Question

Predict whether each of the following solvents is polar or nonpolar. (a) water, \(\mathrm{H}_{2} \mathrm{O}\) (b) hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}\) (c) acetone, \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) (d) chloroform, \(\mathrm{CHCl}_{3}\)

Short answer

Water and acetone are polar; hexane is nonpolar; chloroform is polar.

Step-by-step solution

Understanding Polarity

To determine whether a solvent is polar or nonpolar, we need to look at the molecular structure and the electronegativity differences between the atoms. Polar molecules have an uneven distribution of electrons, leading to partial positive and negative charges.

Analyzing Water

Water ( H_2O ) has a bent shape due to the two lone pairs on the oxygen. The oxygen is much more electronegative than hydrogen, resulting in a significant difference in electronegativity. This gives water a distinct dipole moment, making it a polar solvent.

Checking Hexane

Hexane ( C_6H_{14} ) is a hydrocarbon with a linear chain. The carbon-carbon and carbon-hydrogen bonds have very little difference in electronegativity, leading to an overall nonpolar molecule. Hexane does not have any partial charges or dipole moments.

Consider Acetone

Acetone ( C_3H_6O ) has a carbonyl group (C=O), where oxygen is much more electronegative than carbon, creating a polar bond. The molecular geometry allows these polar bonds to result in a net dipole moment, making acetone a polar molecule.

Evaluating Chloroform

Chloroform ( CHCl_3 ) has three chlorine atoms and one hydrogen atom bound to a central carbon atom. Chlorine is more electronegative than hydrogen and carbon, resulting in polar C-Cl bonds. The asymmetrical distribution of these bonds creates a net dipole moment, making chloroform polar.

Key concepts

Molecular Structure and Polarity

Molecular structure plays a crucial role in determining if a molecule is polar or nonpolar. Polar molecules have an asymmetric shape or arrangement that leads to an unequal distribution of electrons. This difference creates partial charges within the molecule, often referred to as dipoles. For example, water has a bent molecular shape due to its lone pairs, causing an uneven electron distribution.
This shape results in regions of partial positive and negative charges, making water a classic example of a polar solvent. On the other hand, nonpolar molecules like hexane typically have a symmetrical structure. Such arrangements allow electrons to be evenly distributed, negating any dipole moment. Hexane, with its linear configuration, exemplifies a nonpolar solvent, where the balanced charge distribution means it has no regions of partial charges.

Electronegativity in Chemistry

Electronegativity refers to the ability of an atom to attract shared electrons in a chemical bond. It is a key factor in determining the polarity of molecules. Atoms with high electronegativity attract electrons more strongly, causing a bond with an electronegative atom to become polarized.
In a molecule, if there is a significant difference in electronegativity between two bonding atoms, a polar bond is formed. Taking water as an example, oxygen has a higher electronegativity than hydrogen. As such, the electrons are pulled closer to the oxygen atom, creating partial negative and positive charges on the molecule's ends. In contrast, carbon and hydrogen have similar electronegativities, as seen in hexane, resulting in nonpolar bonds.

Dipole Moment in Molecules

The concept of dipole moment is foundational when discussing polarity. A dipole moment arises in a molecule when there is a separation of electric charges, creating a pair of equal and oppositely charged or magnetized poles. This is measured as the product of the charge difference and the distance between the charges.

• When molecular geometry allows polar bonds to align in such a way that they do not cancel out, a net dipole moment is observed. Notably, acetone possesses a net dipole moment due to the polar carbonyl group (C=O), as the electronegative oxygen attracts more electrons, resulting in a polar molecule.
• Similarly, in chloroform, the polar C-Cl bonds have differing electronegativities, creating a net dipole moment.

The direction and magnitude of a molecule's dipole moment further illustrate its polarity status, helping to predict the behavior of the molecule in different chemical environments.