Question

Why is carbon oxidized in the following reaction (i.e., in what sense is carbon losing electrons)? $$ \mathrm{C}+2 \mathrm{~S} \longrightarrow \mathrm{CS}_{2} $$

Short answer

Carbon is oxidized because its oxidation state increases from 0 in elemental carbon to +4 in carbon disulfide (CS2), indicating a loss of electrons.

Step-by-step solution

Identify the oxidation states before and after reaction

Determine the oxidation state of carbon in both the reactants and the products. For the reactants, elemental carbon has an oxidation state of 0. For the products, in the CS2 molecule, sulfur (S) is more electronegative than carbon (C), so sulfur will be assigned a negative oxidation state. Given that each sulfur has an oxidation state of -2 and there are 2 sulfur atoms in CS2, and the total charge must balance to zero, the carbon must have an oxidation state of +4.

Compare the oxidation states

Compare the oxidation states of carbon before and after the reaction. Carbon starts with an oxidation state of 0 and ends with a +4 oxidation state. An increase in oxidation state means an atom has lost electrons.

Determine if carbon is oxidized

Since the oxidation state of carbon has increased from 0 to +4, it has lost electrons. Losing electrons is the definition of oxidation. Therefore, carbon is oxidized in this reaction.

Key concepts

Redox Reactions

Redox reactions, or reduction-oxidation reactions, involve the transfer of electrons between two substances. In a broad sense, these reactions encompass a variety of processes that are all about the movement of electrons from one element to another.

Understanding a redox reaction starts with recognizing the two halves that make up the whole: reduction, where a substance gains electrons, and oxidation, where a substance loses electrons. This electron movement is pivotal because it alters the oxidation states of the substances involved, effectively changing their properties.

Characteristics of Redox Reactions

Redox reactions have several important features:

• Electron movement: Electrons move from one reactant (the oxidizing agent) to another (the reducing agent).
• Oxidation states: These change in the reactants as electrons are transferred, with increases indicating oxidation and decreases indicating reduction.
• Energy changes: Often accompany redox reactions, as breaking and forming bonds involves the transfer of energy.
• Practical applications: They are central to processes like combustion, metabolism, and corrosion, as well as industrial applications such as electroplating and battery operation.

In the given exercise, the reaction between carbon and sulfur to form carbon disulfide (CS₂) is an example of a redox reaction where carbon is oxidized.

Electron Transfer

The concept of electron transfer is key to understanding redox reactions. It's essentially the 'currency exchange' of chemistry, where electrons are traded between atoms and molecules, determining their chemical behavior.

Atoms seek to reach a state of stability, and electron transfers often occur to fulfill that quest. In a redox reaction, one species will lose electrons and another will gain them. The species that loses electrons is oxidized, which means it increases its oxidation state. Conversely, the species that gains electrons is reduced, showing a decrease in its oxidation state.

Significance of Electron Transfer

Electron transfer is significant for several reasons:

• Foundation of redox reactions: Without the transfer of electrons, there would be no redox reactions.
• Energy transfer: The movement of electrons often involves the transfer of energy, which can be harnessed for work in biological and technological systems.
• Creation of ions: Electron transfer can result in the formation of ions, which are pivotal in a wide range of chemical reactions.

In the example from the exercise, carbon loses electrons to sulfur, indicating that an electron transfer has taken place from the carbon to the sulfur, resulting in the oxidation of carbon.

Oxidation Numbers

Oxidation numbers, or oxidation states, are a conceptual method in chemistry used to keep track of electrons during redox reactions. They are assigned to the atoms in a molecule to represent the distribution of electrons among those atoms.

An oxidation number is a positive or negative number that indicates how many electrons an atom has gained, lost, or shared in a compound compared to its neutral state. The rules for assigning oxidation numbers are designed to reflect the electron distribution in chemical bonds and to help predict the reactivity of materials during chemical reactions.

Rules of Oxidation Numbers

When assigning oxidation numbers, keep the following rules in mind:

• The oxidation number of an atom in its elemental form is always zero.
• For monoatomic ions, the oxidation number is the same as the charge on the ion.
• Oxygen usually has an oxidation number of -2, except in peroxides or in combination with fluorine.
• Hydrogen is usually +1 when bonded with nonmetals and -1 when bonded with metals.
• The sum of oxidation numbers in a neutral compound must be zero; in a polyatomic ion, it must equal the charge of the ion.

In our exercise example, the oxidation state of elemental carbon is 0, and the oxidation state of carbon in carbon disulfide is +4. This change indicates that carbon has undergone oxidation, losing electrons to achieve a more positive state in the reaction.