Unit 8: Topic 6 - Molecular Structure of Acids and Bases

Question 1

The hydrogen-halogen acids all completely ionize in water, except for HF. Explain why.

Answer 1

HCl, HBr, and HI are all strong acids, but HF is a weak acid. We can use Coulomb’s law, F = kq1q2/r^2 to explain this. Fluorine has the smallest atomic size of all the halogens, so the 1/r^2 factor is significantly larger than for chlorine, bromine, or iodine. This intramolecular force in HF holds the molecules together more strongly to prevent full dissociation of HF in water, and the hydrogen bonding intermolecular force is also relatively strong. None of these are present in the remaining hydrogen halogen acids.

Question 2

Explain why HNO3 is a strong acid.

Answer 2

We can look at the conjugate base. The stabler the conjugate base, the stronger the original acid, since the conjugate base will not rapidly react with water to form base, and the acid will dissociate in aqueous solution. The conjugate base is NO3-. If we draw out the Lewis structure for this ion, we notice that nitrogen is the central atom, and single bonds are drawn to each terminal oxygen. This is a very stable structure, and the hydrogen in HNO3 is very weakly attached to one of the terminal oxygens, and easily dissociates in water.

Question 3

Give some organic examples of acids and bases.

Answer 3

Carboxylic acids, or compounds with a -COOH group, are weak organic acids. Nitrogenous compounds, or compounds with an -NH2 group, are usually weak bases. For certain weak acids (10^-14 < Ka < 10^-1), including carboxylic acids, their conjugate bases are weak bases, and the conjugate acids of weak bases (10^-14 < Kb < 10^-1), including nitrogenous compounds, are also weak acids. Therefore, protonated nitrogenous compounds are weakly acidic, and carboxylate ions are weakly basic.

Question 4

Rank the compounds ethane, methyl amine, and ethanol from decreasing to increasing acidity.

Answer 4

Here is a diagram depicting each of the following compounds. The blue hydrogen is the acidic hydrogen, and the red highlighted atom is used to highlight the strength of each acid. We can compare electronegativities of the red atom: the more electronegative the atom, the better it can accomodate the negative charge of the conjugate base after the acidic hydrogen dissociates, and the stronger the acid. Since the electronegativities in increasing order are carbon, nitrogen, and oxygen, the most stable conjugate base consists of oxygen, and the least stable consists of carbon. Therefore, ethane, methyl amine, and ethanol are in increasing order of acidity.