Polarity of Bonds

The polarity of a bond can be determined by the difference in electronegativity of the atoms forming the bond. Polarity is a relative property. Following are general guidelines only:

So, for example for the following chemical bonds:

NaCl Delta EN = 3.0-0.9 = 2.1 ionic

HCl        Delta EN = 3.0-2.1 = 0.9 polar

CCl        Delta EN = 3.0-2.5 = 0.4 non-polar

HC        Delta EN = 2.5-2.1 = 0.4 non-polar

HN        Delta EN = 3.0-2.1 = 0.9 polar

HO        Delta EN = 3.5-2.1 = 1.4 polar

etc. (Pair up other atoms for practice)

We represent the polarity of a bond usually by drawing an arrow next to it, with the arrowhead pointing to the more electronegative atom.

The molecular polarity depends on two things. The polarity of the bonds in the molecule and the molecular geometry. The overall polarity, or charge distribution, in the molecule is given by the vector sum of the individual bond polarities. (A vector is a quantity with both a magnitude and a direction).

Some molecules can have polar bonds, but still be non-polar because the vectors cancel each other. (Examples would be BF₃ and CO₂).

Because water is bent, it has a net polarity (called a dipole).

Polar molecules tend to associate with each other as the positive end of one molecule lines up with the negative end of another—like tiny magnets.

Solvent properties of water

Water is often called the universal solvent because it dissolves so many things. But it by no means dissolves everything. Let’s consider separately ionic compound and covalent compounds.

Ionic compounds

Note the video representation (this video requires RealPlayer) of water dissolving the salt sodium chloride. Normally the Na⁺ and Cl^- ions are held together in a lattice arrangement, held together by very strong electrostatic forces. The water molecules, as small dipoles, are able to solvate the ions and break up this attraction. (Note how the molecules orient themselves about the positively charged sodium ion and the negatively charged chloride ion).

Recall that in the formation of NaCl from sodium and chlorine, the process involves a transfer of electrons:

2 Na  ---> 2 Na⁺ + 2 electrons (oxidation)

Cl₂ + 2 electrons   --->  2 Cl^- (reduction)

(The process of losing electrons is called oxidation, while gaining electrons is called reduction).

A solution of ions in water is capable of transmitting an electric current. Hence soluble ionic substances are often called electrolytes.

Demonstration of electrolytes and non-electrolytes.

Note that pure water is a poor conductor of electricity. A solution of sodium chloride conducts electricity ( the light lights up brightly), while a solution of acetic acid conducts electricity to a much lesser extent (the light lights only dimly).  (These links are to videos which require RealPlayer).

The ions conduct an electric current because they are mobile and can be attracted to the electrodes. The negative electrode is called the cathode. It attracts positive ions, so positive ions are called cations. The positive electrode is called the anode. It attracts negative ions, so negative ions are called anions.

A solution of sucrose does not conduct electricity. Therefore we call a sodium chloride solution a strong electrolyte, acetic acid a weak electrolyte, and sucrose a non-electrolyte. We will see in the next chapter that acetic acid dissociates slightly to form ions, while sucrose does not dissociate.

Why does sucrose dissolve in water. If you examine its structure (see figure 5.10, page 165) you see that there are many O-H bonds in the molecule, bonds which are polar and which can interact with the polar water molecules.

There is an additional interaction involved, however, called hydrogen bonding, which we will examine more closely in the next lecture.