Which oxide, \(\mathrm{CrO}_{3}\) or \(\mathrm{CrO}\), is more acidic in water? Why?

Metal oxides are compounds formed when metals react with oxygen. They can exhibit different chemical properties based on the metal they are composed of and its oxidation state. There are three main categories of metal oxides:

• Basic Oxides: These react with acids to form salt and water. Most metal oxides, especially those of alkali and alkaline earth metals, fall into this category.
• Amphoteric Oxides: These can react with both acids and bases to form salts and water.
• Acidic Oxides: These react with water to form acids or with bases to form salts. Non-metal oxides and metal oxides with high oxidation states are typically acidic.

Understanding the type of an oxide is crucial for determining its reactivity and behavior in different chemical environments.

The oxidation state (or oxidation number) signifies the degree of oxidation of an atom in a compound. It can be positive, negative, or zero and is determined based on electron transfer in oxidation-reduction (redox) reactions. In the compound \(\text{CrO}\), the oxidation state of chromium (Cr) is +2. Here’s how you calculate it: Oxygen typically has an oxidation state of -2. Since there is only one oxygen atom, its contribution to the compound’s charge is -2. The total charge of \(\text{Cr = +2}\) must balance the charge from oxygen, making Cr in \(\text{CrO}\) have an oxidation state of +2. For \(\text{CrO}_3\), the oxidation state of Cr is +6. With three oxygen atoms, the total contribution from oxygen is -6. Thus, Cr's oxidation state must be +6 to balance it out.
Oxidation states influence the behavior of metal oxides in chemical reactions. Oxides of elements with higher oxidation states tend to be more acidic, as they can more easily form acids when reacted with water.

There is a profound relationship between the oxidation state of metal in an oxide and its acidity. Generally, metal oxides with higher oxidation states are more acidic. This is because:

• Higher oxidation states increase the polarity of the metal-oxygen bond, enhancing the oxide's ability to attract water and form acids.
• Metal oxides with high oxidation states can establish strong interactions with water molecules, leading to the release of \text{H}^+ ions and forming acidic solutions.

Let's apply this to the given problem. Chromium in \(\text{CrO}\) has an oxidation state of +2, making it less able to form an acid when compared to \(\text{CrO}_3\), where Cr is in a +6 oxidation state. Thus, \(\text{CrO}_3\) is more acidic than \text{CrO}\ since it can better interact with water to produce acidic solutions.
When determining the acidity of metal oxides, always compare the oxidation states to predict which oxide will exhibit greater acidity.