Question

Obtain the oxidation number for the element noted in each of the following. (a) \(\mathrm{Cr}\) in \(\mathrm{CrO}_{3}\) (b) \(\mathrm{Hg}\) in \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\) (c) Ga in \(\mathrm{Ga}(\mathrm{OH})_{3}\) (d) \(\mathrm{P}\) in \(\mathrm{Na}_{3} \mathrm{PO}_{4}\)

Short answer

(a) +6; (b) +1; (c) +3; (d) +5.

Step-by-step solution

Oxidation Number Rules

To find oxidation numbers, apply these key rules: 1. Pure elements have an oxidation number of 0. 2. The sum of oxidation numbers in a compound is the net charge of the compound. 3. Typical oxidation states: Oxygen is usually -2, Hydrogen +1, Alkali metals +1, and Alkaline earth metals +2.

Step 2a: Chromium in \( \text{CrO}_3 \)

The compound \( \text{CrO}_3 \) is neutral. Oxygen typically has an oxidation state of -2. Therefore, setup the equation:\[\text{Oxidation number of Cr} + 3(-2) = 0\]Solve for the oxidation number of Cr:\[\text{Oxidation number of Cr} = +6\]

Step 2b: Mercury in \( \text{Hg}_2\text{Cl}_2 \)

In \( \text{Hg}_2\text{Cl}_2 \), the compound is neutral. Chlorine typically has an oxidation state of -1. Apply the rule:\[2(\text{Oxidation number of Hg}) + 2(-1) = 0\]Solve for the oxidation number of Hg:\[2(\text{Oxidation number of Hg}) = +2\]Thus, the oxidation number of Hg is +1.

Step 2c: Gallium in \( \text{Ga(OH)}_3 \)

The hydroxide ion \( \text{OH}^- \) has a charge of -1 per ion. With \( \text{Ga(OH)}_3 \), which is neutral:\[\text{Oxidation number of Ga} + 3(-1) = 0\]Solve for the oxidation number of Ga:\[\text{Oxidation number of Ga} = +3\]

Step 2d: Phosphorus in \( \text{Na}_3\text{PO}_4 \)

\( \text{Na}_3\text{PO}_4 \) is neutral, where sodium (Na) has an oxidation state of +1 and oxygen -2. Calculate using the rule:\[3(+1) + \text{Oxidation number of P} + 4(-2) = 0\]Simplify:\[3 + \text{Oxidation number of P} - 8 = 0\]Solving gives:\[\text{Oxidation number of P} = +5\]

Key concepts

Redox Chemistry

Redox chemistry is a vital part of studying chemistry, as it deals with the processes where oxidation and reduction occur. In these processes, there is a transfer of electrons between substances, significantly impacting their oxidation states. An essential concept to grasp is that oxidation involves the loss of electrons, resulting in an increase in oxidation number. Conversely, reduction is the gain of electrons, leading to a decrease in oxidation number. Redox reactions can be observed in a variety of everyday occurrences, such as rusting iron, batteries in your gadgets, and even biological processes. By understanding redox chemistry, students learn to balance complex equations and predict changes in chemical reactions.

Chemical Compounds

Chemical compounds are substances formed by the chemical bonding of two or more different elements. These bonds can be covalent, ionic, or metallic, depending on the nature of the elements involved. Chemical compounds are characterized by a definite composition and distinct properties, allowing us to classify various substances based on their building blocks. Each compound has a chemical formula expressing the quantity of each type of atom present. For example, water's chemical formula is \( \text{H}_2\text{O} \), indicating two hydrogen atoms bonded to one oxygen atom. Understanding chemical compounds gives students the foundation to explore more complex chemical interactions and the roles that different elements play in forming stable substances.

Oxidation States

Oxidation states, also known as oxidation numbers, indicate the degree of oxidation of an atom in a chemical compound. These numbers can be positive, negative, or zero, based on the distribution of electrons around an atom. By assigning oxidation states to the elements within a compound, chemists can track electrons in redox reactions and verify that equations are balanced correctly. For example, in the compound \( \text{CrO}_3 \), oxygen has an oxidation state of -2, compelling the chromium to have an oxidation number of +6 to maintain neutrality. Knowing how to determine and apply oxidation states is essential in predicting the behavior of elements in reactions, especially in complex redox processes.

Neutral Compounds

Neutral compounds are those chemical compounds that carry no overall charge. This neutrality results from the balance of positive and negative charges within the compound. Each constituent atom's oxidation numbers must sum to zero, rendering the compound stable. For example, in \( \text{Hg}_2\text{Cl}_2 \), mercury and chlorine's oxidation states are balanced, leading to a neutral compound. Understanding the concept of neutral compounds helps students comprehend how diverse ions and molecules interact to form stable substances. Recognizing and balancing the charges within a compound are foundational skills in chemistry, allowing a deeper insight into molecular structures and interactions.