8.3: Acidic and Basic Anhydrides
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Learning Objectives
- Identify acid anyhdrides and basic anhydrides.
- Know the reactions of acid and basic anhydrides with water.
Metal oxides are substances used in the manufacture of commonly used bases like calcium hydroxide (traditionally known as slaked lime), sodium hydroxide and potassium hydroxide. Acid anhydrides (non-metal oxides) are environmentally relevant. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapor to produce acid rain (Chapter 13).
Nonmetal Oxides: Acid Anhydrides
Acid anhydrides are compounds that turn into an acid when you add water. They are non-metal oxides. The oxides of period three elements demonstrate periodicity with respect to acidity. As you move across the period, the oxides become more acidic. Sodium and magnesium oxides are alkaline. Aluminium oxides are amphoteric (reacting both as a base or acid). Silicon, phosphorus, sulfur, and chlorine oxides are acidic. Some non-metal oxides, such as nitrous oxide (N 2 O) and carbon monoxide (CO), do not display any acid/base characteristics. These are a little more complicated than basic anhydrides, so don't worry too much about them right now.
Several examples of reactions involving nonmetal oxides are as follows:
1. Carbon dioxide is the anhydride of carbonic acid:
CO 2 (g) + H 2 O(l) → H 2 CO 3 (aq)
2. Sulfur dioxide reacts with water to form the weak acid, sulfurous acid:
SO 2 (g) + H 2 O (l)→ H 2 SO 3 (aq)
3. Sulfur trioxide forms the strong sulfuric acid with water (so, sulfur trioxide is the anhydride of sulfuric acid):
SO 3 (g) + H 2 O(l) → H 2 SO 4 (aq)
This reaction is important in the manufacture of the acid.
Metal Oxides: Basic Anhydrides
Oxides of more electropositive elements like the alkali or alkaline earth metals tend to be basic. They are called basic anhydrides . Exposed to water, they may form bases (basic hydroxides). Sodium oxide is basic—when hydrated, it forms sodium hydroxide. Here's another example:
CaO(s) + H 2 O(l) → Ca(OH) 2 (aq)
If the metal is an alkali or alkaline earth, the reaction probably happens quickly and produces a lot of heat. If the metal is a transition metal, the reaction might not happen so easily or at all.
Summary
- Acid anhydrides are nonmetal oxides that react with water to form acids.
- Basic anhydrides are metal oxides that react with water to form bases.