CHEM1P80_Lecture

Lecture 14

Reactions in Aqueous Solutions: Part 2

A. Acids and Bases: Overview

Compound Type
Characteristic Behaviour
Chemical Characteristics

Acids
Taste sour, react with many metals to produce hydrogen gas. React with bases to produce salts in a neutralization reaction.
Produce H⁺ (hydrogen ion or proton) in solution

Bases
Taste bitter, feel slippery, don't react with most metals, but do form insoluble hydroxides with most metal cations. React with acids to produce salts in a neutralization reaction.
Produce OH^- (hydroxide ion) in solution

B. Strong and Weak Acids; Monoprotic and Polyprotic Acids

Strong acids are acids that dissociate completely when they dissolve in water. Among the strong acids are hydrochloric and nitric acid:

HCl (aq) ---> H⁺ (aq) + Cl^- (aq)

HNO₃ (aq) ---> H⁺ (aq) + NO₃^- (aq)

Weak acids are acids that dissociate incompletely upon dissolution in water. Among the weak acids are acetic acid and carbonic acid:

Carbonic acid undergoes dissociation in two stages. The hydrogen carbonate ion produced in the first stage further dissociates:

Monoprotic acids are acids that provide only 1 mole of H⁺ per mole of acid. HCl and HNO₃ are obviously monoprotic. CH₃CO₂H is also monoprotic. Only one of its four hydrogens is ionizable.

Polyprotic acids are acids that can provide more than 1 mole of H⁺ per mole of acid. Carbonic acid (H₂CO₃) is one such acid; sulfuric acid (H₂SO₄) is another. Both carbonic and sulfuric acids contain 2 ionizable hydrogens. Phosphoric acid (H₃PO₄) can ionize to produce 3 H⁺ per mole of acid.

C. Strong and Weak Bases

Strong bases are generally metal hydroxides such as NaOH and Ba(OH)₂. They dissociate completely into their ions in water:

NaOH ---> Na⁺ (aq) + OH^- (aq)

Ba(OH)₂---> Ba²⁺ (aq) + 2 OH^- (aq)

A distinction can be made between strong soluble bases and strong insoluble bases. Not many metal hydroxides are soluble; the ones that are comprise the strong soluble bases. Hydroxides that are only slightly soluble in water (such as calcium hydroxide or iron(III) hydroxide) are strong bases, because whatever amount does dissolve dissociates completely into the ions.

Weak bases include ammonia (NH₃) and organic amines such as methylamine (CH₃NH₂). They do not contain the hydroxide ion, but they produce it when they dissolve in water because they react with water:

For a listing of common acids and bases see Table 5.2 in Kotz and Treichel.

D. Reactions of Acids with Strong Bases (Metal Hydroxides)

The fact that acids react with bases to produce salts is highly important in chemistry, biology, and the earth and environmental sciences.

If the base is a metal hydroxide (soluble or insoluble), water is the other product of the reaction. Thus, an acid reacts with a metal hydroxide to produce a salt and water. Some examples:

(a) reaction of a strong acid with a strong soluble base:

2 HCl (aq) + Ba(OH)₂ (aq) ---> BaCl₂ (aq) + 2 H₂O (l)

(b) reaction of a weak acid with a strong soluble base:

CH₃CO₂H (aq) + NaOH (aq) ---> NaCH₃CO₂ (aq) + H₂O (l)

(c) reaction of a strong acid with a strong insoluble base:

3 H₂SO₄ (aq) + 2 Al(OH)₃ (s) ---> Al₂(SO₄)₃ (aq) + 6 H₂O (l)

The net ionic equations (Lecture 13) for these 3 reactions are:

(a) H⁺ (aq) + OH^- (aq) ---> H₂O (l)

(b) CH₃CO₂H (aq) + OH^- (aq) ---> CH₃CO₂^- (aq) + H₂O (l)

(c) 3 H⁺(aq) + Al(OH)₃ (s) ---> Al³⁺ (aq) + 3 H₂O (l)

If the acid is polyprotic, the neutralization proceeds in stages. The final product depends on the amount of base used in the reaction. Here are the overall equations for the reaction of phosphoric acid (a weak triprotic acid) with different amounts of potassium hydroxide:

(d) H₃PO₄ (aq) + KOH (aq) ---> KH₂PO₄ (aq) + H₂O (l)

(e) H₃PO₄ (aq) + 2 KOH (aq) ---> K₂HPO₄(aq) + 2 H₂O (l)

(f) H₃PO₄ (aq) + 3 KOH (aq) ---> K₃PO₄(aq) + 3 H₂O (l)

The net ionic equations for these 3 reactions are:

(d) H₃PO₄ (aq) + OH- (aq) ---> H₂PO₄^- (aq) + H₂O (l)

(e) H₃PO₄ (aq) + 2 OH- (aq) ---> HPO₄^2- (aq) + 2 H₂O (l)

(f) H₃PO₄ (aq) + 3 OH- (aq) ---> PO₄^3- (aq) + 3 H₂O (l)

E. Reactions of Acids with Weak Bases

Weak bases do not contain hydroxide in their formulas. When they react with acids, they do so by removing a proton (hydrogen ion) from the acid so as to become the cation. A salt is produced, but no water. For example, when aqueous nitric acid reacts with aqueous ammonia, the salt ammonium nitrate is produced. The overall reaction is:

HNO₃ (aq) + NH₃ (aq) ---> NH₄NO₃ (aq)

The net ionic equation for this reaction is

H⁺ (aq) + NH₃ (aq) ---> NH₄⁺ (aq)

Similarly, if carbonic acid reacts with an equimolar amount of ammonia, ammonium hydrogen carbonate is produced:

H₂CO₃ (aq) + NH₃ (aq) ---> NH₄HCO₃ (aq)

The net ionic equation for this reaction is

H₂CO₃ (aq) + NH₃ (aq) ---> NH₄⁺ + HCO₃⁺ (aq)

F. Acidic and Basic Oxides

Oxides that react with water to produce acids are called acidic oxides or acid anhydrides. Carbon dioxide and sulfur trioxide are two such compounds. Carbon dioxide reacts with water to produce carbonic acid:

CO₂ (g) + H₂O (l) ---> H₂CO₃ (aq)

Sulfur trioxide reacts with water to produce sulfuric acid:

SO₃ (g) + H₂O (l) ---> H₂SO₄ (aq)

Most acidic oxides are oxides of nonmetals, or of metals that are in very high oxidation states. An example of an acidic metal oxide is CrO₃, which reacts with water to produce chromic acid, H₂CrO₄.

Oxides that react with water to produce bases are called basic oxides or base anhydrides. Sodium oxide reacts with water to produce the strong soluble base sodium hydroxide:

Na₂O (s) + H₂O (l) ---> NaOH (aq)

Calcium oxide ("lime") reacts with water to produce the insoluble base calcium hydroxide ("slaked lime"):

CaO (s) + H₂O (l) ---> Ca(OH)₂ (s)

G. Gas-forming Reactions

Important gases that may be evolved in chemical reactions are shown in the Table below.

Gas
Produced in this kind of reaction:

Hydrogen (H₂)
Metal + acid
Example: Mg (s) + 2 HCl (aq) ---> MgCl₂ (aq) + H₂ (g)

Carbon dioxide (CO₂)
Metal carbonate or bicarbonate + acid
Example: Na₂CO₃ (aq) + 2 HNO₃ (aq) ---> 2 NaNO₃(aq) + H₂O (l) + CO₂ (g)

Hydrogen sulfide (H₂S)
Metal sulfide + acid
Example: FeS (s) + H₂SO₄ (aq) ---> FeSO₄ (aq) + H₂S (g)

Sulfur dioxide (SO₂)
Metal sulfite or hydrogen sulfite + acid
Example: NaHSO₃ + H₂SO₄ (aq) ---> SO₂ (g) + NaHSO₄ (aq) + H₂O (l)

Ammonia (NH₃)
Ammonium salt + strong base
Example: NH₄NO₃ (aq) + NaOH (aq) ---> NaNO₃ (aq) + NH₃ (g) + H₂O (l)

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Created September 11, 2000 by M. F. Richardson
© Brock University, 2000

Compound Type	Characteristic Behaviour	Chemical Characteristics
Acids	Taste sour, react with many metals to produce hydrogen gas. React with bases to produce salts in a neutralization reaction.	Produce H⁺ (hydrogen ion or proton) in solution
Bases	Taste bitter, feel slippery, don't react with most metals, but do form insoluble hydroxides with most metal cations. React with acids to produce salts in a neutralization reaction.	Produce OH^- (hydroxide ion) in solution

Gas	Produced in this kind of reaction:
Hydrogen (H₂)	Metal + acid Example: Mg (s) + 2 HCl (aq) ---> MgCl₂ (aq) + H₂ (g)
Carbon dioxide (CO₂)	Metal carbonate or bicarbonate + acid Example: Na₂CO₃ (aq) + 2 HNO₃ (aq) ---> 2 NaNO₃(aq) + H₂O (l) + CO₂ (g)
Hydrogen sulfide (H₂S)	Metal sulfide + acid Example: FeS (s) + H₂SO₄ (aq) ---> FeSO₄ (aq) + H₂S (g)
Sulfur dioxide (SO₂)	Metal sulfite or hydrogen sulfite + acid Example: NaHSO₃ + H₂SO₄ (aq) ---> SO₂ (g) + NaHSO₄ (aq) + H₂O (l)
Ammonia (NH₃)	Ammonium salt + strong base Example: NH₄NO₃ (aq) + NaOH (aq) ---> NaNO₃ (aq) + NH₃ (g) + H₂O (l)