Chapter 12: Q12.6 36P (page 525)
Given the following two reactions and corresponding enthalpy changes,
Compute of the reaction ?
The heat change of the reaction, .
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A chemist mixes with an excess of in dilute aqueous solution. He measures the evolution of heat as the two substances react to give Compute that would result from the reaction of with an excess of
The enthalpy of reaction changes somewhat with temperature. Suppose we wish to calculate \(\Delta H\) for a reaction at a temperature \(T\) that is different from \(298\;{\rm{K}}\). To do this, we can replace the direct reaction at \(T\) with a three-step process. In the first step, the temperature of the reactants is changed from \(T\)to \(298\;{\rm{K}}\).\(\Delta H\) for this step can be calculated from the molar heat capacities of the reactants, which are assumed to be independent of temperature. In the second step, the reaction is conducted at \(298\;{\rm{K}}\) with an enthalpy change \(\Delta {H^o}\). In the third step, the temperature of the products is changed from \(298\;{\rm{K}}\) to \(T\). The sum of these three enthalpy changes is \(\Delta H\) for the reaction at temperature \(T\).
An important process contributing to air pollution is the following chemical reaction \({\rm{S}}{{\rm{O}}_2}(g) + \frac{1}{2}{{\rm{O}}_2}(g) \to {\rm{S}}{{\rm{O}}_3}(g)\). For \({\rm{S}}{{\rm{O}}_2}(g)\), the heat capacity \({C_{\rm{p}}}\) is\(39.9\), for \({{\rm{O}}_2}(g)\) it is \(29.4\), and for \({\rm{S}}{{\rm{O}}_3}(g)\) it is \(50.7{\rm{J}}{{\rm{K}}^{ - 1}}\;{\rm{mo}}{{\rm{l}}^{ - 1}}\). Calculate \(\Delta H\) for the preceding reaction at \(500\;{\rm{K}}\), using the enthalpies of formation at \(298.15\;{\rm{K}}\) from Appendix D.
Initially, \(46.0\;{\rm{g}}\) oxygen is at a pressure of 1.00 atm and a temperature of \(400\;{\rm{K}}\). It expands adiabatically and reversibly until the pressure is reduced to 0.60atm, and it is then compressed isothermally and reversibly until the volume returns to its original value. Calculate the final pressure and temperature of the oxygen, the work done and heat added to the oxygen in this process, and the energy change dU. Take \({c_2}\left( {{{\rm{O}}_2}} \right) = 29.4\;{\rm{J}}\;{{\rm{K}}^{ - 1}}\;{\rm{mo}}{{\rm{l}}^{ - 1}}\).
Galen, the great physician of antiquity, suggested scaling temperature from a reference point defined by mixing equal masses of ice and boiling water in an insulated container. Imagine that this is done with the ice at andthe water at . Assume that the heat capacity of the container is negligible, and that it takes 333.4 J of heat to melt 1.000 g ice at to water at . Compute Galen’s reference temperature in degrees Celsius.
Question 85. Find the maximum possible temperature that may be reached when role="math" localid="1663438895360" is allowed to react with role="math" localid="1663438958928" of a solution, both initially at . Assume that the final volume of the solution is , and that the specific heat at constant pressure of the solution is constant and equal to that of water, .
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