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Chapter 17: Problem 15

A mixture of hydrogen gas and chlorine gas remains unreacted until it is exposed to ultraviolet light from a burning magnesium strip. Then the following reaction occurs very rapidly: $$ \mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{HCl}(g) $$ Explain.

Short Answer

Expert verified
The reaction between hydrogen gas (H2) and chlorine gas (Cl2) occurs rapidly when exposed to ultraviolet light from a burning magnesium strip because the high-energy UV light breaks the bonds in the H2 and Cl2 molecules, forming individual hydrogen and chlorine atoms. These highly reactive atoms then quickly combine to form stable hydrogen chloride (HCl) molecules, making the reaction proceed rapidly.

Step by step solution

01

Understand the reactants and products

The initial reactants, hydrogen gas (H2) and chlorine gas (Cl2), are both diatomic molecules. They are stable and do not react with each other in the absence of light. In the reaction, these molecules break apart into individual atoms, which then form hydrogen chloride (HCl) as a new, stable molecule.
02

Discuss the role of ultraviolet light

Ultraviolet (UV) light is a form of electromagnetic radiation with high energy. When the hydrogen gas (H2) and chlorine gas (Cl2) are exposed to UV light, the energy from the light gets absorbed by their molecules, which provides enough energy for the bond in the diatomic molecules to break.
03

Breaking the bond

The absorbed energy from the UV light is enough to break the bond between the hydrogen atoms in the H2 molecule and between the chlorine atoms in the Cl2 molecule. Once these bonds break, individual hydrogen and chlorine atoms are formed. \( H_{2}(g) \rightarrow 2H(g) \) \( Cl_{2}(g) \rightarrow 2Cl(g) \)
04

Formation of HCl

Now that the individual hydrogen and chlorine atoms are formed, they can react with each other to form hydrogen chloride (HCl). This process is very fast due to the high reactivity of the individual atoms. \( H(g) + Cl(g) \rightarrow HCl(g) \)
05

Conclusion

The ultraviolet light from the burning magnesium strip provides enough energy to break the bonds in the hydrogen (H2) and chlorine (Cl2) molecules. Once these bonds break, individual hydrogen and chlorine atoms are formed. These highly reactive atoms quickly combine to form hydrogen chloride (HCl), making the reaction proceed rapidly.

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Most popular questions from this chapter

Gas \(\mathrm{A}_{2}\) reacts with gas \(\mathrm{B}_{2}\) to form gas \(\mathrm{AB}\) at a constant temperature. The bond energy of \(\mathrm{AB}\) is much greater than that of either reactant. What can be said about the sign of \(\Delta H ? \Delta S_{\text {surr }}\) ? \(\Delta S\) ? Explain how potential energy changes for this process. Explain how random kinetic energy changes during the process.

Which of the following processes are spontaneous? a. Salt dissolves in \(\mathrm{H}_{2} \mathrm{O}\). b. A clear solution becomes a uniform color after a few drops of dye are added. c. Iron rusts. d. You clean your bedroom.

Consider the reactions $$ \begin{aligned} \mathrm{Ni}^{2+}(a q)+6 \mathrm{NH}_{3}(a q) & \longrightarrow \mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}^{2+}(a q) \\ \mathrm{Ni}^{2+}(a q)+3 \mathrm{en}(a q) & \longrightarrow \mathrm{Ni}(\mathrm{en})_{3}^{2+}(a q) \end{aligned} $$ where $$ \text { en }=\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2} $$ The \(\Delta H\) values for the two reactions are quite similar, yet \(\mathrm{K}_{\text {reaction } 2}>K_{\text {reaction } 1} .\) Explain.

Predict the sign of \(\Delta S^{\circ}\) and then calculate \(\Delta S^{\circ}\) for each of the following reactions. a. \(\mathrm{H}_{2}(g)+\frac{1}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)\) b. \(2 \mathrm{CH}_{3} \mathrm{OH}(g)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+4 \mathrm{H}_{2} \mathrm{O}(g)\) c. \(\mathrm{HCl}(g) \longrightarrow \mathrm{H}^{+}(a q)+\mathrm{Cl}^{-}(a q)\)

Monochloroethane \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\right)\) can be produced by the direct reaction of ethane gas \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)\) with chlorine gas or by the reaction of ethylene gas \(\left(\mathrm{C}_{2} \mathrm{H}_{4}\right)\) with hydrogen chloride gas. The second reaction gives almost a \(100 \%\) yield of pure \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) at a rapid rate without catalysis. The first method requires light as an energy source or the reaction would not occur. Yet \(\Delta G^{\circ}\) for the first reaction is considerably more negative than \(\Delta G^{\circ}\) for the second reaction. Explain how this can be so.
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