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 [Section 4.4]

Short answer

The water-splitting reaction, \(\mathrm{H}_{2}\mathrm{O}(l) \longrightarrow \mathrm{H}_{2}(g) + \frac{1}{2}\mathrm{O}_{2}(g)\), is (c) a redox reaction, as it involves changes in the oxidation states of hydrogen and oxygen.

Step-by-step solution

Analyze the reactants and products of the reaction

The reactants of the reaction are: \[ \mathrm{H}_{2}\mathrm{O}(l) \] The products of the reaction are: \[ \mathrm{H}_{2}(g) + \frac{1}{2}\mathrm{O}_{2}(g) \] Now that we have the reactants and products, it is time to evaluate the reaction based on the given types.

Identify the type of the reaction as acid-base, metathesis, redox, or precipitation

We now analyze each reaction type: (a) An acid-base reaction is a reaction in which an acid reacts with a base, and the products are a salt and water. In this case, the only reactant is water. The reaction is not an acid-base reaction since there is no acid and base involved in the reaction. (b) A metathesis reaction (also called a double displacement reaction) involves the exchange of two ionic species between two compounds. In this case, there is only one compound, water, with no ions. Thus, this reaction is not a metathesis reaction. (c) A redox reaction is a reaction that involves the transfer of electrons. In the given reaction, hydrogen (H) is changing states from +1 in water (as part of the hydroxyl group) to 0 in the hydrogen gas molecule (\(\mathrm{H}_2\)). Oxygen (O) is also changing states, from -2 in the water to 0 in the oxygen gas molecule (\(\mathrm{O}_2\)). Due to these changes in the oxidation states, the water-splitting reaction is a redox reaction. (d) A precipitation reaction is a reaction in which two soluble salts react to form an insoluble salt (the precipitate) and a soluble salt. There are no salts involved in this reaction, so it is not a precipitation reaction. Therefore, based on our analysis, the water-splitting reaction is:

Answer

(c) a redox reaction

Key concepts

Oxidation States

Oxidation states are a fundamental concept in understanding redox reactions. They serve as a kind of book-keeping system that helps chemists determine the electron ownership of individual atoms within a molecule. In a substance, the oxidation state indicates whether atoms have gained, lost, or shared electrons compared to their elemental form.
In the water-splitting reaction

• Hydrogen starts with an oxidation state of +1 as a part of water (H₂O), and shifts to 0 in hydrogen gas (H₂).
• Oxygen starts with an oxidation state of -2 in water and shifts to 0 in molecular oxygen ( O₂).

This change denotes that oxidation and reduction are occurring, a hallmark characteristic of redox reactions. In oxidation, an atom's oxidation state increases, whereas in reduction, it decreases. Identifying the change in oxidation states is crucial for recognizing redox reactions.

Electron Transfer

Electron transfer is at the heart of redox reactions, driving the processes of oxidation and reduction. In the context of water-splitting, electrons are transferred between atoms, leading to shifts in oxidation states.
During the reaction:

• Hydrogen atoms gain electrons to form H₂, which is a reduction process.
• Conversely, oxygen atoms lose electrons forming O₂, indicating oxidation.

Understanding electron transfer is vital for comprehending why redox reactions, such as this one, are unique. Electrons move between atoms or molecules, causing one to be oxidized and the other to be reduced, which is why these processes are always happening simultaneously. Tracking electron movement enables predictions about the behavior of redox reactions in various chemical scenarios.

Chemical Reactions

Chemical reactions describe the transformation of compounds via the breaking and forming of chemical bonds. In the water-splitting reaction, water molecules are subjected to conditions that compel them to separate into hydrogen and oxygen gases.
Various chemical reactions have unique characteristics. While acid-base reactions involve proton transfers and metathesis reactions pertain to ionic exchanges, redox reactions stand out by involving the transfer of electrons. The water-splitting reaction exemplifies a redox reaction, as it involves electron exchange, but does not align with the characteristics of acid-base, metathesis, or precipitation reactions. By cleavage of O-H bonds and formation of H-H and O-O bonds, this reaction highlights the dynamic nature of chemical bonding and transformations within reactions.