the Zn^2+ ion under normal conditions, and this ion has a full 3d^10 configuration rather than a partially filled d subshell.

Correct: (A)

Typical values are Mg = 738, Al = 578 and Na = 496 kJ mol^-1. Aluminium is lower than magnesium because its outer electron is in a higher-energy 3p subshell.

Correct: (D)

Chlorine is a stronger oxidising agent than bromine, so Cl2 + 2Br^- → 2Cl^- + Br2 occurs.

Correct: (C)

Members share the same functional group and general formula, while physical properties change gradually as chain length increases.

Correct: (C)

Enthalpy change is the heat transferred between a system and its surroundings when a reaction occurs at constant pressure.

Correct: (D)

Magnitude of q = mcΔT = 100.0 × 4.18 × 5.00 = 2090 J = 2.09 kJ.

Correct: (A)

Subtract the second equation from the first: -394 – (-283) = -111 kJ.

Correct: (B)

The strength of a named bond varies slightly with its surroundings, so average bond enthalpies give approximate rather than exact reaction enthalpies.

Correct: (D)

Particles in a gas are far more dispersed than in a liquid or solid, so water vapour has the greatest entropy of the listed samples.

Correct: (A)

At constant temperature and pressure, a process can occur spontaneously when the Gibbs energy change is negative.

Correct: (B)

An exothermic change transfers energy to the surroundings, so its enthalpy change, including an enthalpy of formation, is negative.

Correct: (C)

The equation forms 2 mol of NH3, so ΔH = 2 × (-46) = -92 kJ.

Correct: (A)

A larger surface area exposes more reactant particles, increasing the frequency of collisions at the surface.

Correct: (B)

For rate = k[A], k = rate / concentration, and the concentration units cancel, leaving s^-1.

Correct: (D)

The temperature and energy distribution stay the same, but the lower activation-energy threshold means more molecules exceed it.

Correct: (C)

The rate is proportional to [A]^2 because 2^2 = 4.

Correct: (B)

Kc is temperature dependent. A catalyst speeds up both directions equally, and changing concentrations or pressure changes the position needed to restore equilibrium, not the constant itself.

Correct: (C)

Increasing pressure favours the side with fewer gas molecules. N2O4 is one mole of gas, while the product side has two moles.

Correct: (A)

Rate = 0.500 × 0.200 × 0.300 = 0.0300 mol dm^-3 s^-1.

Correct: (D)

Pure solids are omitted from the equilibrium constant expression because their effective concentrations are constant, leaving only [CO2].

Correct: (C)

Kc = [B] / [A] = 0.300 / 0.700 = 0.429.

Correct: (D)

At dynamic equilibrium, the forward and reverse rates are equal, so composition and macroscopic properties remain constant even though particles continue reacting.

Correct: (B)

A zero-order reaction has a constant rate, so concentration falls by equal amounts in equal time intervals, giving a straight line with negative slope.

Correct: (A)

A catalyst accelerates the forward and reverse reactions equally, so the equilibrium position and Kc are unchanged.

Correct: (D)

A Bronsted-Lowry acid donates a proton to a base during an acid-base reaction.

Correct: (A)

The pH scale is logarithmic, so a decrease of one pH unit corresponds to [H^+] becoming ten times greater.

Correct: (C)

NH4^+ is the conjugate acid of the weak base NH3. It reacts with water: NH4^+ + H2O ⇌ NH3 + H3O^+.

Correct: (B)

The buffer reservoir of conjugate base removes most of the added H^+, forming more weak acid and limiting the pH change.

Correct: (A)

Magnesium atoms lose electrons and increase in oxidation state from 0 to +2, so magnesium is oxidised.

Correct: (D)

A galvanic cell uses a spontaneous redox reaction to produce an electric current.

Correct: (B)

pOH = -log(1.0 × 10^-3) = 3.00. At 298 K, pH + pOH = 14.00, so pH = 11.00.

Correct: (C)

A weak base establishes an equilibrium with water and only a small fraction of its particles form OH^- ions.

Correct: (B)

Hydrogen ions from water are discharged in preference to sodium ions, so hydrogen gas forms at the cathode. Sodium ions are too difficult to reduce in aqueous solution.

Correct: (C)

Let the oxidation state of Mn be x. x + 1 + 4(-2) = 0, so x = +7.

Correct: (D)

The ions formed from a strong acid and a strong base do not hydrolyse appreciably, so the equivalence-point solution is neutral at 25°C.

Correct: (A)

Ultraviolet light produces chlorine radicals, which replace hydrogen atoms in the alkane by a free-radical substitution mechanism.