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Chapter 5: Problem 82

Without doing any calculations, predict the sign of \(\Delta H\) for each of the following reactions: (a) \(2 \mathrm{NO}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{O}_{4}(g)\) (b) \(2 \mathrm{~F}(g) \longrightarrow \mathrm{F}_{2}(g)\) (c) $\mathrm{Mg}^{2+}(g)+2 \mathrm{Cl}^{-}(g) \longrightarrow \mathrm{MgCl}_{2}(s)$ (d) \(\mathrm{HBr}(g) \longrightarrow \mathrm{H}(g)+\mathrm{Br}(g)\)

Short Answer

Expert verified
(a) \(\Delta H < 0\) (exothermic) (b) \(\Delta H < 0\) (exothermic) (c) \(\Delta H < 0\) (exothermic) (d) \(\Delta H > 0\) (endothermic)

Step by step solution

01

(a) Understanding the reaction

For the reaction \(2 \mathrm{NO}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{O}_{4}(g)\), we need to compare the strength and number of bonds in the reactants and products. In the reactants, we have two NO₂ molecules, where a nitrogen molecule is bound to two oxygen molecules. In the product, we have one N₂O₄ molecule, where two nitrogen molecules are bound to four oxygen molecules.
02

(a) Predicting ∆H

When two NO₂ molecules combine to form a single N₂O₄ molecule, new nitrogen-nitrogen and nitrogen-oxygen bonds are being formed. This bond formation generally releases energy, making the reaction exothermic. Therefore, we can predict the sign of ∆H to be negative.
03

(b) Understanding the reaction

The reaction \(2 \mathrm{~F}(g) \longrightarrow \mathrm{F}_{2}(g)\) involves the combination of two separate fluorine atoms to form one F₂ molecule.
04

(b) Predicting ∆H

The formation of a single bond between two fluorine atoms releases energy, making this reaction exothermic. Therefore, the sign of ∆H for this reaction is also negative.
05

(c) Understanding the reaction

The reaction \(\mathrm{Mg}^{2+}(g)+2 \mathrm{Cl}^{-}(g) \longrightarrow \mathrm{MgCl}_{2}(s)\) is a simple combination of charged ions to form a solid ionic compound.
06

(c) Predicting ∆H

When oppositely charged ions combine to form an ionic compound, electrostatic forces are used to create a stable lattice structure. This reaction generally releases energy, making it exothermic. Hence, we can predict the sign of ∆H to be negative for this reaction as well.
07

(d) Understanding the reaction

The reaction \(\mathrm{HBr}(g) \longrightarrow \mathrm{H}(g)+\mathrm{Br}(g)\) involves breaking the hydrogen-bromine bond in HBr to generate separate H and Br atoms.
08

(d) Predicting ∆H

Breaking a bond like this requires input energy to overcome the bond energy, making this reaction endothermic. Therefore, the sign of ∆H for this reaction is positive.

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Most popular questions from this chapter

Assume that 2 moles of water are formed according to the following reaction at constant pressure \((101.3 \mathrm{kPa})\) and constant temnerature $(298 \mathrm{~K});$ $$ 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(l) $$ (a) Calculate the pressure-volume work for this reaction. (b) Calculate \(\Delta E\) for the reaction using your answer to (a).

Use bond enthalpies in Table 5.4 to estimate \(\Delta H\) for each of the following reactions: (a) $\mathrm{H}-\mathrm{H}(g)+\mathrm{Br}-\mathrm{Br}(g) \longrightarrow 2 \mathrm{H}-\mathrm{Br}(g)$ (b)

At the end of 2012, global population was about 7.0 billion people. What mass of glucose in kg would be needed to provide 1500 Cal/person/day of nourishment to the global population for one year? Assume that glucose is metabolized entirely to \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(l)\) according to the following thermochemical equation: $$ \begin{aligned} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s)+6 \mathrm{O}_{2}(g) \longrightarrow 6 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) \\ \Delta H^{\circ} &=-2803 \mathrm{~kJ} \end{aligned} $$

Indicate which of the following is independent of the path by which a change occurs: (a) the change in potential energy when a book is transferred from table to shelf, (b) the heat evolved when a cube of sugar is oxidized to \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(g),(\mathbf{c})\) the work accomplished in burning a gallon of gasoline.

(a) Under what condition will the enthalpy change of a process equal the amount of heat transferred into or out of the system? (b) During a constant-pressure process, the system releases heat to the surroundings. Does the enthalpy of the system increase or decrease during the process? (c) In a constant-pressure process, \(\Delta H=0\). What can you conclude about \(\Delta E, q,\) and \(w ?\)
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