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Ch.5 - Thermochemistry
All textbooksBrown 14th EditionCh.5 - ThermochemistryProblem 103
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Chapter 5, Problem 103

The corrosion (rusting) of iron in oxygen-free water includes the formation of iron(II) hydroxide from iron by the following reaction: Fe(s) + 2 H2O(l) → Fe(OH)2(s) + H2(g). If 1 mol of iron reacts at 298 K under 101.3 kPa pressure, the reaction performs 2.48 J of P-V work, pushing back the atmosphere as the gaseous H2 forms. At the same time, 11.73 kJ of heat is released to the environment. What are the values of _x001F_H and of _x001F_E for this reaction?

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1
Step 1: Understand the problem and identify the given data. The reaction involves the formation of iron(II) hydroxide and hydrogen gas from iron and water. Given data includes: 1 mol of iron, 298 K temperature, 101.3 kPa pressure, 2.48 J of P-V work, and 11.73 kJ of heat released.
Step 2: Recall the relationship between enthalpy change (ΔH), internal energy change (ΔE), work (w), and heat (q). The first law of thermodynamics states: ΔE = q + w. Here, q is the heat exchanged, and w is the work done by the system.
Step 3: Convert all energy values to the same units for consistency. Since the heat released is given in kJ and the work done is in J, convert the work from J to kJ by dividing by 1000. Thus, 2.48 J = 0.00248 kJ.
Step 4: Calculate the change in internal energy (ΔE) using the formula: ΔE = q + w. Substitute the values: q = -11.73 kJ (since heat is released, it's negative), and w = -0.00248 kJ (work done by the system is also negative).
Step 5: Calculate the change in enthalpy (ΔH). For reactions at constant pressure, ΔH is approximately equal to q, the heat exchanged at constant pressure. Therefore, ΔH = q = -11.73 kJ.
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The air bags that provide protection in automobiles in the event of an accident expand because of a rapid chemical reaction. From the viewpoint of the chemical reactants as the system, what do you expect for the signs of q and w in this process?

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Consider a system consisting of the following apparatus, in which gas is confined in one flask and there is a vacuum in the other flask. The flasks are separated by a valve. Assume that the flasks are perfectly insulated and will not allow the flow of heat into or out of the flasks to the surroundings. When the valve is opened, gas flows from the filled flask to the evacuated one. (a) Is work performed during the expansion of the gas? (b) Why or why not?

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Open Question
Both oxyhydrogen torches and fuel cells use the following reaction to produce energy: 2 H2(g) + O2(g) → 2 H2O(l). Both processes occur at constant pressure. In both cases, the change in state of the system is the same: the reactant is oxyhydrogen (“Knallgas”) and the product is water. Yet, with an oxyhydrogen torch, the heat evolved is large, and with a fuel cell, it is small. If heat at constant pressure is considered to be a state function, why does it depend on path?
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Textbook Question

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