SRJC H1 CHEM P2 Suggested Solutions

SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 1 CHEMISTRY 8873/02 JC2 Preliminary Examination 12 September 2018 Paper 2 Structured Questions 2 hours Section A 1 Combustion data can be used to calculate the empirical formula, molecular formula and relative molecular mass of many organic compounds. (a) Define the term relative molecular mass. [1] Relative molecular mass is the average mass of one molecule of the substance compared to 12 1 the mass of an atom of carbon-12. [1] (b) B is an alcohol, CxHyO. A 20 cm 3 gaseous sample of B was completely burnt in 200 cm 3 of oxygen (an excess). The final volume, measured under the same conditions as the gaseous sample, was 250 cm 3. Under these conditions, all water present was vaporised. Removal of the water vapour from the gaseous mixture decreased the volume to 170 cm 3. The remaining gaseous mixture was treated with concentrated alkali, and the eventual volume was decreased to 110 cm 3. Given the equation for the complete combustion of B, answer the questions below. C xHyO + zO2  xCO2 + ௬ ଶH2O (i) Calculate the value of x and y. [2] Volume of CO2 = 170 – 110 = 60 cm3 Using Avogadro’s Law, ஼௫ு௬ை ஼ைଶ = ଶ଴ ଺଴ = ଵ ௫ x = 3 [1] Volume of water vapour = 250 – 170 = 80 cm3 ஼௫ு௬ை ுଶை = ଶ଴ ଼଴ = ଵ ௬/ଶ y = 8 [1] (c) Period 3 elements also react with oxygen to form oxides. Sodium oxide and sulfur dioxide are two such oxides.

(i) Write chemical equations, with state symbols, when each of the above oxides react with water. [2] Na2O(s) + H2O(l)  2NaOH(aq) [1] SO2(g) + H2O(l) ⇌ H2SO3(aq) [1] (ii) State and explain the pattern of change of oxidation number which occurs to both oxygen and the Period 3 elements (sodium to silicon) when they react together. [2] • The oxidation number of O does not change (-2 throughout). • Oxidation number of Na to Si increases from +1 to +4. • The number of oxygen attached to the element matches the respective group numbers [2] for 2 points (d) The first ionisation energies of elements across Period 3 show a general increase. Aluminium and sulfur do not follow this general trend. (i) Write an equation to define the first ionisation energy of aluminium. [1] Al (g)  Al+ (g) + e [1] with state symbols (ii) Explain the general increase in ionisation energy across Period 3. [2] Across period 3, nuclear charge increases while shielding effect is similar . [1] Thus, overall effective nuclear charge increases, resulting in stronger electrostatic forces of attraction between nucleus and valence electrons. More energy is needed to remove the electron [1] across Period 3. (iii) Explain why aluminium has a lower first ionisation energy than magnesium. [1] Al: [Ne]3s23p1; Mg: [Ne]3s2 Electron is removed from the outer 3p subshell for Al, which is further away from the attraction of the nucleus. Thus, less energy is needed to do so. [1] (e) Silicon is an element in Period 3 displaying unique properties such as electrical conductivity at high temperatures and high melting point. (i) Complete the sketch of the melting point trend for Period 3 elements in Figure 1.1. [1] [1] Na Mg Al Si P S Cl Melting point / °C

(ii) With the aid of a well-labelled diagram, explain the abnormally high melting point of silicon, using concepts of structure and bonding. [3] [1] for labelled diagram Silicon has a giant covalent structure [1] with strong covalent bonds between Si atoms. A lot of energy is needed to overcome these strong bonds, hence high melting point is observed. [1] (ii) When silicon reacts with magnesium, Mg2Si forms. Mg2Si is thought to contain the Si4– ion. State the full electronic configuration of Si 4–. [1] 1s22s22p63s23p6 [1] (iii) Solid Mg 2Si reacts with dilute hydrochloric acid to form gaseous SiH 4 and a solution of magnesium chloride. Write an equation to show the reaction described above. Include state symbols. [1] Mg2Si (s) + 4HCl (aq)  SiH4 (g) + 2 MgCl2 (aq) [1] with correct S.S. [Total: 17] 2 Hydrazine has many industrial uses such as rocket fuel, pesticides and to prepare gas precursors used in air bags. Liquid hydrazine undergoes combustion according to the following equation: N2H4 (l) + O2 (g) → N2 (g) + 2H2O (l) A chemist conducted an experiment to de termine the standard enthalpy change of combustion of hydrazine where 0.420 g of hydrazine was burnt to heat up a beaker containing 200 cm 3 of water. The temperature of water in the beaker was recorded as follows: Initial temperature /ºC 29.3 Final temperature / ºC 37.4 (a) (i) Draw the dot-and-cross diagram of hydrazine. [1] [1] Si atom covalent bond

(ii) Explain what is meant by standard enthalpy change of combustion of hydrazine. [1] Standard enthalpy change of combustion (ΔHcθ) of hydrazine is the energy released when one mole of the hydrazine is completely burnt in oxygen at (standard conditions) 298K and 1 atm. [1] (iii) Calculate the standard enthalpy change of combustion of hydrazine. You may assume the process has 80 % efficiency [2] Q = 200 x (37.4 – 29.3) x 4.18 = 6771.6 J Amount of heat released by reaction, Q’ = Q / 0.8 = 8464.5 J [1] No of moles of N2H4 = 0.420 / 32 = 1.313 x 10 ⎯2 mol ΔHc(N2H4) = − 6771.6 / 1.313 x 10⎯2 = − 644914 J mol⎯1 = −645 kJ mol⎯1 [1] (iv) Given the following data: enthalpy change of formation of steam = −242 kJ mol⎯1 enthalpy change of vapourisation of water = +44 kJ mol⎯1 Using the value you have calculated in (iii), complete the following energy cycle to determine the standard enthalpy of formation of hydrazine. [2] M[1] By Hess’ law, ∆Hf (N2H4(l)) = 2(−242) – (−645) – 2(+44) = + 73.0 kJmol⎯1 [1] N2 (g) + 2H2O (l) N2 (g) + 2H2O (g) N2 (g) + 2H2 (g) N 2H4 (l) N2 (g) + 2H2 (g) N 2H4 (l) 2 xΔHf(H2O (g)) N2 (g) + 2H2O (l) ΔHc(N2H4 (l) N2 (g) + 2H2O (g) ΔHf(N2H4 (l) 2 xΔHv(H2O (l))

(b) (i) The standard enthalpy change of formation of hydrazine gas is +235 kJ mol⎯1. Using appropriate data from the Data Booklet , calculate the bond energy of N−H in hydrazine gas. [2] ∆Hf (N2H4) = [944 + 2(436)] – [(160) + 4 x B.E(N−H)] + 235 +(+160) + 4 x B.E(N−H) = +944 + 2(+436) [M1] B.E (N−H) = +355 kJ mol−1 [1] (ii) Suggest a reason for the difference in the N −H bond energy value obtained from (b)(i) with the value given in the Data Booklet. [1] The bond energy values obtained from the Data Booklet are average values and would differ from the experimental values. [1] [Total: 9] 3 This question is about nanoparticles and its applications. (a) Finely divided nanoparticles of nickel is found to be an extremely good heterogeneous catalyst for many organic synthesis reactions. (i) Define the term catalyst. [1] a substance that increases the rate of a chemical reaction by providing an alternative pathway with a lower activation energy, itself being chemically unchanged. [1] (ii) State the difference between a nanoparticle and a nanomaterial. [1] Nanoparticles ALL dimensions 1 to 100 nm on the nanoscale Nanomaterial (at least) one dimension 1 to 100 nm on the nanoscale . [1] (iii) Briefly explain how nickel nanoparticles operate as a catalyst. [2] Nanopartic