Question

Assign an oxidation state to each atom in each compound. (a) \(\mathrm{CH}_{4}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{CuCl}_{2}\) (d) HI

Short answer

The oxidation states are (a) C: -4, H: +1, (b) C: 0, H: +1, Cl: -1, (c) Cu: +2, Cl: -1, (d) H: +1, I: -1.

Step-by-step solution

Understanding Oxidation State Rules

The oxidation state (or number) is a concept that provides a measure of the degree of oxidation of an atom in a substance. Rules to assign oxidation states include: elements in their natural state have an oxidation state of 0, the sum of the oxidation states in a neutral compound must equal 0, the sum in a polyatomic ion equals the ion's charge, Group 1 metals have an oxidation state of +1, Group 2 metals +2, hydrogen +1 (except metals hydrides where it's -1), oxygen -2 (except peroxides and F2O), halogens -1 (except when combined with more electronegative elements), and for carbon it varies depending on what it's bonded to.

Assigning Oxidation States for CH4

For methane (CH4), carbon is bonded to four hydrogen atoms. By rule, hydrogen has an oxidation state of +1. Since there are four H atoms in total, the sum of the oxidation states of the hydrogen is +4. To make the compound neutral, carbon must have an oxidation state of -4 to balance the +4 from hydrogen.

Assigning Oxidation States for CH2Cl2

For dichloromethane (CH2Cl2), hydrogen still has an oxidation state of +1. With two hydrogens, this is +2. Chlorine typically has an oxidation state of -1. With two chlorine atoms, this totals -2. To keep the molecule neutral, carbon must have an oxidation state of 0, since +2 from hydrogen balances with -2 from chlorine.

Assigning Oxidation States for CuCl2

In copper(II) chloride (CuCl2), each chlorine has an oxidation state of -1. Having two chlorines gives a total of -2. To make the compound neutral, copper must have an oxidation state of +2 to balance the two -1 charges from the chlorines.

Assigning Oxidation States for HI

In hydrogen iodide (HI), by rule, hydrogen has an oxidation state of +1, and iodine is a halogen which typically has an oxidation state of -1. The sum of the oxidation states equals zero, which matches the compound being neutral.

Key concepts

Redox Chemistry

Redox chemistry is the study of oxidation-reduction reactions where the oxidation states of atoms change. These reactions are fundamental in various chemical and biological processes, including metabolism, corrosion, and combustion. The oxidation state, often called the oxidation number, is an indicator of the degree of oxidation of an atom within a molecule or ion. Oxidation involves the loss of electrons, leading to an increase in oxidation state, while reduction involves the gain of electrons, resulting in a decrease in oxidation state.

To understand redox reactions, it is crucial to assign oxidation states properly, just as we did for the compounds in the provided exercise. Recognizing the changes in these oxidation states allows chemists to identify which species are oxidized and which are reduced in a chemical reaction.

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules. There are several types of chemical bonds, including ionic, covalent, and metallic bonds. The type of bond formed depends on the elements involved and their valence electrons. For example, in the methane molecule (\(CH_{4}\)), carbon and hydrogen form covalent bonds where electrons are shared between atoms.

In a covalent bond, the shared electrons spend more time with one atom than the other, based on electronegativity differences, which can induce partial oxidation states. When elements form ionic bonds, as in copper(II) chloride (\(CuCl_{2}\)), one atom donates electrons, becoming positively charged, while the other gains electrons, becoming negatively charged.

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom and are involved in chemical bonding. The number of valence electrons determines an atom's chemical properties and how it will interact with other atoms. For instance, in the periodic table, Group 1 elements have one valence electron, often resulting in a +1 oxidation state because they tend to lose this electron to achieve a stable electron configuration.

Valence electrons explain why hydrogen, with one valence electron, is commonly found with a +1 oxidation state in most compounds, such as hydrogen iodide (\(HI\)). However, there are exceptions, like when hydrogen is bonded to a metal, in which it takes a -1 oxidation state due to its metallic hydride form.

Periodic Table Groups

The periodic table is organized into groups (columns) that share similar chemical properties. These groups are numbered from 1 to 18 and often have predictable oxidation states. For example, Group 1 metals (alkali metals) typically have a +1 oxidation state, and Group 2 metals (alkaline earth metals) have a +2 oxidation state. This is because atoms in these groups have one and two valence electrons, respectively, and they tend to lose them to achieve stable configurations.

Understanding the typical oxidation states of these groups can simplify the process of assigning oxidation states in compounds. Halogens, found in Group 17, usually have a -1 oxidation state as they need one electron to complete their valence shell, like chlorine in dichloromethane (\(CH_{2}Cl_{2}\)), highlighting the reliability of periodic trends in predicting chemical properties.