Question

For the reaction of glucose with oxygen to produce carbon dioxide and water,Glucose \(+6 \mathrm{O}_{2} \rightarrow 6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O}\).the \(\Delta G^{\circ}\) is \(-2880 \mathrm{kJ} \mathrm{mol}^{-1}\), a strongly exergonic reaction. However. a sample of glucose can be maintained indefinitely in an oxygencontaining atmosphere. Reconcile these two statements.

Short answer

The high activation energy prevents the reaction from occurring spontaneously at room temperature, ensuring glucose remains stable in oxygen.

Step-by-step solution

Understand \(\rightarrow\) the Reaction

The given reaction is the cellular respiration process where glucose \((\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6)\) reacts with oxygen \((\mathrm{O}_2)\) to form carbon dioxide \((\mathrm{CO}_2)\) and water \((\mathrm{H}_2\mathrm{O})\). The reaction is highly exergonic with a \( \Delta G^\circ = -2880 \mathrm{kJ}\mathrm{mol}^{-1}\), meaning it releases a significant amount of energy.

Identify the Nature of the Reaction

Despite being highly exergonic, the reaction between glucose and oxygen has a high activation energy. The activation energy is the energy barrier that must be overcome for the reaction to occur.

High Activation Energy's Effects

Because of the high activation energy, the reaction does not proceed spontaneously at room temperature. This means that glucose can remain unchanged in an oxygen-containing atmosphere at ambient conditions.

Role of Catalysts in Biological Systems

In biological systems, this reaction is catalyzed by enzymes that lower the activation energy, allowing the reaction to proceed rapidly at body temperature. In the absence of these enzymes, the high activation energy prevents the reaction.

Conclusion

The glucose sample remains stable in an oxygen-containing atmosphere because the high activation energy barrier prevents the reaction from occurring without a catalyst. This reconciles the fact that while the reaction is exergonic, it does not proceed spontaneously under normal conditions.

Key concepts

Activation Energy

Activation energy is the amount of energy needed to start a chemical reaction. Think of it as the hurdle you have to jump over for the reaction to begin. Even if a reaction is highly exergonic, like the conversion of glucose and oxygen to carbon dioxide and water, it won't start on its own if it has a high activation energy.
Without enough energy to overcome this barrier, the reaction remains inactive. This is precisely what happens when glucose remains stable in an oxygen-containing atmosphere; the activation energy is too high, so the reaction doesn't proceed.
To visualize this, imagine trying to push a large boulder over a hill (activation energy). Once you get it over the hill, it rolls down easily. But until then, pushing it uphill is a formidable challenge.

Exergonic Reactions

Exergonic reactions release energy. The reaction of glucose with oxygen to produce carbon dioxide and water is a perfect example. It has a \( \Delta G^\circ = -2880 \text{kJ mol}^{-1} \), meaning it releases a lot of energy. In simpler terms, after the reaction starts, it gives off energy that can be used by biological systems.
However, just being exergonic doesn't mean the reaction will spontaneously happen. This is because of the high activation energy barrier that must be overcome first. Therefore, while the reaction has the potential to release a lot of energy, it remains dormant without the necessary initial input of energy.

Enzyme Catalysis

Enzymes are biological catalysts that speed up chemical reactions. They work by lowering the activation energy, making it easier for the reaction to proceed. In the example of glucose and oxygen reacting to form carbon dioxide and water, enzymes play a critical role. They reduce the energy hurdle, allowing the reaction to occur rapidly at body temperature.
Without these enzymes, the high activation energy would prevent the reaction from proceeding at a noticeable rate, especially at room temperature. This is why your sample of glucose can remain unchanged even in an environment rich with oxygen.
The enzymes essentially provide an easier path for the reaction, like lowering the height of the hill the boulder needs to go over, making it much easier to push it over and allowing the reaction to proceed efficiently.