Properties of Acids and Bases
Acids and bases have characteristics different from most other materials. The following table gives properties and examples of each:
Property
Taste Touch Reactivity with metals Conducts electricity? pH Reaction with phenolphthalien indicator Reaction with universal indicator Examples |
Acid
Tart Will sting, especially on a cut Vigorously and produces bubbles Yes 0 to below 7 Colorless Red, orange, yellow Fruits, soft drinks, coffee, milk |
Base
Bitter Feels slippery, because it is dissolving skin No reaction Yes above 7 to 14 Pinkish-purple Blue, violet Blood, eggs, soap, ammonia, drain cleaner |
Definitions of Acids and Bases
There are two definitions for the classification of acids and bases. The first is called the Arrhenius Definition (1884). Arrhenius said that acids and bases release specific ions in water:
There are two definitions for the classification of acids and bases. The first is called the Arrhenius Definition (1884). Arrhenius said that acids and bases release specific ions in water:
Acids - dissociate to produce H+ ions in water
Bases - dissociate to produce OH- (hydroxide) ions in water
Or in simpler terms, a substance that begins with H+ will be an acid. like HCl (stomach acid) and H2SO4 (battery acid). A substance that ends with an OH- will be a base, like NaOH and Ca(OH)2 (drain cleaners).
Bases - dissociate to produce OH- (hydroxide) ions in water
Or in simpler terms, a substance that begins with H+ will be an acid. like HCl (stomach acid) and H2SO4 (battery acid). A substance that ends with an OH- will be a base, like NaOH and Ca(OH)2 (drain cleaners).
This definition will help to identify many acids and bases. However, there are several materials that have acidic and basic properties which do not fit in this definition, so another, more complete definition was designed. This more complete definition is called the Bronsted-Lowery Definition (1923)
Bronsted and Lowery independently said that acids and bases can be thought of H+ donors and acceptors:
Acids donate H+ ions
Bases accept H+ ions
Acids donate H+ ions
Bases accept H+ ions
For example, when nitrogen trihydride is placed in water, it undergoes this reaction:
NH3 (g) + H2O (l) --> NH4+ (aq) + OH- (aq)
NH3 changes in to NH4+, thereby accepting an H+ ion. Therefore, NH3 is a base. H2O changes into OH-, thereby donating an H+ ion. Therefore, H2O is an acid. Water can either accept or donate H+ ions. When water accepts a H+ ion, it becomes H3O+, which is called hydronium.
Neutralization
When acids and bases are mixed, they retain none of the properties of either an acid or base. The reaction of an acid and a base always produce two materials: water and a “salt”. The salt will not always be sodium chloride.
Examples:
When acids and bases are mixed, they retain none of the properties of either an acid or base. The reaction of an acid and a base always produce two materials: water and a “salt”. The salt will not always be sodium chloride.
Examples:
HCl (aq) + NaOH (aq) --> HOH (l) + NaCl (aq)
H3PO4 (aq) + 3 KOH (aq) --> 3 HOH (l) + K3PO4 (aq)
H3PO4 (aq) + 3 KOH (aq) --> 3 HOH (l) + K3PO4 (aq)
The above equations are color coded. Red is an acid, purple is a base, blue is water (written HOH instead of H2O: either way is acceptable) and green is the “salt”.
Conjugate Acids and Bases
Many acid/base reactions are reversible. Here is an example:
Many acid/base reactions are reversible. Here is an example:
NH3 (g) + H2O (l) <==> NH4+ (aq) + OH- (aq)
A reversible reaction is a reaction in which the products can reform the reactants. When this happens, we can label the products as acid and base just like we did the reactants (see Bronsted-Lowery definition above).
However, when we label these products as acid and base, we add the word “conjugate” to indicate this is an acid or base acting in a reversible reaction; So when we label these substances, we get:
However, when we label these products as acid and base, we add the word “conjugate” to indicate this is an acid or base acting in a reversible reaction; So when we label these substances, we get:
NH3 (g) + H2O (l) <==> NH4+ (aq) + OH- (aq)
base acid conjugate acid conjugate base
base acid conjugate acid conjugate base
Another example:
H2SO3 (g) + H2O (l) <==> HSO3- (aq) + H3O+ (aq)
acid base conjugate based conjugate acid
acid base conjugate based conjugate acid
2 things. For these, I will write the reactions for you, so don't worry about predictions. Just identify the acids and bases. Also, notice that if you identify the acid on the left, what it turns into is always the conjugate base on the right. Conjugates are always on the right.