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Chapter 9: Problem 72

Explain why the bond enthalpy of a molecule is usually defined in terms of a gas-phase reaction. Why are bond-breaking processes always endothermic and bond-forming processes always exothermic?

Short Answer

Expert verified
Bond enthalpy is defined for gas-phase reactions because gases have simpler molecular structures and their behavior is easier to predict thermodynamically. Bond-breaking processes are endothermic as they require energy to overcome the forces holding the atoms in a molecule together, hence absorbing this energy from the surroundings. On the other hand, bond-forming processes are exothermic because they release energy upon forming new, more stable bonds, transferring the heat to the surroundings.

Step by step solution

01

Concept of Bond Enthalpy

Bond enthalpy, or bond energy, is a measure of the amount of energy required to break one mole of a bond in a gaseous molecule. It is typically used for gas-phase reactions because gases tend to have simple molecular structures, and their behaviors are easier to predict and measure in terms of thermodynamics.
02

Explanation of Endothermic process

Endothermic process refers to a reaction where heat is absorbed. In a bond-breaking process, energy is required to overcome the forces of attraction between atoms. This energy is absorbed from the surroundings, causing the reaction to be endothermic.
03

Explanation of Exothermic process

An exothermic process is a reaction that releases heat to its surroundings. In bond-forming processes, atoms come together to form a more stable molecule, releasing the energy that was previously used to keep them apart. This released energy is transferred to the surroundings as heat, leading to an exothermic reaction.

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Most popular questions from this chapter

Write Lewis structures for the following molecules: (a) ICl, (b) \(\mathrm{PH}_{3}\), (c) \(\mathrm{P}_{4}\) (each \(\mathrm{P}\) is bonded to three other \(\mathrm{P}\) atoms (d) \(\mathrm{H}_{2} \mathrm{~S}\), (e) \(\mathrm{N}_{2} \mathrm{H}_{4}\), (f) \(\mathrm{HClO}_{3}\), (g) \(\mathrm{COBr}_{2}\) (C is bonded to \(\mathrm{O}\) and \(\mathrm{Br}\) atoms).

Which of the following species are isoelectronic: \(\mathrm{NH}_{4}^{+}, \mathrm{C}_{6} \mathrm{H}_{6}, \mathrm{CO}, \mathrm{CH}_{4}, \mathrm{~N}_{2}, \mathrm{~B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6} ?\)

Write Lewis structures for \(\mathrm{BrF}_{3}, \mathrm{ClF}_{5},\) and \(\mathrm{IF}_{7} .\) Identify those in which the octet rule is not obeyed.

Write three resonance structures for hydrazoic acid, \(\mathrm{HN}_{3}\). The atomic arrangement is HNNN. Show formal charges.

Oxygen forms three types of ionic compounds in which the anions are oxide \(\left(\mathrm{O}^{2-}\right),\) peroxide \(\left(\mathrm{O}_{2}^{2-}\right),\) and superoxide \(\left(\mathrm{O}_{2}^{-}\right) .\) Draw Lewis structures of these ions.
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