Question

Reduction was once defined as chemically adding hydrogen to a substance. Use this reaction to argue that this definition is consistent with the modern definition of reduction. $$ \mathrm{C}_{2} \mathrm{H}_{2}+2 \mathrm{H}_{2} \rightarrow \mathrm{C}_{2} \mathrm{H}_{6} $$

Short answer

Adding hydrogen aligns with modern reduction as it indicates electron gain, lowering oxidation state.

Step-by-step solution

Understanding the Old Definition

The old definition of reduction involves the chemical addition of hydrogen to a substance. In the given reaction, acetylene (\( \mathrm{C}_2 \mathrm{H}_2 \)) is combined with hydrogen (\( \mathrm{H}_2 \)) to form ethane (\( \mathrm{C}_2 \mathrm{H}_6 \)). According to the old definition, this reaction would be considered reduction because hydrogen is being added.

Exploring the Modern Definition

The modern definition of reduction refers to the gain of electrons by a substance during a chemical reaction. This definition is focused more on changes in oxidation states and electron transfer.

Oxidation State Analysis

To connect the modern definition to the old one, analyze the oxidation states of carbon in the equation: In \( \mathrm{C}_2 \mathrm{H}_2 \), carbon has an oxidation state of -1. In \( \mathrm{C}_2 \mathrm{H}_6 \), carbon has an oxidation state of -3. Thus, carbon undergoes a reduction in its oxidation state from -1 to -3, indicating a gain of electrons.

Connection Between Definitions

The reduction in oxidation state (from -1 to -3) suggests a gain of electrons. Since adding hydrogen in terms of chemistry often involves adding electrons (as seen here through the lowering of oxidation state), the older definition of reduction, involving the addition of hydrogen, aligns with the modern definition, which involves the gain of electrons.

Key concepts

Oxidation States

Oxidation states, also known as oxidation numbers, are a way to keep track of electrons in chemical reactions. They can be quite helpful in understanding which atoms are gaining or losing electrons during these reactions. By assigning oxidation states, chemists can identify if a substance undergoes oxidation or reduction based on changes in these numbers.
In the reaction from acetylene ( C_2H_2 ) to ethane ( C_2H_6 ), the oxidation states of carbon shift from -1 to -3. A decrease in oxidation state indicates a gain of electrons, signaling that reduction has occurred. This concept of oxidation states is crucial in understanding redox reactions, as it provides a quantitative approach to tracking electron movement.

• The oxidation state is a theoretical charge on an atom.
• Lowering of oxidation states signals a reduction.
• Tracking these numbers helps identify electron transfer in reactions.

By mastering the concept of oxidation states, you will better grasp how substances are transformed in chemical reactions, and you will build a solid foundation for further studies in chemistry.

Electron Transfer

Electron transfer is the key process in redox (reduction-oxidation) reactions. It involves the movement of electrons from one chemical species to another. When an element gains electrons, it undergoes reduction. Conversely, when an element loses electrons, it undergoes oxidation.
In the acetylene to ethane reaction, a reduction happens as the molecule of carbon gains electrons. This electron gain makes the carbon move from an oxidation state of -1 to -3. As electrons are transferred to carbon, it enhances its electron cloud, showcasing how closely related electron gain is to reduction.

• Redox reactions include both oxidation and reduction.
• Electron transfer is the driving force behind these reactions.
• Understanding electron transfer provides insight into energy changes during reactions.

Knowing electron transfer helps you decipher complex reactions and predict the flow of electrons, connecting with broader topics in chemical energetics.

Hydrogen Addition

Adding hydrogen to a substance is a historic way to define reduction, and it remains important in modern chemistry. When hydrogen atoms are added, it typically results in a gain of electrons, reflecting the new, broader understanding of reduction through electron transfer.
In our focus reaction, acetylene ( C_2H_2 ) reacts with hydrogen to form ethane ( C_2H_6 ). Here, each added hydrogen atom brings an electron, completing carbon's valence shell and aligning with its lower oxidation state (-3). This demonstrates the classic concept of hydrogen addition aligning with modern reduction meanings.

• Adding hydrogen typically involves electron gain.
• Hydrogen addition aligns old and new reduction definitions.
• This concept is central to many biological and industrial processes.

By viewing hydrogen addition through the lens of electron transfer, you can relate traditional ideas to modern chemical principles, enhancing your understanding of fundamental chemistry concepts.