Properties of Acidic Oxides

Generally, Groups 14-17 (non-metals) react to produce acidic oxides. These acid anhydrides or acidic oxides form acids with water

Example: SO₃ (g) + H₂O (l) → H₂SO₄ (aq) 

Example: CO₂ (g) + H₂O (l) → H₂CO₃ (aq)

Properties of Basic Oxides

Generally Group 1 and Group 2 elements form bases called base anhydrid or basic oxides.

Example: K₂O (s) + H₂O (l) → 2KOH (aq)  

Group 1 metals react rapidly with oxygen to produce several different ionic oxides, usually in the form of M₂O.

Example: 4 Li + O₂ → 2Li₂O    

• Peroxides:
  • Often Lithium and Sodium reacts with excess oxygen to produce the peroxide, M₂O₂.
  • The oxidation number of the oxygen is -1.

General Example: 2M + O₂→ M₂O₂

Example: H₂ + O₂ → H₂O₂

• Superoxides:
  • Often Potassium, Rubidium, and Cesium react with excess oxygen to produce the superoxide, MO₂.
  • The oxidation number of the oxygen is -1/2.

General Example: M + O₂→ MO₂

Example: Cs + O₂ → CsO₂

Properties of Amphoteric Oxides

An amphoteric solution is a substance that can chemically react as either acid or base. For example, when HSO₄^- reacts with water it can make hydroxide or hydronium ions. 

HSO₄^- + H₂O → SO₄^2- + H₃O⁺

HSO₄^- + H₂O → H₂SO⁴ + OH^-

Amphoteric oxides have both acidic and basic properties. A common example of an amphoteric oxide is aluminum oxide. In general, amphoteric oxides form with metalloids. (see chart below for more detail). Example with acidic properties:

Al₂O₃ + H₂O → 2 Al(OH) + 2 H⁺

Example with basic properties:

Al₂O₃ + H₂O → 2 Al³⁺ + 3 OH^-

Trends in Acid-Base Behavior  

If we take a closer look at a specific period, we may better understand the acid-base properties of oxides. It may also help to examine the physical properties of oxides, but it is not necessary. Metal oxides on the left side of the periodic table produce basic solutions in water (e.g. Na₂O and MgO). Non-metal oxides on the right side of the periodic table produce acidic solutions (e.g. Cl₂O, SO₂, P₄O₁₀). There is a trend within acid-base behavior: basic oxides are present on the left side of the period and acidic oxides are found on the right side. However, this trend yields the question of where and when does the shift occur? The figure below shows how, as we move from left to right, the oxides are more acidic and as we move from top to bottom, the oxides are more basic.

Aluminum oxide shows acid and basic properties of an oxide, it is amphoteric. Thus Al₂O₃ entails the marking point at which a change over from a basic oxide to acidic oxide occurs. It is important to remember that the trend only applies for oxides in their highest oxidation states. The individual element must be in its highest possible oxidation state because the trend does not follow if all oxidation states are included. Notice how the amphoteric oxides (shown in blue) of each period signify the change from basic to acidic oxides.

                                                                            Groups: 

   The figure above show oxides of the s- and p-block elements.
               purple: basic oxides blue: amphoteric oxides pink: acidic oxides             

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