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Below are some examples:

  1. HCl ( g ) + NH 3 ( g ) NH 4 + + 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: HCl Cl - + H + and NH 3 + H + 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. CH 3 COOH + H 2 O H 3 O + + CH 3 COO - The reaction can be broken down as follows: CH 3 COOH CH 3 COO - + H + and H 2 O + H + H 3 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. NH 3 + H 2 O NH 4 + + OH - The reaction can be broken down as follows: H 2 O OH - + H + and NH 3 + H + 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 .

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:

HCl + NH 3 NH 4 + + 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:

HCl Cl - + H + and

NH 3 + H + 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:

NH 4 + NH 3 + H + and

Cl - + H + 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.

Khan academy video on acids and bases

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. H 2 SO 4 + Ca ( OH ) 2 CaSO 4 + 2 H 2 O
    2. CuO + H 2 SO 4 CuSO 4 + H 2 O
    3. H 2 O + C 6 H 5 OH H 3 O + + C 6 H 5 O -
    4. HBr + C 5 H 5 N ( C 5 H 5 NH + ) Br -
  2. In each of the following reactions, label the conjugate acid-base pairs.
    1. H 2 SO 4 + H 2 O H 3 O + + HSO 4 -
    2. NH 4 + + F - HF + NH 3
    3. H 2 O + CH 3 COO - CH 3 COOH + OH -
    4. H 2 SO 4 + Cl - HCl + 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:

Questions & Answers

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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