Question

In which of the following processes is it necessary to break covalent bonds as opposed to simply overcoming intermolecular forces? (a) subliming dry ice (b) vaporizing chloroform \(\left(\mathrm{CHCl}_{3}\right)\) (c) decomposing water into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) (d) changing chlorine molecules into chlorine atoms

Short answer

Answer: The processes that require breaking covalent bonds are: (c) decomposing water into H₂ and O₂, and (d) changing chlorine molecules into chlorine atoms.

Step-by-step solution

Understanding the Processes

Let's analyze each process one by one. (a) Subliming dry ice: Dry ice is solid carbon dioxide (CO₂), and sublimation is the process of turning a solid directly into a gas. During this process, only the intermolecular forces holding the CO₂ molecules together in the solid state are broken, and the covalent bonds within each CO₂ molecule remain intact. (b) Vaporizing chloroform (CHCl₃): Chloroform is a chemical compound with the formula \(\mathrm{CHCl}_{3}\). Vaporizing chloroform involves turning a liquid into a gas. This process only requires breaking the intermolecular forces between the molecules, not the covalent bonds within the \(\mathrm{CHCl}_{3}\) molecules. (c) Decomposing water into H₂ and O₂: Water (H₂O) molecules are composed of hydrogen and oxygen atoms, held together by covalent bonds. To decompose water into hydrogen (H₂) and oxygen (O₂) gas molecules, we need to break the covalent bonds between the H and O atoms in the water molecules. (d) Changing chlorine molecules into chlorine atoms: Chlorine molecules consist of two chlorine atoms, which are held together by a covalent bond. To convert a chlorine molecule into separate chlorine atoms, we need to break the covalent bond between the atoms.

Determine the Answer

Based on our analysis, we can see that: - (a) and (b) only involve overcoming intermolecular forces, so they do not require breaking covalent bonds. - (c) and (d) involve breaking covalent bonds within the molecules or between atoms. Thus, the processes in which it is necessary to break covalent bonds are: (c) decomposing water into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\), and (d) changing chlorine molecules into chlorine atoms.

Key concepts

Covalent Bonds

Covalent bonds are strong chemical bonds formed by the sharing of electron pairs between atoms. This occurs primarily between nonmetal atoms that have similar electronegativities. These bonds create stable molecules, because the shared electrons act to bind the atoms closely together.
- The strength of covalent bonds is due to the shared pairs of electrons creating a strong attraction between the atoms, making it difficult to break these bonds without significant energy input. - A key example of covalent bonding is in water ( H_2O ), where oxygen shares electrons with hydrogen atoms. - Breaking covalent bonds, as seen in the decomposition of water, requires energy to overcome the strong attraction between the shared electrons, a process often achieved through methods like electrolysis.

Intermolecular Forces

Intermolecular forces are forces of attraction that exist between molecules, rather than within them like covalent bonds. These forces are generally weaker than covalent bonds and influence the physical state and properties of substances.
- There are various types of intermolecular forces, including London dispersion forces, dipole-dipole interactions, and hydrogen bonding, each with varying strengths and roles. - For example, in the vaporization of chloroform, the intermolecular Van der Waals forces between the chloroform molecules are broken, allowing it to transition from liquid to gas. - While important for states of matter (solid, liquid, gas), these forces require less energy to overcome compared to the breaking of covalent bonds within molecules.

Sublimation

Sublimation is the process where a solid changes directly into a gas, bypassing the liquid state. This phase change occurs when the particles in a solid gain enough energy to overcome the intermolecular forces holding them in place without needing to break the covalent bonds.
- A common example of sublimation is seen with dry ice, or solid carbon dioxide ( CO_2 ). - During sublimation, the molecules of dry ice gain energy and escape into the air as gas, while the covalent bonds within the CO_2 molecules remain intact. - Sublimation is often used to illustrate the difference between intermolecular and intramolecular forces, as it highlights how the latter (covalent bonds) remains unbroken during this phase change.

Decomposition Reaction

A decomposition reaction involves breaking down a compound into simpler substances or constituent elements. This often requires breaking covalent bonds within the molecules, which is a distinctive feature from processes involving only changes in state or intermolecular interactions.
- Decomposition reactions, such as the breakdown of water ( H_2O ) into hydrogen ( H_2 ) and oxygen ( O_2 ), are vital in both natural and industrial chemical processes. - This process can be endothermic, meaning it requires energy input, such as electrical energy in electrolysis, to break the covalent bonds. - Unlike sublimation or fusion processes, the purpose of decomposition is to chemically change the substance, rather than merely altering its physical state. Understanding these reactions is crucial in studying chemical changes and energy requirements.