Practice Exam IV Fall 2004 -- ANSWERS UPDATED III

# Practice Exam IV Fall 2004 -- ANSWERS UPDATED III -...

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Practice Exam IV 1) Calculate the heat required to melt 10.0 grams of ice (originally at 0.0°C) and then heat the water to 25° C. ΔH fus for ice = 6.02kJ/Mol Specific Heat of liquid water is 4.184 J/g K q for melting = 10.0g x 1mol/18g x 6020 J/Mol = 3344 J q for heating = 10g(4.184J/g K)(25-0) = 1046 J q total = 4390 J 2) Calculate ΔH° for the reaction 2Al(s) + 3Cl 2 (g) 2AlCl 3 (s) From the following data: 2Al(s) + 6HCl(aq) 2AlCl 3 (aq) + 3H 2 (g) ΔH° = -1049 kJ HCl(g) HCl(aq) ΔH° = -74.8 kJ AlCl 3 (s) AlCl 3 (aq) ΔH° = -323 kJ And ΔH f ° of HCl(g) = -922.5 kJ The reaction corresponding to ΔH f °(HCl) is 1/2H 2 (g) + 1/2Cl 2 (g) HCl(g) You need to add reaction 1 - 2x reaction 3 + 6x reaction 2 + 6x reaction 4 ΔH° = -1049 -2(-323) +6(-74.8) + 6(-922.5) = -6387 kJ/mol 3) Predict which material has a higher entropy and explain your reasoning a) A solid, paramagnetic metal at 298 K with NO permanent magnetic field present (other than that of the earth which we can ignore for this question) b) The same solid, paramagnetic metal at 298 K in the presence of a large magnetic field from a nearby electromagnet A has higher entropy because in part B the spins of the electrons have been aligned with the external magnetic field

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Practice Exam IV Fall 2004 -- ANSWERS UPDATED III -...

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