Acid-base reactions  (Page 2/5)

Below are some examples:

1. $\mathrm{HCl}\left(\mathrm{g}\right)+{\mathrm{NH}}_3\left(\mathrm{g}\right)\to {\mathrm{NH}}_4^{+}+{\mathrm{Cl}}^{-}$ In order to decide which substance is a proton donor and which is a proton acceptor, we need to look at what happens to each reactant. The reaction can be broken down as follows: $\mathrm{HCl}\to {\mathrm{Cl}}^{-}+{\mathrm{H}}^{+}$ and ${\mathrm{NH}}_3+{\mathrm{H}}^{+}\to {\mathrm{NH}}_4^{+}$ From these reactions, it is clear that HCl is a proton donor and is therefore an acid , and that NH ${}_3$ is a proton acceptor and is therefore a base .
2. ${\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{CH}}_3{\mathrm{COO}}^{-}$ The reaction can be broken down as follows: ${\mathrm{CH}}_3\mathrm{COOH}\to {\mathrm{CH}}_3{\mathrm{COO}}^{-}+{\mathrm{H}}^{+}$ and ${\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+}\to {\mathrm{H}}_3{\mathrm{O}}^{+}$ In this reaction, CH ${}_3$ COOH (acetic acid) is a proton donor and is therefore the acid . In this case, water acts as a base because it accepts a proton to form H ${}_3$ O ${}^{+}$ .
3. ${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NH}}_4^{+}+{\mathrm{OH}}^{-}$ The reaction can be broken down as follows: ${\mathrm{H}}_2\mathrm{O}\to {\mathrm{OH}}^{-}+{\mathrm{H}}^{+}$ and ${\mathrm{NH}}_3+{\mathrm{H}}^{+}\to {\mathrm{NH}}_4^{+}$ In this reaction, water donates a proton and is therefore an acid in this reaction. Ammonia accepts the proton and is therefore the base . Notice that in the previous equation, water acted as a base and that in this equation it acts as an acid. Water can act as both an acid and a base depending on the reaction. This is also true of other substances. These substances are called ampholytes and are said to be amphoteric .

Amphoteric

An amphoteric substance is one that can react as either an acid or base. Examples of amphoteric substances include water, zinc oxide and beryllium hydroxide.

Conjugate acid-base pairs

Look at the reaction between hydrochloric acid and ammonia to form ammonium and chloride ions:

$\mathrm{HCl}+{\mathrm{NH}}_3\leftrightharpoons {\mathrm{NH}}_4^{+}+{\mathrm{Cl}}^{-}$

Looking firstly at the forward reaction (i.e. the reaction that proceeds from left to right ), the changes that take place can be shown as follows:

$\mathrm{HCl}\to {\mathrm{Cl}}^{-}+{\mathrm{H}}^{+}$ and

${\mathrm{NH}}_3+{\mathrm{H}}^{+}\to {\mathrm{NH}}_4^{+}$

Looking at the reverse reaction (i.e. the reaction that proceeds from right to left ), the changes that take place are as follows:

${\mathrm{NH}}_4^{+}\to {\mathrm{NH}}_3+{\mathrm{H}}^{+}$ and

${\mathrm{Cl}}^{-}+{\mathrm{H}}^{+}\to \mathrm{HCl}$

In the forward reaction , HCl is a proton donor (acid) and NH ${}_3$ is a proton acceptor (base). In the reverse reaction , the chloride ion is the proton acceptor (base) and NH ${}_4^{+}$ is the proton donor (acid). A conjugate acid-base pair is two compounds in a reaction that change into each other through the loss or gain of a proton. The conjugate acid-base pairs for the above reaction are shown below.

The reaction between ammonia and water can also be used as an example:

Conjugate acid-base pair

The term refers to two compounds that transform into each other by the gain or loss of a proton.

Acids and bases

1. In the following reactions, identify (1) the acid and the base in the reactants and (2) the salt in the product.
   1. ${\mathrm{H}}_2{\mathrm{SO}}_4+\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\to {\mathrm{CaSO}}_4+2{\mathrm{H}}_2\mathrm{O}$
   2. $\mathrm{CuO}+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{CuSO}}_4+{\mathrm{H}}_2\mathrm{O}$
   3. ${\mathrm{H}}_2\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{O}}^{-}$
   4. $\mathrm{HBr}+{\mathrm{C}}_5{\mathrm{H}}_5\mathrm{N}\to \left({\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{NH}}^{+}\right){\mathrm{Br}}^{-}$
2. In each of the following reactions, label the conjugate acid-base pairs.
   1. ${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}\leftrightharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{HSO}}_4^{-}$
   2. ${\mathrm{NH}}_4^{+}+{\mathrm{F}}^{-}\leftrightharpoons \mathrm{HF}+{\mathrm{NH}}_3$
   3. ${\mathrm{H}}_2\mathrm{O}+{\mathrm{CH}}_3{\mathrm{COO}}^{-}\leftrightharpoons {\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{OH}}^{-}$
   4. ${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{Cl}}^{-}\leftrightharpoons \mathrm{HCl}+{\mathrm{HSO}}_4^{-}$

Acid-base reactions

When an acid and a base react, they neutralise each other to form a salt . If the base contains hydroxide (OH ${}^{-}$ ) ions, then water will also be formed. The word salt is a general term which applies to the products of all acid-base reactions. A salt is a product that is made up of the cation from a base and the anion from an acid. When an acid reacts with a base, they neutralise each other. In other words, the acid becomes less acidic and the base becomes less basic. Look at the following examples: