Problem 9 - Chemistry The Central Science In Si Units

Question

What kind of reaction is the "water-splitting" reaction? \(\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2}(g)+1 / 2 \mathrm{O}_{2}(g)\) (a) an acid-base reaction (b) a metathesis reaction (c) a redox reaction (d) a precipitation reaction

Step-by-Step Solution

Answer

The water-splitting reaction is a redox reaction (option c).

Step 1: Understand the Reaction Context

The given reaction is \( \text{H}_2\text{O}(l) \rightarrow \text{H}_2(g) + \frac{1}{2}\text{O}_2(g) \). This involves the decomposition of water into hydrogen gas \( \text{H}_2 \) and oxygen gas \( \text{O}_2 \). It's important to identify what type of reaction this is.

Step 2: Analyze the Reaction Features

The reaction is breaking down water into its elemental components. This involves hydrogen atoms going from a bonded state in water to a free molecular form, \( \text{H}_2 \), and the oxygen from a bonded state in water to molecular oxygen, \( \text{O}_2 \). This suggests the involvement of electron transfer, a sign of a redox reaction.

Step 3: Determine the Role of Electrons

In the reaction, electrons must be transferred to break the \( \text{O-H} \) bonds in water. This transfer is characteristic of oxidation-reduction (redox) processes: \( \text{H}_2\text{O} \) is oxidized to \( \text{O}_2 \), and \( \text{H}^+ \) ions are reduced to form \( \text{H}_2 \).

Step 4: Conclude the Type of Reaction

Given the electron exchange from water decomposing into \( \text{H}_2 \) and \( \text{O}_2 \), the reaction is classified as a redox reaction. The overall process involves oxidation of water and reduction of hydrogen ions.

Key Concepts

Electron Transfer

Electron transfer is a fundamental concept in chemistry, especially when discussing redox reactions. It involves the movement of electrons from one atom or molecule to another. In the context of the water-splitting reaction, this idea is crucial as it forms the basis of the reaction's classification.

In the given reaction, the transfer of electrons results in the breaking of the \( \text{O-H} \) bonds in water. As water breaks down, or decomposes, electrons are transferred such that:

• The oxygen atoms in water lose electrons, which is termed oxidation.
• The hydrogen ions gain electrons, being reduced to form hydrogen gas \( \text{H}_2 \).

Electron transfer plays a pivotal role in enabling water to split into hydrogen and oxygen gases. This movement of electrons is what defines the reaction as a redox process. Understanding electron transfer helps in visualizing how the atoms change bonds and states during the reaction.

Decomposition Reaction

A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. In the water-splitting reaction, the compound water \( \text{H}_2\text{O} \) breaks apart into hydrogen gas \( \text{H}_2 \) and oxygen gas \( \text{O}_2 \).

This reaction can be categorized as a decomposition because:

• The starting substance is one compound—water.
• The products are two distinct gases from the starting liquid form.

Decomposition reactions are generally driven by an input of energy which provides the necessary conditions for the compound to break apart. In practical applications, the energy for water splitting can come from electricity or perhaps solar energy. Recognizing water splitting as a decomposition reaction aids in understanding the energy flow and matter transformation occurring in this redox process.

Oxidation and Reduction

Oxidation and reduction, often referred to together as redox, are processes that occur simultaneously in many chemical reactions. In these reactions, the oxidation state of atoms changes due to the exchange of electrons.

In the case of the water-splitting reaction:

• Oxidation involves the increase in oxidation state by losing electrons. Here, oxygen in water is oxidized to form\( \text{O}_2 \), losing electrons in the process.
• Reduction involves the decrease in oxidation state by gaining electrons. Hydrogen ions \( \text{H}^+ \) in water gain electrons to form \( \text{H}_2 \).

It is important to note that in any redox reaction, the total number of electrons lost in oxidation equals the number of electrons gained in reduction. These paired processes illustrate the conservation of charge and meticulously balance the electron book-keeping, allowing for clear classification and understanding of such reactions as seen in the decomposition of water.