Question

Which of the following reactions shows reduction of water? (a) \(2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{Na} \rightarrow 2 \mathrm{NaOH}+\mathrm{H}_{2}\) (b) \(6 \mathrm{CO}_{2}+12 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{H}_{2} \mathrm{O}+6 \mathrm{O}_{2}\) (c) \(2 \mathrm{~F}_{2}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{H}^{+}+4 \mathrm{~F}^{-}+\mathrm{O}_{2}\) (d) \(\mathrm{P}_{4} \mathrm{O}_{10}+6 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{H}_{3} \mathrm{PO}_{4}\)

Short answer

None of the reactions provided shows the classical reduction of water, but some might consider reaction (d) as a non-typical example of water reduction.

Step-by-step solution

Understanding Reduction

Reduction in chemistry refers to the gain of electrons by an atom or molecule. In reactions involving water, we are looking for a process where water (H2O) gains electrons or hydrogen is added to it.

Analyzing Reaction (a)

In reaction (a), water reacts with sodium (Na) forming sodium hydroxide (NaOH) and hydrogen gas (H2). Here, the water is losing hydrogen in the form of H2 gas, not gaining electrons. Thus, no reduction of water occurs.

Analyzing Reaction (b)

Reaction (b) is the photosynthesis equation where carbon dioxide and water are converted into glucose, water, and oxygen. There is no net change in the oxidation state of water, so there's no reduction of water in this reaction.

Analyzing Reaction (c)

In reaction (c), water (H2O) is losing hydrogen ions (H+) and producing fluoride ions (F-) and oxygen (O2). This process involves the loss of hydrogen ions from water rather than the gain of electrons - this is an oxidation of water, not a reduction.

Analyzing Reaction (d)

In reaction (d), P4O10 reacts with water to form phosphoric acid (H3PO4). Here, water provides oxygen and hydrogen that are used to form the H3PO4. Although water is consuming electrons to form the bonds with phosphorus, this is more of a contribution to forming a new compound than a classic reduction process.

Identifying Reduction of Water

Reviewing all options, none of the reactions clearly shows the reduction of water in the classical sense, since none involve a direct gain of electrons by water molecules. However, some may argue that reaction (d) can be considered as involving a reduction since water molecules provide oxygen to form phosphoric acid, but this interpretation is not typical for reduction processes.

Key concepts

Redox Reactions

When exploring the fascinating world of chemistry, redox reactions are part and parcel of the core foundational concepts. The term 'redox' is a fusion of two processes: reduction and oxidation. A redox reaction involves the movement of electrons between two species. The substance that donates electrons undergoes oxidation, while the one that gains electrons is said to undergo reduction.

Understanding redox reactions is crucial as they occur in various domains, from industrial processes to biological systems. For instance, cellular respiration, which allows animals to convert food into energy, is a chain of redox reactions. Similarly, redox reactions are central to the functioning of batteries where electrons flow from one material to another, enabling the generation of an electric current.

In the context of our exercise, identifying the reduction of water means scouting for a process where water, comprised of H₂O molecules, embraces extra electrons or has an influx of hydrogen atoms, indicating a gain in electrons. Let's keep this in mind as we analyze each reaction.

Chemical Reactions

Broadening our horizon, chemical reactions are the bread and butter of chemistry. These are processes in which chemical bonds between atoms are broken and new ones are formed, resulting in one or more new substances. Chemical reactions can be classified into several types, including combination, decomposition, single-replacement, double-replacement, and combustion reactions.

The distinction among types of chemical reactions helps us predict the outcome of certain reactants under specific conditions. For instance, in our original question, we encounter a variety of reactions. Reaction (a) illustrates a single-replacement reaction, where sodium replaces hydrogen in water. Reaction (b) depicts a combination reaction, crucial for life, known as photosynthesis. Reaction (c) represents a decomposition reaction, and reaction (d) is akin to a combination reaction where phosphoric acid is formed.

Each reaction involves different elements and compounds coming together or breaking apart and gives us insight into the possible changes that might occur, including changes in oxidation states, which are essential in identifying redox reactions. It's through these transformations that we can decipher the overall changes taking place within a reaction, such as the reduction of a substance like water.

Oxidation State Changes

Diving deeper into the ocean of chemical reactions, let's consider oxidation state changes, which are at the core of understanding redox reactions. The oxidation state, or oxidation number, is a concept that helps chemists keep track of electrons in atoms and molecules. It's essentially a bookkeeping tool that reflects the degree of oxidation or reduction an atom has undergone when forming compounds.

In simple terms, if an atom's oxidation state decreases, it means it has gained electrons, hinting at a reduction. Conversely, an increase in oxidation state signifies a loss of electrons, indicating oxidation. When evaluating redox reactions, we meticulously examine how the oxidation states of different elements change as reactants turn into products.

To bring this concept to life, let's revisit our textbook example. Upon close analysis, we observe that none of the given reactions show a clear reduction (gain of electrons) of water. The oxidation state of oxygen in water remains at -2, while the oxidation state of hydrogen is +1, and there are no noteworthy electrons being accepted by water molecules in any of these reactions. Our quest for the reduction of water in these specific reactions proves futile as there is no obvious decrease in the oxidation state of water molecules, ensuring that our understanding of oxidation state changes is consistent and accurate.