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: