12th Grade IB HL Chemistry

Solution

Correct: D

The Arrhenius equation, k = Ae^(-Ea/RT), shows that the rate constant (k) increases exponentially with increasing temperature (T). A is the pre-exponential factor, Ea is the activation energy, and R is the gas constant. As T increases, the exponential term becomes larger, and therefore k increases.

Solution

Correct: D

The standard cell potential (E°cell) is calculated as E°(cathode) - E°(anode). In this cell, Cu is being reduced (cathode) and Zn is being oxidized (anode). Therefore, E°cell = E°(Cu²⁺/Cu) - E°(Zn²⁺/Zn) = 0.34 V - (-0.76 V) = +1.10 V.

Solution

Correct: C

Geometric isomerism occurs when atoms or groups of atoms are arranged differently in space around a central metal ion, and this is possible when there are at least two different ligands. Tetrahedral (like Zn and often Ag) and linear complexes generally do not exhibit geometric isomerism due to the free rotation of ligands. [Co(NH₃)₅Cl]²⁺ cannot exhibit geometric isomerism due to having multiple of the same ligand. Square planar complexes with the general formula MA₂B₂ (like trans-[Pt(NH₃)₂Cl₂] ) exhibit cis-trans isomerism (geometric isomerism).

Solution

Correct: B

An alcoholic solution of KOH promotes elimination (E2) reactions. CH₃CH₂CH₂Br is a primary alkyl halide, and in the presence of a strong base like KOH in ethanol, it will undergo an elimination reaction to form the alkene, CH₃CH=CH₂ (propene).

Solution

Correct: C

For a weak acid, [H⁺] ≈ √(Ka * [HA]). So, [H⁺] ≈ √(1.8 x 10⁻⁵ * 0.02) = √(3.6 x 10⁻⁷) ≈ 6 x 10⁻⁴ M. pH = -log[H⁺] = -log(6 x 10⁻⁴) ≈ 3.22. The closest choice is 3.37. However, it's important to verify the 5% rule: (6 x 10⁻⁴ / 0.02) * 100% = 3%. Since it's less than 5%, the approximation is valid.

Solution

Correct: B

Boiling point increases with increasing intermolecular forces. Alkanes (CH₃CH₂CH₃ and CH₃CH₂CH₂CH₃) exhibit only London dispersion forces. Ethers (CH₃OCH₃) exhibit dipole-dipole interactions, in addition to London dispersion forces. Alcohols (CH₃CH₂OH) exhibit hydrogen bonding, which is the strongest intermolecular force. Furthermore, CH₃CH₂CH₂CH₃ has a greater molecular weight than CH₃CH₂CH₃. Hydrogen bonding is the dominant factor in the question; therefore, CH₃CH₂OH will have the highest boiling point.

Solution

Correct: C

The overall order of a reaction is the sum of the exponents in the rate law. In this case, the exponent of [A] is 2, and the exponent of [B] is 1. Therefore, the overall order is 2 + 1 = 3, which is third order.

Solution

Correct: B

The compound CH₃CH₂COCH₃ contains a carbonyl group (C=O) bonded to two alkyl groups, making it a ketone. The longest carbon chain containing the carbonyl group has four carbons. The carbonyl group is on the second carbon. Therefore, the IUPAC name is butanone.

Solution

Correct: A

In CO₂, the central carbon atom forms two double bonds with the oxygen atoms. This means there are two sigma bonds and no lone pairs. The carbon atom needs two hybrid orbitals to form these sigma bonds. Therefore, the hybridization is sp.

Solution

Correct: C

Freezing point depression is a colligative property, meaning it depends on the number of solute particles in the solution. The more particles, the greater the depression. Glucose and sucrose are non-electrolytes and dissolve into 1 particle. NaCl dissolves into 2 particles (Na⁺ and Cl⁻). MgCl₂ dissolves into 3 particles (Mg²⁺ and 2Cl⁻). Therefore, 1.0 m MgCl₂ will have the lowest freezing point.

Solution

Correct: A

In the presence of peroxides, the addition of HBr to an alkene follows an anti-Markovnikov rule via a radical mechanism. This means the bromine atom will add to the carbon with more hydrogens. 2-methylpropene is (CH₃)₂C=CH₂. The carbon with the most hydrogens is the CH₂ group, so the bromine will add to this carbon. This yields 1-bromo-2-methylpropane.

Solution

Correct: B

An oxidizing agent is a substance that gains electrons and is reduced. The stronger the oxidizing agent, the more readily it is reduced. Fluorine (F₂) has a very high electronegativity and a strong tendency to gain electrons. Li⁺ and K⁺ are already in their highest oxidation state, so cannot act as oxidizing agents. I₂ is a weaker oxidizing agent than F₂.

Solution

Correct: C

After one half-life (10 days), 1/2 of the isotope remains. After two half-lives (20 days), (1/2) * (1/2) = 1/4 remains. After three half-lives (30 days), (1/2) * (1/2) * (1/2) = 1/8 remains.

Solution

Correct: B

When a strong acid is titrated with a strong base, the equivalence point pH is 7. When a strong acid is titrated with a weak base, the equivalence point pH is less than 7 because the conjugate acid of the weak base will hydrolyze to produce H⁺ ions. When a weak acid is titrated with a strong base, the equivalence point pH is greater than 7 because the conjugate base of the weak acid will hydrolyze to produce OH⁻ ions. A strong acid with a strong acid will not react at all, so no equivalence point exists.

Solution

Correct: C

Ethylenediamine (en) is a bidentate ligand, meaning it binds to the central metal ion through two donor atoms. In this complex, there are two ethylenediamine ligands, each contributing two donor atoms, and two chloride ligands, each contributing one donor atom. Therefore, the coordination number is (2 * 2) + (2 * 1) = 6.

Solution

Correct: B

The salt bridge allows ions to flow between the two half-cells in order to maintain electrical neutrality. Without the salt bridge, the build-up of charge in the half-cells would quickly stop the flow of electrons and the reaction would cease. The salt bridge does not directly provide electrons or increase the reaction rate.

Solution

Correct: D

We use the equation ΔG = -RTlnK. Rearranging for K, we get K = exp(-ΔG/RT). R = 8.314 J/(mol*K). Therefore, K = exp(-(-35000 J/mol) / (8.314 J/(mol*K) * 298 K)) = exp(14.12) ≈ 1.4 x 10⁶.

Solution

Correct: C

In this reaction, ethanol (CH₃CH₂OH) is heated with concentrated sulfuric acid (H₂SO₄), resulting in the formation of ethene (CH₂=CH₂) and water. This is an elimination reaction, specifically a dehydration reaction, as water is removed from the ethanol molecule to form the alkene.

Solution

Correct: B

The Gibbs free energy equation is ΔG = ΔH - TΔS, where ΔG is the Gibbs free energy change, ΔH is the enthalpy change, T is the absolute temperature, and ΔS is the entropy change. This equation describes the spontaneity of a reaction.

Solution

Correct: B

Diamond is a network solid. Each carbon atom is sp³ hybridized and covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a strong, rigid, three-dimensional network. Graphite has layers of carbon atoms, and C₆₀ is buckminsterfullerene.