Question

In the process of respiration $$\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{s})+6 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 6 \mathrm{CO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\ell)$$ which molecule undergoes oxidation? Which molecule undergoes reduction?

Short answer

Glucose (C6H12O6) undergoes oxidation; Oxygen (O2) undergoes reduction.

Step-by-step solution

Identify Oxidation and Reduction

Oxidation refers to the loss of electrons, and reduction means the gain of electrons. To identify these processes, analyze changes in oxidation states of involved elements.

Determine Oxidation States in Reactants

For glucose (\(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6\)), carbon is less electronegative than oxygen and hydrogen, typically bearing an oxidation state of 0 in glucose due to resonance. Oxygen (O2) has an oxidation state of 0 as it is in its elemental form.

Determine Oxidation States in Products

In carbon dioxide (\(6 \mathrm{CO}_{2}\)), carbon has an oxidation state of +4, and in water (\(6 \mathrm{H}_{2} \mathrm{O}\)), oxygen usually has a -2 oxidation state, and hydrogen a +1 oxidation state.

Analyze Changes in Oxidation States

In glucose, carbon changes from an oxidation state of 0 to +4 in carbon dioxide, meaning it undergoes oxidation (electron loss). Conversely, in oxygen, it changes from an oxidation state of 0 in \(\mathrm{O}_2\) to a -2 in water, indicating reduction (electron gain) occurs.

Key concepts

Oxidation

Oxidation is a fundamental chemical concept involving the loss of electrons from a substance. During oxidation, an element or compound increases its oxidation state. In simpler terms, the substance involved loses electrons, leading to an increase in energy. An easy way to remember oxidation is through the phrase: "LEO says GER"—where LEO means "Loss of Electrons is Oxidation."
In the respiration process of glucose, carbon atoms in glucose (\(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6\)) experience oxidation. This is because the carbon atoms change from an oxidation state of 0 in glucose to +4 in carbon dioxide (\(\mathrm{CO}_2\)). This change signifies that electrons have been removed from carbon, causing oxidation.

Reduction

Reduction is the companion process to oxidation, involving the gain of electrons by a molecule, atom, or ion. During reduction, the oxidation state of a substance decreases, meaning it gains electrons and potentially energy, which is an integral part of many chemical processes.
Let's refer back to the mnemonic "LEO says GER," where GER stands for "Gain of Electrons is Reduction." In the respiration equation involving glucose and oxygen, oxygen is the molecule that undergoes reduction. Initially, the oxygen molecule (\(\mathrm{O}_2\)) has an oxidation state of 0. During the chemical reaction, it becomes part of water (\(\mathrm{H}_2 \mathrm{O}\)), and its oxidation state shifts to -2, clearly indicating it has gained electrons, hence undergoing reduction.

Oxidation States

Oxidation states, or oxidation numbers, help us understand how electrons are shared between atoms in a molecule. They represent the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. This concept is a neat bookkeeping method to track electron transfer during reactions.
The sum of oxidation states in a neutral compound should be zero. Here's how it applies in respiration:

• In glucose (\(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6\)), carbon is generally considered to have an oxidation state of 0 due to how the molecule is structured.
• Oxygen in its diatomic elemental state (\(\mathrm{O}_2\)) naturally has an oxidation state of 0.
• In carbon dioxide (\(\mathrm{CO}_2\)), carbon changes to +4, signifying oxidation, while oxygen in water (\(\mathrm{H}_2 \mathrm{O}\)) assumes a -2 state, indicating reduction.

Understanding oxidation states is crucial when analyzing redox reactions like respiration.

Glucose

Glucose is a simple sugar and a key energy source for many living organisms. It plays an essential role in cellular respiration, providing the fuel necessary for cells to generate ATP (adenosine triphosphate). Glucose molecules are composed of carbon, hydrogen, and oxygen, represented by the molecular formula \(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6\).
In the respiration equation, glucose acts as a reactant undergoing oxidation. The carbon in glucose provides the electrons that are transferred to oxygen, forming carbon dioxide and water in the process.
Glucose’s role in oxidation is a pivotal step that leads to energy conversion in cells. Each glucose molecule becomes an important energy source through a series of reactions that are part of cellular respiration.

Respiration

Respiration is a biochemical process in which cells convert the energy stored in glucose into usable energy in the form of ATP. This process is crucial for life, as ATP powers many cellular activities.
During cellular respiration, glucose combines with oxygen to release carbon dioxide, water, and energy. This can be summed up by the chemical equation:

• \[\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\,(\mathrm{s}) + 6 \mathrm{O}_{2} \, (\mathrm{g}) \rightarrow 6 \mathrm{CO}_{2} \,(\mathrm{g}) + 6 \mathrm{H}_{2} \mathrm{O} \,(\ell) + \text{energy (ATP)}\]

Respiration involves both oxidation and reduction reactions. It efficiently harnesses the potential energy stored in glucose by systematically breaking it down with oxygen. As glucose undergoes oxidation, and oxygen undergoes reduction, the energy released is conserved in ATP, which cells use for work. Thus, respiration highlights the important interplay of redox reactions in biological systems.