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5 Thermochemistry Solutions to Exercises surroundings will increase. The system loses heat by decreasing its temperature, so the sign of qsys is (-). The surrounding gains heat by increasing its temperature, so the sign of is (+). From the system's perspective, the process is exothermic because it loses heat. (b) If neither volume nor pressure of the system changes, W = 0 and = q = The change in internal energy is equal to the change in enthalpy. 5.10 (a) N₂(g) + O₂(g) 2NO(g). Since V = 0, W = 0. (b) AH = = 90.37 kJ for production of 1 mol of NO(g). The definition of a formation reaction is one where elements combine to form one mole of a single product. The enthalpy change for such a reaction is the enthalpy of formation. 5.11 (a) = + Hc. The net enthalpy change associated with going from the initial state to the final state does not depend on path. The diagram shows that the change can be accomplished via reaction A, or via two successive reactions, B then C, with the same net enthalpy change. = + Hc because AH is a state function, independent of path. (b) = Hx + The diagram indicates that Reaction Z can be written as the sum of reactions X and Y. (c) Hess's law states that the enthalpy change for a net reaction is the sum of the enthalpy changes of the component steps, regardless of whether the reaction actually occurs via this path. The diagrams are a visual statement of Hess's law. 5.12 Since mass must be conserved in the reaction A B, the component elements of A and B must be the same. Further, if > 0 for both A and B, the energies of both A and B are above the energies of their component elements on the energy diagram. (a) The bold arrow shows the reaction as written; A combination of the two thin arrows shows an alternate route from A to B. B elements (b) = - A. If the overall reaction is exothermic, the sign of AH is (-) and A > B. This means that the enthalpy of A is the highest energy level on the diagram. This is the situation pictured in the diagram above, but nothing in the given information requires this arrangement. If the reaction is endothermic, B > A and the enthalpy of B would be the highest energy level on the diagram. The Nature of Energy (section 5.1) 5.13 An object can possess energy by virtue of its motion or position. Kinetic energy, the energy of motion, depends on the mass of the object and its velocity. Potential energy, stored energy, depends on the position of the object relative to the body with which it interacts. 109