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 mo re heat exchange bet ween the reaction and the environment, this is why there is a need to measure the weighing bottle again to calculate the actual mass transfer. Pls do not suggest rinsi ng the remaining solid into the cup as it will cause the same problem of increasing heat exchange between reaction and environment and it will also add extr a volume to the reaction mixture and the calculation to determine enthalpy change is affected. 3. Using a 50 ml measuring cylinder, measure 50 cm 3 of HCl into a styrofoam cup supported in beaker. 4. Measure initial temperature of HCl. 5. Add solid Mg(OH) 2 into the styrofoam cup, taking care to replace the lid immediately. 6. Stir (stirring should be done continuously so that reaction can be completed within the shortest time possible so that heat exchange with the environment is minimized. ) with thermometer and measure the highest temperature reached/ final constant te mp attained (a final constant temp indicates that the reaction is completed). 7. Weigh mass of weighing bottle and residual solid and calculate the actual mass transfer. 8. Wash the styrofoam cup and dry it thoroughly before the next experiment. 9. Repeat expt till ∆T/ m is within 5% difference. (Solid reactant is limiting, therefore ∆T/m is used to check consistency between different sets of experiments as the mass of solid reactant transferred to HCl solution for each expt may not be exactly the same.) Comments:

3 A1 “taking care to replace the lid immediately”, “Stir with thermometer” and “measure the highest temp attained” are steps taken to ensure ∆T measured for this expt is reliable. (heat exchange with the environment is kept to minimal therefore result is reliable) A2 However, student may choose to monitor temp change with time and it is in general divided into three segments. (i) before addition of 2 nd reagent (ii) during the reaction (iii) when reaction mixture st arts to cool down. The extrapolation from the cooling curve and its intersection with the time you start the reaction determines the actual ∆T as heat exchange with environment is already taken into account. This will also ensure reliability of ∆T. This will also mean that step 4, 5 and 6 of the above procedure will be replaced with those underlined above. B1 Some students suggested adding solid reactant to the Styrofoam cup followed by addition of HCl solution. It is actually alright if the student has measured the initial temp of the HCl acid before it is added to the solid Mg(OH) 2 in the styrofoam cup. However some students only started measuring the initial temp when HCl is added to solid Mg(OH) 2 in the styrofoam cup, this is wrong as reaction has already started and results will become unreliable. B2 In fact, addition of solid to solution is a better method than the reverse. This is because tipping the solid into the solution is much faster than adding a big volume of solution into the Styrofoam cup. This is crucial as the longer you need to transfer the second reactant, the more heat exchange can take place between the reaction mixture and the environment. [5] (c) Given the following data, show how the enthalpy change of reaction between solid Mg(OH) 2 and hydrochloric acid can be determined accurately. Expt Mass of weighing bottle and solid sample/g a Mass of weighing bottle and residual solid /g b Initial temp of 50.0 cm3 of hydrochloric acid T1 Final constant temp of hydrochloric acid T 2 > T1 21 21 21 50 4.18 ( ) (24.3 34.0) 209( ) 58.3 ()12185 TTH ab TT ab TT ab          [1] -ve or +ve sign of enthalpy change is important as it will affect the calculation for the unknown enthalpy later. Heat released from reaction is absorbed by solution, therefore it should be 50 cm3 of HCl solution. Since it’s a dilute solution, its density is similar to water, mass of 50 cm3 HCl solution = 50.0 x 1gcm-3 = 50 g

4 1 (d) 2HCl (aq) + Mg(OH)2 (s) + aq  Mg2+ (aq) + 2OH (aq) + 2 HCl (aq) ∆H solution = ∆H(c)  2 x ∆H1 [2] (e) Wear gloves to avoid direct contact with HCl as it is of high concentration . [1] 2 (a) Reddish brown/ brick red ppt formed. [1] [1] O O + 2Cu2 + 5OH warm O O + Cu2O + 3 H2O O brick red ppt Comments: Warming with Fehling’s solution (alkaline solution of Cu 2+ complex) is to test for aliphatic aldehydes. Many students cannot balance the equation. A number wrote copper(II) oxide instead of copper(I) oxide. In writing the equation, first write out the reagents and products, ensure that the atoms and charges are balanced. Warming with Tollen’s reagent (ammonical solution of Ag(NH3)2 + complex) is to test for aldehydes. R C O + 2 Ag(NH3)2 + 3OH warm R C O + 2 Ag + 4NH3 + 2 H2O O silver mirrorH MgCl2 (aq) + 2 H2O (l) ∆H from part (c) 2 x ∆H1

5 2 (b) (i) Reaction I : Hydrazine, N2H4, H2N-