Question

Assign an oxidation state to each element in each reaction and use the change in oxidation state to determine which element is being oxidized and which element is being reduced. (a) \(\mathrm{CH}_{4}(g)+2 \mathrm{H}_{2} \mathrm{~S}(g) \longrightarrow \mathrm{CS}_{2}(g)+4 \mathrm{H}_{2}(g)\) (b) \(2 \mathrm{H}_{2} \mathrm{~S}(g) \longrightarrow 2 \mathrm{H}_{2}(g)+\mathrm{S}_{2}(g)\) (c) \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s)+6 \mathrm{O}_{2}(g) \longrightarrow 6 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(g)\) (d) \(\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{4} \mathrm{Cl}_{2}(g)\)

Short answer

In (a) carbon is oxidized and hydrogen is reduced. In (b) sulfur is oxidized and hydrogen is reduced. In (c) carbon is oxidized and oxygen is reduced. In (d) carbon is oxidized and chlorine is reduced.

Step-by-step solution

Identify Oxidation States for Reaction (a)

For the reaction \(\mathrm{CH}_{4}(g) + 2 \mathrm{H}_{2} \mathrm{S}(g) \longrightarrow \mathrm{CS}_{2}(g) + 4 \mathrm{H}_{2}(g)\), assign oxidation states. Carbon in \(\mathrm{CH}_{4}\) is -4. In \(\mathrm{H}_{2}\mathrm{S}\), hydrogen is +1 and sulfur is -2. In \(\mathrm{CS}_{2}\), carbon is +4 and sulfur is -2. In \(\mathrm{H}_{2}\) gas, hydrogen is 0.

Identify the Redox Changes for Reaction (a)

Compare the oxidation states of elements before and after the reaction. Carbon goes from -4 to +4, indicating oxidation (loss of electrons). Sulfur remains at -2, so it's neither oxidized nor reduced. Hydrogen goes from +1 in \(\mathrm{H}_{2}\mathrm{S}\) to 0 in \(\mathrm{H}_{2}\), which is reduction (gain of electrons).

Identify Oxidation States for Reaction (b)

For the reaction \(2 \mathrm{H}_{2} \mathrm{S}(g) \longrightarrow 2 \mathrm{H}_{2}(g) + \mathrm{S}_{2}(g)\), assign oxidation states. In \(\mathrm{H}_{2}\mathrm{S}\), hydrogen is at +1, and sulfur is at -2. In \(\mathrm{H}_{2}\), hydrogen is at 0, and in \(\mathrm{S}_{2}\) the sulfur is at 0 because it's a diatomic molecule in its elemental form.

Identify the Redox Changes for Reaction (b)

Compare the oxidation states of the elements before and after the reaction. Sulfur goes from -2 to 0, indicating oxidation (loss of electrons). Hydrogen goes from +1 to 0, which indicates reduction (gain of electrons).

Identify Oxidation States for Reaction (c)

For the reaction \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s) + 6 \mathrm{O}_{2}(g) \longrightarrow 6 \mathrm{CO}_{2}(g) + 6 \mathrm{H}_{2} \mathrm{O}(g)\), assign oxidation states. In glucose (\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}), oxidation states can vary but carbons are oxidized in the reaction. Oxygen is -2 in both \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2}\mathrm{O}\), and in \(\mathrm{O}_{2}\), it's 0. Hydrogen is +1 in both glucose and water.

Identify the Redox Changes for Reaction (c)

Look for changes in oxidation states; carbon is being oxidized from various states in glucose to +4 in \(\mathrm{CO}_{2}\). Oxygen in \(\mathrm{O}_{2}\) is reduced to -2 in both \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2}\mathrm{O}\). Hydrogen remains at +1 throughout the reaction.

Identify Oxidation States for Reaction (d)

For the reaction \(\mathrm{C}_{2} \mathrm{H}_{4}(g) + \mathrm{Cl}_{2}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{4} \mathrm{Cl}_{2}(g)\), typi_individual oxidation numbers within ethylene (\mathrm{C}_{2} \mathrm{H}_{4}) can be challenging to assign, but overall, the carbons are at a lower oxidation state than when they bind to chlorine. Chlorine in \(\mathrm{Cl}_{2}\) is at 0.

Identify the Redox Changes for Reaction (d)

Carbons in ethylene are oxidized as they form bonds with two chlorines, each giving the carbon a partial positive charge. Chlorine is reduced from 0 to -1 as it forms \(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{Cl}_{2}\).

Key concepts

Oxidation-Reduction (Redox) Chemistry

Redox reactions are fundamental chemical processes where the oxidation states of atoms are changed. These reactions involve the transfer of electrons between species; oxidation is the loss of electrons, while reduction is the gain of electrons. It's essential to understand that in a redox process, one species is oxidized and another is reduced - they occur simultaneously. These reactions are vital in fields like electrochemistry, metabolism, and industrial processes.

For example, in a simple redox reaction, a metal that loses electrons (oxidation) can transfer them to a non-metal that gains those electrons (reduction). Recognizing redox reactions is crucial in predicting the behavior of chemical systems and in balancing chemical equations.

Assigning Oxidation Numbers

Oxidation numbers or states are assigned to the elements in a compound or molecule to help keep track of electron movement during chemical reactions. These numbers are based on a set of rules, such as oxygen usually having an oxidation state of -2, hydrogen +1, and elements in their elemental form having an oxidation state of 0. Assigning these numbers is a critical step in analyzing redox reactions.

To assign oxidation numbers, we also consider electronegativity, ionic character of bonds, and the overall charge of a molecule. In a completed reaction, we can compare the oxidation states of atoms before and after the reaction to identify which atoms have been oxidized or reduced.

Chemical Reaction Analysis

Analyzing chemical reactions involves identifying reactants, products, and the types of reactions occurring. In redox chemistry, a detailed analysis allows us to see which electrons are transferred and how the oxidation states of atoms change. This information is critical to balance redox reactions, where the number of electrons lost must equal the number gained.

In the redox reactions provided, for instance, we observed changes in oxidation states of carbon and hydrogen among other elements, which led to the determination of which species were being oxidized or reduced. This kind of analysis is not only necessary for academic understanding but also for practical applications like corrosion prevention, battery design, and chemical synthesis.

Identification of Oxidizing and Reducing Agents

Identifying the oxidizing and reducing agents is crucial in a redox reaction. The oxidizing agent is the substance that gets reduced (gains electrons) and in doing so, it causes another species to be oxidized. Conversely, the reducing agent is the substance that gets oxidized (loses electrons) and thereby reduces another species. These agents are key players in driving redox reactions forward.

In the given examples, we see that when carbon goes from an oxidation state of -4 to +4, as in reaction (a), it is the reducing agent as it loses electrons. The hydrogen, which goes from +1 to 0, is being reduced and is thus the oxidizing agent.