2025 H2 Chem Prelim P1 detailed ans VJC

1 VICTORIA JUNIOR COLLEGE 2025 JC2 H2 CHEMISTRY PRELIM EXAM PAPER 1 ANSWERS 1 B 7 D 13 C 19 C 25 D 2 A 8 A 14 D 20 D 26 B 3 C 9 A 15 B 21 C 27 C 4 D 10 B 16 A 22 C 28 B 5 B 11 A 17 D 23 A 29 A 6 B 12 C 18 D 24 A 30 D 1 B species Bk2+ Fm3+ protons 97 100 neutrons 249 – 97 = 152 252 – 100 = 152 electrons 97 – 2 = 95 100 – 3 = 97 Hence, Fm3+ has more electrons than Bk2+ but both ions have the same number of neutrons 2 A According to Hund’s rule, electrons will occupy the subshells singly first before pairing up. All the elements in Period 3 have principal quantum 3 as its valence shell. There is 1 orbital for s subshell and 3 orbitals for p subshell. element electronic configuration no. of paired electrons Na [Ne]3s1 0 Mg [Ne]3s2 2 Al [Ne]3s23p1 2 Si [Ne]3s23p2 2 P [Ne]3s23p3 2 S [Ne]3s23p4 4 Cl [Ne]3s23p5 6 Ar [Ne]3s23p6 8 Total no. of paired electrons = 0 + 2 + 2 + 2 + 2 + 4 + 6 + 8 = 26 3 C All three molecules have instantaneous dipole -induced dipole and permanent dipole -permanent dipole interactions. Statement 1 (incorrect): Chlorine is more electronegative than bromine and thus there is a larger electronegativity difference between hydrogen and chlorine than between hydrogen and bromine. Correspondingly, there is a larger overall dipole moment in HC l, which leads to stronger permanent dipole - permanent dipole interactions between molecules of HCl than between molecules of HBr. Statement 2 (correct): Fluorine has less electrons than bromine. Thus, the total number of electrons in HF is less than that in HBr. This leads to HF having a smaller electron cloud size and less extent of electron cloud polarisation, resulting in weaker instantaneous dipole - induced dipole interactions between HF molecules. Statement 3 (Correct): Fluorine is more electronegative than chlorine and thus there is a larger electronegativity difference between hydrogen and fluorine than between hydrogen and chlorine. Correspondingly, there is a larger overall dipole moment in HF, which leads to stronger permanent dipole -permanent dipole interactions between molecules of HF than between molecules of HCl. 4 D 5 B Option A (incorrect): Only molecules of ideal gases are assumed to have elastic collisions i.e. collisions are associated with no loss of kinetic energy. Option B (correct): Molecules of both ideal gases and real gases are in constant random motion. Option C (incorrect): Only molecules of ideal gases are assumed to have no intermolecular forces of attraction. Option D (incorrect): Molecules of ideal gases only are assumed to have negligible volume (size) compared to the volume (size) of the container. 6 B Option A (incorrect): A compound containing a hydrogen atom is not necessarily an Arrhenius acid. Option B (correct): As acetylene is unable to react with water, it is unable to dissociate in water to form H 3O+, which is characteristic of an Arrhenius acid. Option C (incorrect): The conjugate base of acetylene is HC≡C–. Option D (incorrect): A Lewis acid is an electron pair acceptor. Acetylene contains carbon and hydrogen that have full valence electron shells and cannot further accept an electron pair. 7 D The effectiveness of each of the three minerals as fire retardant is dependent on its ease of thermal decomposition to produce CO2, which smothers the fire. The ease of thermal decomposition of the minerals is dependent on the charge density and hence the polarising power of the respective Group II metal ions (Ba2+, Ca2+ and Mg2+). The order of effectiveness as fire retardant, from best to worst, corresponds to the order of decreasing polarising power of the Group II metal ions: Mg2+ > Ca2+ > Ba2+. Huntite is a more effective fire retardant than dolomite as huntite contains more MgCO 3 and hence it produces more CO 2 upon complete thermal decomposition. Dolomite is a more effective fire retardant than norsethite as CaCO3 has higher ease of thermal decomposition that BaCO3. 8 A P4O10 dissolves in water to give H3PO4. P4O10, an acidic oxide, reacts with NaOH to give soluble Na3PO4. Even though P4O10 has not reaction with an acid HCl(aq), it can dissolve in the water present. mineral chemical formula behaves as a mixture of dolomite CaMg(CO3)2 1 mol of CaCO3 & 1 mol of MgCO3 huntite Mg3Ca(CO3)4 1 mol of CaCO3 & 3 mol of MgCO3 norsethite BaMg(CO3)2 1 mol of BaCO3 & 1 mol of MgCO3

2 9 A Molecular Formula of G = C7H12O3 C7H12O3 + 8.5O2 → 7CO2 + 6H2O 10 B 𝛥𝐻𝐿𝐸𝛼 𝑞+𝑞− 𝑟+ + 𝑟− Since the charge of cation in MgCl is +1 while that in MgCl2 and SrCl2 is +2, MgCl has the smallest magnitude of lattice energy. Since ionic radius of Sr 2+ is larger than that of Mg 2+ due to additional quantum shell, SrC l2 has a smaller magnitude of lattice energy than that of MgCl2. |L.E|: MgCl < SrCl2 < MgCl2 11 A Statement 1 (correct) ∆Hro = ∑n∆Hfo (products) − ∑m∆Hfo (reactants) = −1273 – [6×(−394) + 6×(−286)] = +2807 kJ mol−1 Statement 2 (correct): The photosynthesis process has no change in number of particles of gases, but there is a change of state (liquid water in the reactant to solid C6H12O6 in the product). There is a decrease in overall entropy and thus, ∆S has a negative sign (e.g. less disordered). Statement 3 (correct): Since ∆G = ∆H (+ve) – T∆S (– ve), the ∆G for the reaction will always be positive at all temperatures as ∆H and – T∆S is always positive. 12 C Option A (incorrect): Since H+ is a catalyst, the [H+] (and hence pH) will not change during the reaction. Option B (incorrect): To find the order with respect to H+, the concentration that needs to be varied (for different sets of experiments) should be [H +] and not [CH3CO2CH2CH3]. Option C (correct): This is using the initial rate method. Option D (incorrect): Since H+ is a catalyst, the [H+] (and hence volume of NaOH required) will not change during the reaction. 13 C Step 2 is the rate –determining step and rate = k’[CHCl3][Cl]. Since C l is an intermediate, it cannot be in the rate equation and [Cl] is dependent on [Cl2] in step 1. Kc = [Cl]2 [Cl2]  [Cl] = (Kc)1/2 [Cl2]1/2 Thus, rate = k’[CHCl3] ( Kc)1/2 [Cl2]1/2 = k[CHCl3] [C l2]1/2 where k = k’(Kc)1/2. 14 D Option A & B (correct): Increasing the temperature and adding a catalyst will lead to an increase in rate constant, according to Arrhenius equation. Hence there is a greater proportion of particles having energy greater than the activation energy Option C (correct): Increase in temperature will lead to an increase in rate constant Option D (incorrect): Concentration of the reactants affects the rate of reaction but it does not affect the rate constant. 15 B Statement 1 (correct) : pKa has an inverse relationship with Ka and hence with acid strength. A lower p Ka denotes a stronger acid. –COOH has the lowest pKa and hence is the most acidic group. Statement 2 (correct): At pH 8, –COOH and –SeH will mostly be in the deprotonated form while amine will be mostly in the protonated form ( –NH3+). This is because when pH = pKa, the concentration of the protonated and deprotonated form is equal. When pH > p Ka, the concentration of the deprotonated is higher than the protonated. When pH < p Ka, the concentration of the deprotonated is lower than the protonated. This is based on the formula: pH = pKa + lg [A-] [HA] Hence, at pH 8, there are 2 negative charge and 1 positive charge in Selenocysteine, giving an overall nett charge of –1. Selenocysteine will migrate towards the positive terminal. Statement 3 (incorrect): A change in the concentration of [H +] at fixed temperature only shifts the position of equilibrium of the dissociation of the acid, but it does not change the value of Ka. 16 A Option A (correct): Adding Pb(NO3)2 introduces the common ion Pb2+. By Le Chatelier’