THE BRØNSTED-LOWRY THEORY
17.2. THE BRØNSTED-LOWRY THEORY
In the Brønsted-Lowry theory, (often called the Brønsted theory for short), an acid is defined as a substance that donates a proton to another substance. In this sense, a proton is a hydrogen atom that has lost its electron; it has nothing to do with the protons in the nuclei of other atoms. (The nuclei of 2 H are also considered protons for this purpose; they are hydrogen ions.) A base is a substance that accepts a proton from another substance. The reaction of an acid and a base produces another acid and base. The following reaction is thus an acid-base reaction according to Brønsted:
The HC 2 H 3 O 2 is an acid because it denotes its proton to the H 2 O to form C 2 H 3 O 2 − and H 3 O + . The H 2 O is a base because it accepts that proton. But this is an equilibrium reaction, and C 2 H 3 O 2 − reacts with H 3 O + to form HC 2 H 3 O 2 and H 2 O. The C 2 H 3 O 2 − is a base because it accepts the proton from H 3 O + ; the H 3 O + is an acid because it donates a proton. H 3 O + is called the hydronium ion. It is the combination of a proton and a water
CHAP. 17]
ACID-BASE THEORY
molecule, and is the species that we have been abbreviating H + thus far in this book. (H + is not stable; it does not have the configuration of a noble gas; see Sec. 5.4.)
The acid on the left of this equation is related to the base on the right; they are said to be conjugates of each other. The HC 2 H 3 O 2 is the conjugate acid of the base C 2 H 3 O 2 − . Similarly, H 2 O is the conjugate base of H 3 O + . Conjugate differ in each case by H + .
EXAMPLE 17.1. Write an equilibrium equation for the reaction of NH 3 and H 2 O, and label each of the conjugate acids and bases.
base 3 +H 2 O acid −→ ←− NH 4 acid + + OH − base
Conjugates
The NH 3 is a base because it accepts a proton from water, which is therefore an acid. The NH 4 + is an acid because it can donate a proton to OH − , a base.
We have now labeled water as both an acid and a base. It is useful to think of water as either, because it really has no more properties of one than the other. Water is sometimes referred to as amphiprotic. It reacts as an acid in the presence of bases, and it reacts as a base in the presence of acids.
Various acids have different strengths. Some acids are strong; that is, they react with water completely to form their conjugate bases. Other acids are weak, and they form conjugate bases that are stronger than the conjugate bases of strong acids. In fact, the stronger the acid, the weaker its conjugate base. Some acids are so extremely weak that they do not donate protons at all at ordinary temperatures. In this text we refer to these conjugates as feeble. We can classify the acids and the related conjugate bases as follows:
Conjugate Acid
Conjugate Base
The same reasoning applies to bases that are molecules and their conjugate acids (that are ions). Note especially that weak acids do not have strong conjugate bases, as stated in some texts. For example, acetic acid is weak, and its conjugate base, the acetate ion, is certainly not strong. It is even weaker as a base than acetic acid is as an acid.
EXAMPLE 17.2. Classify the following acids and bases according to their strength: HNO 3 , HC 2 H 3 O 2 , KOH, and NH 3 . Ans.
HNO 3 is a strong acid; HC 2 H 3 O 2 is a weak acid; KOH is a strong base; NH 3 is a weak base. EXAMPLE 17.3. Classify the conjugates of the species in the prior example according to their strength.
Ans. NO 3 − is a feeble base; C 2 H 3 O 2 − is a weak base; K + is a feeble acid; NH 4 + is a weak acid. This classification indicates that NO − 3 and K + have no tendency to react with water to form their conjugates.
The C 2 H 3 O 2 − does react with water to some extent to form HC 2 H 3 O 2 and OH − . The NH 4 + reacts with water to
a small extent to form H 3 O + and NH 3 . Consider the following equation:
HNO 3 +H 2 O −→
base ←− NO 3 − +H 3 O base + acid
acid
Since HNO 3 reacts with water 100%, NO 3 − does not react with H 3 O + at all. If the nitrate ion cannot take a proton from the hydronium ion, it certainly cannot take the proton from water, which is a much weaker acid than the hydronium ion is.
ACID-BASE THEORY
[ CHAP. 17
EXAMPLE 17.4. What is the difference between the reaction of HC 2 H 3 O 2 with H 2 O and with OH − ? Ans.
The first reaction takes place to a slight extent; HC 2 H 3 O 2 is a weak acid. The second reaction goes almost 100%. Even weak acids react almost completely with OH − .
The acidity of a solution is determined by the hydronium ion concentration of the solution. The greater the [H 3 O + ], the more acidic the solution; the lower the [H 3 O + ], the more basic the solution. Other substances, for example, OH − , affect the acidity of a solution by affecting the concentration of H 3 O + . The presence in water of OH − in greater concentration than H 3 O + makes the solution basic. If the relative concentrations are reversed, the solution is acidic.
EXAMPLE 17.5. Explain why KC 2 H 3 O 2 tests basic in water solution.
Ans. The K + does not react with water at all. The C 2 H 3 O 2 − reacts with water to a slight extent:
C 2 H 3 O 2 − +H 2 O −→ ←− HC 2 H 3 O 2 + OH −
The excess OH − makes the solution basic.