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Chapter 6: Problem 48

Predict the value of \(\Delta H_{\mathrm{f}}^{\circ}\) (greater than, less than, or equal to zero) for these elements at \(25^{\circ} \mathrm{C}\) : (a) \(\mathrm{Br}_{2}(g)\) and \(\mathrm{Br}_{2}(l)\) (b) \(\mathrm{I}_{2}(g)\) and \(\mathrm{I}_{2}(s)\)

Short Answer

Expert verified
The \(\Delta H_{\mathrm{f}}^{\circ}\) for \(Br_{2}(g)\) and \(I_{2}(g)\) is greater than zero, while for \(Br_{2}(l)\) and \(I_{2}(s)\) the \(\Delta H_{\mathrm{f}}^{\circ}\) is equal to zero.

Step by step solution

01

Evaluate Bromine

First consider bromine(\(Br_2\)). The standard state of bromine at \(25^{\circ} \mathrm{C}\) is liquid. Thus, for the formation of bromine in the gaseous state, \(\Delta H_{\mathrm{f}}^{\circ}\) would be greater than zero as energy is needed to convert it from the standard state to gaseous state. On the other hand, for the formation of bromine in the liquid state, which is its standard state, \(\Delta H_{\mathrm{f}}^{\circ}\) would be equal to zero, because there is no change needed for it to reach the standard state.
02

Evaluate Iodine

Next, consider iodine(\(I_2\)). The standard state of iodine at \(25^{\circ} \mathrm{C}\) is in its solid state. Therefore, for the formation of iodine in the gaseous state, \(\Delta H_{\mathrm{f}}^{\circ}\) would be greater than zero, because energy is required to convert it from the standard state to gaseous state. In contrast, for the formation of iodine in the solid state, which is its standard state, \(\Delta H_{\mathrm{f}}^{\circ}\) would be equal to zero, as there's no change needed for it to reach the standard state.

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Most popular questions from this chapter

Explain the meaning of this thermochemical equation: $$ \begin{aligned} 4 \mathrm{NH}_{3}(g)+5 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{NO}(g) &+6 \mathrm{H}_{2} \mathrm{O}(g) \\ \Delta H=&-904 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$

The combustion of \(0.4196 \mathrm{~g}\) of a hydrocarbon releases \(17.55 \mathrm{~kJ}\) of heat. The masses of the products are \(\mathrm{CO}_{2}=1.419 \mathrm{~g}\) and \(\mathrm{H}_{2} \mathrm{O}=0.290 \mathrm{~g} .\) (a) What is the empirical formula of the compound? (b) If the approximate molar mass of the compound is \(76 \mathrm{~g}\), calculate its standard enthalpy of formation.

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.

Calculate the internal energy of a Goodyear blimp filled with helium gas at \(1.2 \times 10^{5} \mathrm{~Pa}\). The volume of the blimp is \(5.5 \times 10^{3} \mathrm{~m}^{3} .\) If all the energy were used to heat 10.0 tons of copper at \(21^{\circ} \mathrm{C},\) calculate the final temperature of the metal. (Hint: See Section 5.6 for help in calculating the internal energy of a gas. 1 ton \(\left.=9.072 \times 10^{5} \mathrm{~g} .\right)\)

Suggest ways (with appropriate equations) that would enable you to measure the \(\Delta H_{\mathrm{f}}^{\circ}\) values of \(\mathrm{Ag}_{2} \mathrm{O}(s)\) and \(\mathrm{CaCl}_{2}(s)\) from their elements. No calculations are necessary.
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