TJC_H2_CHEM_P2_MARK_SCHEME Prelim

Mark Scheme (2011 H2 Structured Prelims) 2011 Prelims JC2 H2 Chemistry Paper 2 [Turn Over 1 1 (a) HO reaction Mg(s) + Cu(s) + 2HCl (aq) CuC l2(aq) + H2(g) + Mg(s) MgCl2(aq) + H2(g) + Cu(s)  By Hess’ law, HO reaction = HO 1 - HO 2 = (x - y) kJ mol-1  Correct energy cycle with balanced equations and state symbols [2] (b) Example: Assuming 100 cm 3 of acid is used and a temperature raise of 10 oC is to be achieved. macidCT = m Mg/Mr(Mg) x 460 000 100 x 4.2 x 10 = m Mg x 460 000/24.3 m Mg = (24.3)[100 x 4.2 x 10)/460000] = 0.22 g No. of moles of acid required to completely react off all Mg(s) = 2(0.22/24.3) = 0 . 0 1 8 1 m o l No. of moles of HCl in 100 cm3 = 5 x 100/1000 = 0.5 mol Hence, there is sufficient acid in 100 cm3 to react with all Mg(s). Marking points [Justification]:  Appropriate change in temperature (any value in the range 5 – 15 oC)  Appropriate mass and volume of acid used to achieve the change in temperature.  Appropriate mass and volume of acid in terms of appropriate quantity i.e. not too small or large amount (at least 0.1 g and 20 cm3) Procedure:  Use a 100 cm 3 measuring cylinder to measure out 100 cm 3 of HC l(aq) HO 2 HO 1

Mark Scheme (2011 H2 Structured Prelims) 2011 Prelims JC2 H2 Chemistry Paper 2 [Turn Over 2 acid into a styrofoam cup. Record the initial temperature of the solution.  Using an electronic balance, weigh out accurately about 0.2 g of powdered Mg(s) into a dry empty weighing bottle.  Use a -10 to 100 oC thermometer to measure the temperature of the 100 cm3 of HCl(aq) acid in the styrofoam cup.  Pour the powered Mg(s) into the Styrofoam cup containing the HCl(aq), stir and measure the highest temperature reached. Record the final temperature reached.  Reweigh the weighing bottle to determine the exact amount of Mg(s) that was poured into the styrofoam cup. Marking points [Procedures]:  Correct sequence of steps that will lead to useful data.  Selection of apparatus is appropriate and types of apparatus used are clearly stated. (Note: this mark is only awarded if the suggested procedure is appropriate.) [5] (c) Marking points [Tables]:  Correct headers and units for both tables to record data about mass and temperature. Mass of weighing bottle + Mg(s) / g Mass of weighing bottle + residual Mg(s) after transferring / g Mass of Mg(s) / g Initial temperature of HCl(aq) / oC Final temperature of HCl(aq) / oC Temperature change / oC [1] (d) Mass of Mg(s) = a g

Mark Scheme (2011 H2 Structured Prelims) 2011 Prelims JC2 H2 Chemistry Paper 2 [Turn Over 3 T = b oC Volume of acid = c cm 3 Heat change = mC