HCI_H2_CHEM_P2_ANSWERS Prelim

Hwa Chong Institution 2011 Prelim Paper 2 Answers 1(a) (b) (i) 100 cm 3 (accept 10 cm 3 to 200 cm 3) (ii) The maximum mass that can dissolve in 100 cm 3 of water = (6.95 / 1000) x 100 x 53.5 = 37.2 g (c) Assuming a temperature change of 5 oC to be measured and no heat loss to surroundings, nsalt x 15 000 = 100 x 4.3 x 5 ⇒ nsalt = 0.143 mol minimum mass to use = 0.143 x 53.5 = 7.65 g (d) 1. Weigh accurately 8.00 g of ammonium chloride in a weighing bottle, using a weighing balance. 2. Using a 100 cm 3 measuring cylinder, add 100 cm 3 of water into a dry styrofoam cup calorimeter. Support the styrofoam cup on a 250 cm 3 beaker. 3. Stir the water gently using the thermometer. 4. Start the stopwatch. 5. Record the temperature of the water in the styro foam cup using a 0.1 oC thermometer, at 30 s intervals ( accept 1 min ) until 2.5 min. 6. At exactly 3 min ( accept 1–5 min ), tip the ammonium chloride into the water. Do not measure the temperature at this time. 7. Stir the solution gently and record the temperat ure of the solution at 3.5 min. Continue to stir and record the temperature at 30 s intervals until solution returns to room temperature. 8. Reweigh the weighing bottle. thermometer lid styrofoam cup calorimeter 250 cm 3 beaker ammonium chloride and water

Table with headings: Mass of weighing bottle and salt / g m 1 Mass of weighing bottle after experiment / g m 2 Time / min Temperature of solution / oC 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 . (e) (f) ∆ H solution (in J mol –1 ) = +v × 4.3 × T m 53.5       (g) The salt with the more endothermic enthalpy change of solution will be the more effective ingredient in the cold pack. OR The salt that gives the fastest drop in temperatur e will be the more effective ingredient.

2 (a) (i) Examples of possible answers : Category of reaction Equation from Group II or Group VII Decomposition 2Mg(NO 3)2 (s) /barb2right 2MgO(s) + 4NO 2(g) + O 2(g) 2HC l(g) /barb2right H2(g) + C l2(g) Single displacement Ca (s) + 2H 2O(l) /barb2right Ca(OH) 2(aq) + H 2(g) Cl2(g) + 2KI(aq) /barb2right 2KC l(aq) + I2(aq) Double displacement Na 2SO 4(aq) + Ba(NO 3)2(aq) /barb2rightBaSO 4(s) + 2NaNO 3(aq) NaCl(aq) + AgNO 3(aq) /barb2right AgC l(s) + NaNO 3(aq) (ii) Type of reaction : redox Equation : 4K I(aq) + 2CuSO 4(aq) /barb2right 2K 2SO 4(aq) + 2Cu I(s) + I2(aq) (iii) NaBr(s) + H 2SO 4(l) /barb2right HBr(g) + NaHSO 4(s) 2HBr(g) + H 2SO 4(l) /barb2right Br 2(g) + SO 2(g) + 2H 2O(l) (b) (i) Ka = ])([ ])()(][ [ 2 62 52 + ++ OHMg OH OHMg H (ii) I. The solubility equilibrium Mg 2+ (aq) + 2OH _(aq) Mg(OH) 2(s) keeps [OH -] constant (pH is constant at 9). II. The Mg 2+ has been completely precipitated / reacted