Question

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

Short answer

Although the reaction of glucose with oxygen is exergonic, it has a high activation energy barrier that prevents it from occurring spontaneously, allowing glucose to remain stable in an oxygen atmosphere.

Step-by-step solution

Understand Exergonic Reactions

An exergonic reaction is one in which energy is released. The given reaction of glucose with oxygen has a \( \Delta G^{\circ} \) value of -2880 kJ \( \mathrm{mol}^{-1} \), indicating it is highly exergonic.

Reaction Kinetics vs. Thermodynamics

Thermodynamics tells us whether a reaction is energetically favorable (exergonic or endergonic), but it doesn't provide information about the reaction speed. The reaction kinetics determines the reaction rate or speed.

Activation Energy Barrier

Despite being exergonic, the reaction may have a high activation energy barrier that prevents it from occurring spontaneously under normal conditions. The glucose molecule might not react without a sufficient energy input to surpass this barrier.

Stability in Oxygen Atmosphere

In an oxygen-containing atmosphere, the glucose can persist indefinitely because the ambient temperature and conditions aren't sufficient to overcome the activation energy barrier needed to start the reaction with oxygen spontaneously.

Key concepts

Exergonic Reactions

In an exergonic reaction, the process releases energy to its surroundings, making the reaction energetically favorable. This is often measured by the change in Gibbs free energy, denoted as \(\rightarrow \Delta G\). For a reaction to be exergonic, \(\rightarrow \Delta G\) must be negative. In the case of glucose oxidation, the \(\rightarrow \Delta G^{\text{\textdegree}}\) is -2880 kJ/mol, which is highly negative. This significant release of energy indicates that the reaction has a strong potential to occur and release energy. However, the term *exergonic* only tells us that the reaction can release energy, not whether it will happen quickly or at all without some initial input.Key Points:

• Negative \(\rightarrow \Delta G\) indicates an exergonic reaction
• Energy is released to the surroundings
• Does not imply reaction speed

Remember that *exergonic* only addresses the energy aspect, so we need to explore further elements to understand why glucose remains stable initially.

Thermodynamics vs. Kinetics

Thermodynamics and kinetics are two sides of the same coin but deal with different aspects of a chemical reaction. Thermodynamics tells us if a reaction *can* happen, while kinetics tells us *how fast* it proceeds. For the oxidation of glucose:Thermodynamics:Thermodynamics indicates that the reaction is highly favorable due to its strongly negative \(\rightarrow \Delta G^{\text{\textdegree}}\).Kinetics:Kinetics, on the other hand, focuses on the steps the molecules must take to reach the final products. In simpler terms, thermodynamics tells us the potential energy outcome, whereas kinetics measures the rate at which the reaction occurs.Key Points:

• Thermodynamics (* if the reaction is energetically favorable*)
• Kinetics (* how fast the reaction occurs*)

While glucose oxidation is thermodynamically favorable, its reaction kinetics might involve slow or unlikely steps under normal conditions.

Activation Energy Barrier

Even if a reaction is exergonic or thermodynamically favorable, an activation energy barrier can prevent it from occurring spontaneously. The activation energy is the minimum energy required to kick-start the reaction. Think of it as a hill that the molecules must get over before they can react.In the glucose oxidation reaction, the activation energy barrier is high. This barrier ensures that glucose does not spontaneously react with oxygen at room temperature. To overcome this barrier, either a catalyst or an increase in temperature would be necessary.Key Points:

• An activation energy barrier prevents spontaneous reactions despite being energetically favorable
• This barrier ensures that reactions do not happen until sufficient energy is provided
• Catalysts or increased temperatures can help overcome this barrier

Understanding the activation energy barrier helps us see why exergonic reactions like glucose oxidation might not occur instantly or without additional energy input.

Reaction Kinetics

Reaction kinetics examines the factors that affect the speed or rate of a chemical reaction. It is influenced by several conditions:Temperature:Higher temperatures generally increase the rate of reaction by providing more kinetic energy to the reacting molecules.Concentration of Reactants:Higher concentration means more reactant molecules which can collide and react, increasing the reaction rate.Catalysts:A catalyst can lower the activation energy required, thereby speeding up the reaction without being consumed in it.Surface Area:More surface area allows for more collisions and interactions between reactants, enhancing the reaction rate.In the context of glucose oxidation, these kinetic considerations explain why the reaction does not readily occur under standard conditions but can be accelerated with the appropriate catalysts or physical conditions.Key Points:

• Higher temperatures and concentrations generally increase the reaction rate
• Catalysts can lower the activation energy needed for the reaction

By understanding reaction kinetics, we see that the favorable energy change alone does not determine a reaction's occurrence; additional factors play crucial roles.