Question

What is an oxidation-reduction reaction? What is transferred during such a reaction?

Short answer

An oxidation-reduction (redox) reaction is a chemical reaction involving the transfer of electrons between two species. The species that loses electrons is oxidized, while the species that gains electrons is reduced. In a redox reaction, the oxidation states of atoms change as electrons are transferred. For example, in the reaction of hydrogen (H) and chlorine (Cl) gas, where \( 2H_2(g) + 2Cl_2(g) \rightarrow 4HCl(g) \), hydrogen atoms are oxidized as their oxidation states increase from 0 to +1, and chlorine atoms are reduced when their oxidation states decrease from 0 to -1.

Step-by-step solution

Introduction to Oxidation-Reduction Reactions

An oxidation-reduction reaction, or redox reaction, is a chemical reaction in which electrons are transferred between two species. The species that loses electrons is being oxidized, and the species that gains electrons is being reduced. To understand this concept, we should first learn about the term "oxidation state."

Oxidation State

The oxidation state is a number assigned to atoms in a compound, according to certain rules. It represents the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. Although actual charges in compounds are often not integer values, oxidation states can be used as a tool to keep track of electron transfer in redox reactions.

Identifying Oxidation and Reduction

In a redox reaction, one species donates electrons (oxidation) and another species accepts them (reduction). To identify which species is being oxidized and which is being reduced, follow these steps: 1. Determine the oxidation state of each atom in the reactants and products of the reaction. 2. Identify the changes in oxidation states between the reactants and products. 3. The atom that has an increase in oxidation state is being oxidized, while the atom that has a decrease in oxidation state is being reduced.

Example of a Redox Reaction

Let's take the reaction between hydrogen (H) and chlorine (Cl) as an example: \[ 2H_2(g) + 2Cl_2(g) \rightarrow 4HCl(g) \] 1. Determine the oxidation states: In H2 and Cl2 molecules, the oxidation state for each atom is zero since they are in their elemental form. In HCl, H has an oxidation state of +1 and Cl has an oxidation state of -1. 2. Identify the changes in oxidation states: The hydrogen atoms go from oxidation state 0 to +1, while the chlorine atoms go from oxidation state 0 to -1. 3. Identify oxidation and reduction: Hydrogen atoms are being oxidized (increase in oxidation state), and chlorine atoms are being reduced (decrease in oxidation state). So, during this reaction, electrons are transferred from hydrogen atoms to chlorine atoms.

Key concepts

Redox Reaction

Understanding the intricacies of a redox reaction is pivotal in chemistry, as many processes — from industrial manufacturing to biological respiration — hinge upon these reactions. Simply put, a redox (reduction-oxidation) reaction is a chemical process in which one substance gives up electrons, undergoing oxidation, while another substance gains electrons, known as reduction. These two processes always occur simultaneously; one species can't be oxidized without another being reduced. This is because electrons cannot exist free in solution; they must be transferred from one atom or molecule to another.

Consider the example of a reaction between hydrogen and chlorine to form hydrochloric acid \(2H_2(g) + 2Cl_2(g) \rightarrow 4HCl(g)\). Here, the hydrogen atoms lose electrons (indicated by an increase in oxidation state from 0 to +1) thus, they undergo oxidation. Concurrently, chlorine atoms gain those electrons (a decrease in their oxidation state from 0 to -1), and reduction occurs. In this process, each hydrogen atom has one electron stripped away, which is then picked up by a chlorine atom, perfectly demonstrating the concept of a coupled redox reaction with direct electron transfer between reactants.

Oxidation State

Diving a bit deeper, the concept of oxidation state or oxidation number is a construct that helps chemists keep track of these electron transfers in a redox reaction. The oxidation state is an assigned value that reflects an atom's degree of oxidation or reduction in a particular molecule or ion. Importantly, it's a hypothetical value that assumes bonds between atoms are purely ionic, even though real-world compounds often have covalent character. For example, atoms in their elemental form, like \(H_2\) and \(Cl_2\), have an oxidation state of 0. Understanding how to determine oxidation states allows for the identification of which atoms are oxidized and which are reduced during a reaction.

To enumerate oxidation states, there are simple rules such as oxygen usually having an oxidation state of -2, except in peroxides, and hydrogen typically having an oxidation state of +1 when bonded to non-metals. By comparing the oxidation states of reactants and products, like in our \(H_2\) and \(Cl_2\) reaction, we can see that the oxidation state of hydrogen increases while that of chlorine decreases, an indication of their respective oxidation and reduction.

Electron Transfer

The core of a redox reaction is electron transfer — without it, no oxidation or reduction can occur. The term refers to the movement of electrons from one atom or molecule to another. It's also the foundation for how we classify agents in a reaction: the species donating electrons is known as the reducing agent, as it causes reduction to occur in another species, while the recipient of electrons is the oxidizing agent. Reducing agents get oxidized, and oxidizing agents get reduced.

Electron transfer can occur directly between reacting species, as in the reaction between hydrogen and chlorine, where electrons move from hydrogen to chlorine atoms. Alternatively, it can involve a series of intermediate steps, often seen in complex biological systems or electrochemical cells. By focusing on which species gains or loses electrons and how their oxidation states change, we gain a comprehensive view of the redox process, and can use it to predict reactions, balance chemical equations, and understand a vast array of natural and industrial processes.