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Chapter 5: Problem 81

Without doing any calculations, predict the sign of \(\Delta H\) for each of the following reactions: $$\begin{array}{l}{\text { (a) } \mathrm{NaCl}(s) \longrightarrow \mathrm{Na}^{+}(g)+\mathrm{Cl}^{-}(\mathrm{g})} \\ {\text { (b) } 2 \mathrm{H}(g) \longrightarrow \mathrm{H}_{2}(g)} \\ {\text { (c) } \mathrm{Na}(g) \longrightarrow \mathrm{Na}^{+}(g)+\mathrm{e}^{-}} \\ {\text { (d) } \mathrm{I}_{2}(s) \longrightarrow \mathrm{I}_{2}(l)}\end{array}$$

Short Answer

Expert verified
For the given reactions, the signs of \(\Delta H\) are as follows: - Reaction (a): Positive \(\Delta H\) - endothermic - Reaction (b): Negative \(\Delta H\) - exothermic - Reaction (c): Positive \(\Delta H\) - endothermic - Reaction (d): Positive \(\Delta H\) - endothermic

Step by step solution

01

Analyze the reaction (a) NaCl(s) -> Na+(g) + Cl-(g)

In this reaction, a solid ionic compound (NaCl) is being dissociated into its gas-phase ions (Na+ and Cl-). This process requires energy to break the ionic bonds between the solid lattice. Therefore, this reaction will absorb heat from the surroundings, making it an endothermic reaction.
02

Predict the sign of ΔH for reaction (a)

Since reaction (a) is endothermic, the change in enthalpy, \(\Delta H\), is positive.
03

Analyze the reaction (b) 2 H(g) -> H2(g)

In this reaction, two gaseous hydrogen atoms are combining to form a gaseous hydrogen molecule (H2). The process involves the formation of a bond between two hydrogen atoms, which releases energy. Therefore, this reaction is exothermic.
04

Predict the sign of ΔH for reaction (b)

Since reaction (b) is exothermic, the change in enthalpy, \(\Delta H\), is negative.
05

Analyze the reaction (c) Na(g) -> Na+(g) + e-

In this reaction, a neutral gaseous sodium atom is being ionized to lose an electron, forming a gaseous sodium ion (Na+). Energy is required to overcome the electrostatic attraction between the electron and the positively charged nucleus. Therefore, this reaction is endothermic.
06

Predict the sign of ΔH for reaction (c)

Since reaction (c) is endothermic, the change in enthalpy, \(\Delta H\), is positive.
07

Analyze the reaction (d) I2(s) -> I2(l)

In this reaction, solid iodine (I2) is being converted into liquid iodine. When a substance changes from a solid to a liquid phase, energy is required to overcome the forces keeping the particles organized in the solid phase. This is called the heat of fusion. Therefore, this reaction is endothermic.
08

Predict the sign of ΔH for reaction (d)

Since reaction (d) is endothermic, the change in enthalpy, \(\Delta H\), is positive. In summary: - Reaction (a): Positive \(\Delta H\) - Reaction (b): Negative \(\Delta H\) - Reaction (c): Positive \(\Delta H\) - Reaction (d): Positive \(\Delta H\)

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Most popular questions from this chapter

A magnesium ion, Mg \(^{2+},\) with a charge of \(3.2 \times 10^{-19} \mathrm{Cand}\) an oxide ion, \(\mathrm{O}^{2-},\) with a charge of \(-3.2 \times 10^{-19} \mathrm{C}\) , are separated by a distance of 0.35 \(\mathrm{nm}\) . How much work would be required to increase the separation of the two ions to an infinite distance?

A 2.200 -g sample of quinone \(\left(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{O}_{2}\right)\) is burned in a bomb calorimeter whose total heat capacity is 7.854 \(\mathrm{kJ} / \mathrm{c}\) . The temperature of the calorimeter increases from 23.44 to \(30.57^{\circ} \mathrm{C}\) . What is the heat of combustion per gram of quinone? Per mole of quinone?

Consider the following reaction: $$2 \mathrm{CH}_{3} \mathrm{OH}(g) \longrightarrow 2 \mathrm{CH}_{4}(g)+\mathrm{O}_{2}(g) \quad \Delta H=+252.8 \mathrm{kJ}$$ (a) Is this reaction exothermic or endothermic? (b) Calculate the amount of heat transferred when 24.0 of \(\mathrm{CH}_{3} \mathrm{OH}(g)\) is decomposed by this reaction at constant pressure. (c) For a given sample of \(\mathrm{CH}_{3} \mathrm{OH},\) the enthalpy change during the reaction is 82.1 kJ. How many grams of methane gas are produced? (\mathbf{d} ) How many kilojoules of heatare released when 38.5 \(\mathrm{g}\) of \(\mathrm{CH}_{4}(g)\) reacts completely with \(\mathrm{O}_{2}(g)\) to form \(\mathrm{CH}_{3} \mathrm{OH}(g)\) at constant pressure?

Suppose that the gas-phase reaction \(2 \mathrm{NO}(g)+\mathrm{O}_{2}(g) \longrightarrow\) 2 \(\mathrm{NO}_{2}(g)\) were carried out in a constant-volume container at constant temperature. (a) Would the measured heat change represent \(\Delta H\) or \(\Delta E ?\) (b) If there is a difference, which quantity is larger for this reaction? (c) Explain your answer to part (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: $$\begin{aligned} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s)+6 \mathrm{O}_{2}(g) \longrightarrow 6 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) & \\ \Delta H^{\circ}=&-2803 \mathrm{kJ} \end{aligned}$$
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