In Questions 6.1-6.4, you will be presented with four samples of student work related to problems in thermodynamics and energy transfer. In each case, your task is to determine if the student’s conclusion is true or false. If you determine their conclusion to be “false,” you must then point out the inaccurate parts of their statement and/or suggest a more accurate version. If you determine their conclusion to be “true,” your work is done Question 6.1: Consider the hypothetical reaction XY2(g) –>X(s) + Y2(g) with AHrxn = -325 kJ/mol XY2. Can you predict whether this reaction favors the products or the reactants? Student Answer: The negative reaction enthalpy suggests an exothermic reaction. Because the products are more stable than the reactants, the products are favored. Question 6.2: Consider the hypothetical reaction XY2(g) –> X(s) + Y2(g) with AHrxn = -325 kJ/mol XY2. Was energy released or consumed when the X-Y bond was broken? Student Answer: The negative reaction enthalpy suggests an exothermic reaction, or one that releases energy. That energy must have been stored in the X-Y bond.

Question 6.3: Consider the hypothetical reaction XY2(g) –> X(s) + Y2(g) with AHrxn = -325 kJ/mol XY2. How much energy must be supplied in order for the reaction to begin? Student Answer: The negative reaction enthalpy suggests an exothermic reaction, but even exothermic reactions require an initial input of twice the bond energy of the X-Y bond. This activation energy is necessary to break XY2 into its individual atoms before the reaction can begin. Question 6.4: Consider the hypothetical reaction XY2(g) –> X(S) + Y2(g) with AG° = +23 kJ/mol XY2. If you start with 1 atm of XY2(g), will the reaction occur? Student Answer: No reaction will occur because AG° > 0. When AG° is positive, the reverse reaction is favored, but that cannot happen here because there are no products to convert into reactants.