We have already looked at the Bronsted-Lowry definitions of acids and bases in the Bronsted-Lowry Theory Study Guide. Recall, an acid is a species that is a proton donor, and a base is a species that is a proton acceptor. A positively charged hydrogen ion is a proton. In the reaction below, HBr is an acid because it donates a proton to H2O. In this reaction water acts as a base because it accepts a proton.
The products also include an acid and a base. Br– is a base and H3O+ is an acid. In the reverse reaction, H3O+ donates a proton to the base, Br–. Any species that has one or more lone pairs of electrons can act as a base.
Bronsted-Lowry acid-base reactions are also called proton transfer reactions. In the reaction above, Br- is the conjugate base of HBr and H3O+ is the conjugate acid for water. NH4+ ion and NH3 are also a conjugate acid-base pair. Their chemical formulas only differ by one hydrogen (proton). For example, the conjugate base of HPO42- is PO43-. The conjugate acid of HPO42- is H2PO4–. The conjugate base of HCl is Cl–. The conjugate base of H2O is OH–.
Another proton transfer reaction is that between NH3 and water. Ammonia is the base and water is the acid in this reaction.
Hydroxide ion, OH– is the conjugate base of H2O, and NH4+ is the conjugate acid of NH3. We have seen in these two reactions that water acts as an acid when reacted with a base, and it acts as a base when reacted with an acid. Water and other substances that can act as both an acid or a base are said to be amphoteric.
Most acid-base reactions are reversible indicated by the double arrow in the chemical equation. The longer arrow determines which side of the reaction is favored. Recall, weak acids only partially dissociate while strong acids essentially dissociate to completion.
A stronger acid will have a weaker conjugate base while a weaker acid will have a stronger conjugate base. A stronger acid more easily loses a proton, therefore, its conjugate base will have little affinity for accepting a proton. A weaker acid will hold on to its proton, therefore, the conjugate base will have a higher affinity for a proton.
Curved arrows are used to show the movement of electrons. The arrow points from the electron donor to the electron acceptor. In the acid-base reaction below, an arrow points from the lone pair of electrons on oxygen in water to the proton of the acid. The second arrow points from the hydrogen to the bond between hydrogen and chlorine. The hydrogen-chlorine bond breaks, hydrogen leaves its bonding electrons with chlorine, and hydrogen (the proton) forms a bond with the oxygen atom of water via a lone pair of electrons. Curved arrows allow us to see bonds that are broken and bonds that form.
Exercises
Exercise 1. For each of the following conjugate acid-base pairs, indicate which species is the acid and which is the base. Draw Lewis structures that show all valence electrons and the formal charges.
a) CO32- and HCO3–
b) CH3OH and CH3OH2+
c) CH3NH3+ and CH3NH2
Check Answer/Solution to Exercise 1
Exercise 2. What is the conjugate acid of the following?
b) Br–
c) CH3CH2OH
d) HO–
Check Answer/Solution to Exercise 2
Exercise 3.What is the conjugate base of the following?
b) HCO3–
c) HI
d) H2O
e) H3O+
Check Answer/Solution to Exercise 3
Exercise 4.Use curved arrows to show the movement of electrons in the following equations. Label the acids and bases.
b) CH3NH2 + H2O → CH3NH3+ + OH–
Check Answer/Solution to Exercise 4
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