NJC_Prelim_P2_SOLUTIONS

1 2011 NJC Prelim H2 Chemistry Paper 2 Solutions 1 (a) Mg(OH)2 is not soluble in water therefore ∆H cannot be determined directly. [1] (b) (i) Mg(OH)2 (s) + 2HCl (aq)  MgCl2 (aq) + 2 H2O (l) ∆H < 0 [1] Comments:  Thermochemical eqn must be accompanied with the associated enthalpy change, indicating whether it is endo or exo when there is no ambiguity.  Pls note that this is enthalpy change of reaction, not enthalpy change of neutralization. It defines the heat released when 1 mole of Mg(OH) 2 and 2 moles of HCl are reacted together, it is NOT heat released when 1 mole of water is formed. (ii) Amt of Mg(OH) 2 = 1.17/ (24.3 + 17.0 x 2) = 0.02007 mol Amt of HCl = 50/1000 x 2.0 = 0.100 mol 0.02007mol of Mg(OH) 2 needs 0.04 mol of HCl for reaction, therefore Mg(OH)2 is limiting. Heat released by reaction = heat absorbed by solution 30000 x 0.02007 = 50 x 4.18 x ∆T ∆T = 2.9 oC Amt of solid Mg(OH) 2 not suitable as ∆T is not within 5-10 oC. Comments:  To judge whether the quantities of reagent used is suitable, you must prove whether the data you are collecting from this quantities used is of suitable range. For energetics, the data you are collecting is change in temperature and it must be of this range: 5 oC < ∆T < 10oC  This calculation actually leads you to preliminary consideration of more suitable working quantities of the reagents.  First of all recognize that the solid Mg(OH) 2 being the limiting agent is the one that determines the amt of heat released from reaction.  Therefore the easiest method to increase ∆T it to use (i) more solid Mg(OH) 2 or (ii) use half the volume of HCl solution.  Method (ii) is rejected as half the volume of 50 cm 3 HCl causes the depth of solution to be too shallow, thus not able to cover the bulb of thermometer to ensure reliability of results.  For method (i), it is logical for students to use multiple of 1.17 g of Mg(OH) 2 so that ∆T is also the same multiple of 2.9 oC. However, students must make sure that the increase in mass of solid Mg(OH) 2 used cannot cause amt of Mg(OH) 2 to be in

2 excess of the amt of HCl present as the linear relationship between mass of Mg(OH)2 used and ∆T is no longer linear. This is because when HCl becomes limiting, it will be the regent that determines the magnitude of ∆T.  For example, a student suggesting triple amt of solid Mg(OH) 2 will expect ∆T to be 3 x 2.9 = 8.7 oC. Student has failed to realize that HCl has become the limiting agent and the temp expected is no longer 8.7 oC. Do you know what will be the ∆T instead? (iii) 1. Use double the mass of Mg(OH) 2 solid so that ∆T is doubled to become 5.8 oC and dil HCl is still present in excess. 2. Weigh accurately about 2.34 g of Mg(OH)2 using a weighing bottle. Pls do not tap the weighing bottle to ensure complete transfer of solid and this will result in m