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Chapter 6: Problem 21

Define these terms: enthalpy, enthalpy of reaction. Under what condition is the heat of a reaction equal to the enthalpy change of the same reaction?

Short Answer

Expert verified
Enthalpy is a property of a system, calculated as the sum of its internal energy and the product of its pressure and volume. Enthalpy of reaction is the change in enthalpy during a chemical reaction, calculated as the difference of enthalpy of the products and reactants. The heat of a reaction equals enthalpy change when the reaction occurs under constant pressure and no non-pressure-volume work is done on or by the system.

Step by step solution

01

Define Enthalpy

Enthalpy (\(H\)) is a thermodynamic property of a system. It is the sum of the internal energy (\(U\)) of the system and the product of its pressure (\(P\)) and volume (\(V\)). This can be represented by the formula: \(H = U + PV\).
02

Define Enthalpy of Reaction

Enthalpy of reaction, or heat of reaction, is the change in enthalpy of a chemical reaction that occurs at a constant pressure. It is represented as \(螖H\) and calculated as the difference in enthalpy of the products and reactants: \(螖H = H_{products} - H_{reactants}\).
03

Conditions for Heat of Reaction Equal to Enthalpy Change

The heat of a reaction (\(q\)) is equal to the enthalpy change of the reaction (\(螖H\)) under conditions of constant pressure and no non-pressure-volume work being done on or by the system. When these conditions are met, we can write: \(q_p = 螖H\).

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Most popular questions from this chapter

A 0.1375 -g sample of solid magnesium is burned in a constant-volume bomb calorimeter that has a heat capacity of \(3024 \mathrm{~J} /{ }^{\circ} \mathrm{C}\). The temperature increases by \(1.126^{\circ} \mathrm{C}\). Calculate the heat given off by the burning \(\mathrm{Mg}\), in \(\mathrm{kJ} / \mathrm{g}\) and in \(\mathrm{kJ} / \mathrm{mol}\).

Consider two metals A and B, each having a mass of \(100 \mathrm{~g}\) and an initial temperature of \(20^{\circ} \mathrm{C}\). The specific heat of \(A\) is larger than that of \(B\). Under the same heating conditions, which metal would take longer to reach a temperature of \(21^{\circ} \mathrm{C} ?\)

When \(1.034 \mathrm{~g}\) of naphthalene \(\left(\mathrm{C}_{10} \mathrm{H}_{8}\right)\) are burned in a constant-volume bomb calorimeter at \(298 \mathrm{~K}, 41.56 \mathrm{~kJ}\) of heat are evolved. Calculate \(\Delta E\) and \(\Delta H\) for the reaction on a molar basis.

Why are cold, damp air and hot, humid air more uncomfortable than dry air at the same temperatures? (The specific heats of water vapor and air are approximately \(1.9 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\) and \(1.0 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\), respectively.

From these data, $$ \begin{aligned} \mathrm{S}(\text { rhombic })+\mathrm{O}_{2}(g) \longrightarrow \mathrm{SO}_{2}(g) \\ \Delta H_{\mathrm{rxn}}^{\circ} &=-296.06 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{S}(\text { monoclinic })+\mathrm{O}_{2}(g) & \longrightarrow \mathrm{SO}_{2}(g) \\ \Delta H_{\mathrm{rxn}}^{\circ} &=-296.36 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ calculate the enthalpy change for the transformation $$ S \text { (rhombic) } \longrightarrow \mathrm{S} \text { (monoclinic) } $$ (Monoclinic and rhombic are different allotropic forms of elemental sulfur.)
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