TA practice exam 3 key


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Unformatted text preview: K 1 HCN ( g ) 135.1 201.78 HCN (l ) 108.87 112.84 Estimate the boiling point of HCN at 1 atm. 1. 313 K correct 2. 100 K 3. 473 K 4. 253 K 5. 573 K At a phase transition, G 0 , which means both phases can coexist. We can write the reaction either way, but just need to make sure that we are self‐consistent. Thus, we will do the problem as if the gas is the product and the liquid is the reactant. We know that G H T S In this case, H 135.1 kJ mol 1 108.87 kJ mol 1 H 25 kJ mol 1 and S 202.78 J mol 1 K 1 112.84 J mol 1 K 1 S 80 J mol 1 K 1 Using the fact that G 0 we get 0 25 kJ mol 1 T 80 J mol 1 K 1 which can be rearranged and solved to find 25 kJ mol 1 T 80 J mol 1 K 1 T 313 K 26. Calculation involving the second law A process that has an entropy change of the system of 37 J K 1 also absorbs 30 kJ from the surroundings at 300 K. What is Suniverse for this process? 1. 137 J K 1 correct 2. 63 J K 1 1 3. 63 J K 4. 137 J K 1 We know that Suniverse Ssystem Ssurr In this case H system Ssurr Tsurr 30 kJ Ssurr 300 K Ssurr 100 J K 1 Adding them up we get Suniverse 37 J K 1 100 J K 1 Suniverse 63 J K 1 27. A reaction occurs spontaneously at 298 K. Which of the following statements could not possibly be true for this system? 1. Both S and H are positive 2. Both S and H are negative 3. S must be positive and H must be negative 4. S must be negative and H must be positive correct If statement 4 were true, then the reaction could never be spontaneous, which is contradicted in the question. Statements 1 and 2 are possible if certain values for the enthalpy and entropy change are taken. 28. A reaction does not occur spontaneously at 100 K but does occur at 1000 K. What are the signs of S and H for this reaction? S H 1. + + correct 2. + ‐ 3. ‐ + 4. ‐ ‐ Remember that G H T S and that for the reaction to occur G must be negative. At high T the entropy contribution becomes more important. In order for it to be spontaneous at high T the entropy must be positive. In order for the reaction to be nonspontaneous at low T the entha...
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