Question

Predict which substance in each pair has the highest surface tension. (a) \(\mathrm{CCl}_{4}\) or \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) (b) ethanol ( \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) ) or ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\)

Short answer

(a) CH2Br2, (b) Ethylene Glycol.

Step-by-step solution

Understand Surface Tension

Surface tension is a property of liquids that describes the elastic tendency of a fluid surface. It is influenced by the intermolecular forces in the liquid, such as hydrogen bonding, Van der Waals forces, and molecular weight.

Analyze Pair (a) CCl4 or CH2Br2

Compare the two substances for intermolecular forces. CCl4 has London dispersion forces because it is nonpolar, while CH2Br2 is polar and can also experience dipole-dipole interactions, which are stronger than dispersion forces. Thus, CH2Br2 is expected to have a higher surface tension due to stronger intermolecular forces.

Analyze Pair (b) Ethanol or Ethylene Glycol

Examine the types of intermolecular forces. Ethanol (CH3CH2OH) has hydrogen bonding because of the hydroxyl group. Ethylene Glycol (HOCH2CH2OH) has two hydroxyl groups, allowing for more extensive hydrogen bonding than ethanol. This means ethylene glycol would have a higher surface tension due to increased hydrogen bonding capacity.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules. These forces are crucial in determining various physical properties of substances, such as boiling points, melting points, and surface tension. While you might hear about chemical bonds within molecules (like covalent or ionic bonds), intermolecular forces are different—they occur between separate molecules.
There are several types of intermolecular forces:

• Dispersion (London) Forces: These are the weakest type of intermolecular force. They occur due to the instantaneous and temporary distribution of electrons around a molecule, causing an induced dipole effect. Despite being weak, they are present in all molecules.
• Dipole-Dipole Interactions: These forces occur between polar molecules. They arise from the attractive forces between the permanent positive and negative dipoles within the molecules.
• Hydrogen Bonds: These are a special, stronger type of dipole-dipole interaction. They occur when a hydrogen atom that is covalently bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) is attracted to another electronegative atom.

Understanding the concept of intermolecular forces is fundamental to predicting the behavior of liquids and their surface tension.

Hydrogen Bonding

Hydrogen bonding is a notable intermolecular force that significantly affects the physical properties of substances. It is especially strong compared to other dipole-dipole interactions. For a hydrogen bond to occur:

• There must be a hydrogen atom bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine.
• This hydrogen atom will then interact with another electronegative atom, creating a bridge between molecules.

Hydrogen bonds play a key role in determining the surface tension of a liquid. In the case of exercise pair (b), ethylene glycol and ethanol both involve hydrogen bonding. However, ethylene glycol contains two hydroxyl groups, allowing for more extensive hydrogen bonding compared to ethanol, resulting in higher surface tension. This is because more hydrogen bonds lead to a greater cohesive force at the surface of a liquid.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are present in all molecules, whether polar or nonpolar. They are the result of temporary shifts in the electron density in a molecule, creating temporary dipoles that induce dipoles in neighboring molecules.
Even though dispersion forces are the weakest type of intermolecular force, they can be significant in larger molecules with higher molar mass due to more electrons and larger electron clouds.For example:

• In the comparison between \(\mathrm{CCl}_{4}\) and \(\mathrm{CH}_{2}\mathrm{Br}_{2}\), \(\mathrm{CCl}_{4}\) relies solely on dispersion forces, making its intermolecular attractions weaker than those in \(\mathrm{CH}_{2}\mathrm{Br}_{2}\), which also includes dipole-dipole interactions.

While dispersion forces are weak individually, in large numbers, especially in heavy atoms or molecules, they can lead to significant attractions and influence properties like the boiling points and viscosities of substances.

Dipole-Dipole Interactions

Dipole-dipole interactions are intermolecular forces that occur between two polar molecules. These occur when the partial positive charge of one molecule is attracted to the partial negative charge of another. Such interactions are stronger than dispersion forces but weaker than hydrogen bonds.Consider the example from the exercise:

• The molecule \(\mathrm{CH}_{2}\mathrm{Br}_{2}\) is polar, allowing it to engage in dipole-dipole interactions.

Because of these interactions, \(\mathrm{CH}_{2}\mathrm{Br}_{2}\) will have stronger intermolecular attractions compared to nonpolar molecules like \(\mathrm{CCl}_{4}\). This results in \(\mathrm{CH}_{2}\mathrm{Br}_{2}\) having a higher surface tension than \(\mathrm{CCl}_{4}\). Understanding dipole-dipole interactions is crucial for predicting the boiling points, melting points, and surface tension of substances.