Problem 93 - Chemistry and Chemical Reactivity

Question

The Tollen's test for the presence of reducing sugars (say, in a urine sample) involves treating the sample with silver ions in aqueous ammonia. The result is the formation of a silver mirror within the reaction vessel if a reducing sugar is present. Using glucose, \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6},\) to illustrate this test, the oxidation-reduction reaction occurring is $$\begin{aligned} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{aq})+& 2 \mathrm{Ag}^{+}(\mathrm{aq})+2 \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{7}(\mathrm{aq})+2 \mathrm{Ag}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\ell) \end{aligned}$$ What has been oxidized, and what has been reduced? What is the oxidizing agent, and what is the reducing agent?

Step-by-Step Solution

Answer

Glucose is oxidized; Ag+ is reduced. Ag+ is the oxidizing agent; glucose is the reducing agent.

Step 1: Determine the substance oxidized

In the given reaction, we have glucose (\[\text{C}_6 \text{H}_{12} \text{O}_6\]) on the reactant side being converted into \[\text{C}_6 \text{H}_{12} \text{O}_7\] on the product side. An increase in oxidation state of carbon atoms in glucose indicates its oxidation during the process. Thus, glucose is oxidized to form \[\text{C}_6 \text{H}_{12} \text{O}_7\].

Step 2: Determine the substance reduced

On the reactant side, silver ions (\[\text{Ag}^+\]) are present, which are being converted to silver metal (\[\text{Ag}\]) on the product side. The reduction of silver ions involves a decrease in their oxidation state from +1 to 0. This indicates that silver ions are being reduced.

Step 3: Identify the oxidizing agent

The oxidizing agent is the substance that is reduced during the reaction. Here, silver ions (\[\text{Ag}^+\]) get reduced to silver metal (\[\text{Ag}\]). Therefore, silver ions are the oxidizing agent.

Step 4: Identify the reducing agent

The reducing agent is the substance that is oxidized in the reaction. In this case, glucose (\[\text{C}_6 \text{H}_{12} \text{O}_6\]) is oxidized to \[\text{C}_6 \text{H}_{12} \text{O}_7\]. Hence, glucose acts as the reducing agent.

Key Concepts

Oxidation-Reduction Reaction

The Tollen's test for detecting reducing sugars, such as glucose, is a classic example of an oxidation-reduction (redox) reaction. In a redox reaction, one substance loses electrons (oxidation) while another gains electrons (reduction). These two processes occur simultaneously. In the context of the reaction in Tollen's test, glucose undergoes oxidation. It transforms from \(\text{C}_6 \text{H}_{12} \text{O}_6\) to \(\text{C}_6 \text{H}_{12} \text{O}_7\) by gaining additional oxygen, which is a hallmark of oxidation.
Silver ions from silver nitrate, which is present in the reaction, are reduced from \(\text{Ag}^+\) to silver metal \(\text{Ag}\). This reduction involves the gain of electrons.
Understanding this electron transfer is key to grasping the concept of a redox reaction.

The Tollen's test visually demonstrates this effect, manifesting as the formation of a silver mirror on the inner surface of the reaction vessel.

Reducing Sugars

Reducing sugars, such as glucose, are capable of donating electrons to other molecules. This ability makes them ideal candidates for oxidation as they easily give up electrons, participating actively in redox reactions. In the Tollen's test, a reducing sugar like glucose reduces silver ions to silver metal by transferring electrons.

Some common reducing sugars include:

• Glucose
• Fructose
• Maltose
• Lactose

These sugars have free aldehyde or ketone groups, which facilitate the redox reactions essential for tests like Tollen's. Recognizing reducing sugars is important in biochemistry and various diagnostic tests, since their presence and concentration can indicate certain health conditions.

Oxidizing Agent

An oxidizing agent plays a crucial role in oxidizing another substance by accepting electrons from it. In the Tollen's test, the silver ions (\(\text{Ag}^+\)) serve as the oxidizing agent. They accept electrons from glucose, leading to their reduction into metallic silver (\(\text{Ag}\)).

This interaction demonstrates the function of an oxidizing agent in a chemical reaction.

• In donating electrons, a reducing agent itself gets oxidized.
• The silver ions gain these electrons as they are reduced to solid silver.

The reaction is easy to see, due to the deposit of shiny silver, vividly highlighting the role of the oxidizing agent.

Glucose Oxidation

During glucose oxidation, glucose acts as the reducing agent. It donates electrons to another molecule in the reaction, in this case, the silver ions. When glucose is oxidized, it transitions from its aldehyde form, \(\text{C}_6 \text{H}_{12} \text{O}_6\), to \(\text{C}_6 \text{H}_{12} \text{O}_7\), reflecting its oxidation state change.
This process involves the addition of oxygen. As electron exchange accompanies this action, glucose loses electrons, necessary for its conversion to an oxidized product.

Understanding the pathway of glucose oxidation is crucial in fields like metabolism and energy production, as these reactions are common in biological systems. Glucose oxidation forms part of a larger network of biochemical processes critical for life.