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 barrier prevents the thermodynamically favorable reaction from proceeding at a noticeable rate.

Step-by-step solution

Understand Gibbs Free Energy \(\Delta G^{\circ}\)

Given that the reaction has a \(\Delta G^{\circ}\) of \(-2880 \mathrm{kJ} \mathrm{mol}^{-1}\), it indicates that the reaction is strongly exergonic. This means it releases a significant amount of energy and is thermodynamically favorable.

Reaction Kinetics vs. Thermodynamics

Thermodynamics tells us whether a reaction is energetically favorable, but it does not provide any information about the rate at which the reaction occurs. Kinetics, on the other hand, deals with the rate of reactions.

Activation Energy Barrier

Even though the reaction is exergonic, it may have a high activation energy barrier. The activation energy is the energy required to initiate the reaction. If the activation energy is high, the reaction will proceed very slowly, or not at all, under normal conditions.

Stability of Glucose in the Atmosphere

A sample of glucose can be maintained indefinitely in an oxygen-containing atmosphere because, despite the reaction being thermodynamically favorable, the high activation energy prevents the reaction from occurring at a noticeable rate. Hence, glucose does not spontaneously react with oxygen at a significant rate under standard conditions.

Key concepts

exergonic reactions

Exergonic reactions are chemical reactions where the change in Gibbs Free Energy (ΔG) is negative. These reactions release energy to the surroundings. In the case of glucose reacting with oxygen, the ΔG is -2880 kJ/mol, indicating a strongly exergonic reaction.

Such reactions are thermodynamically favorable. The release of energy helps in driving the reaction forward. Therefore, they occur spontaneously under the right conditions. Despite this, other factors like kinetics can influence the actual rate of these reactions.

thermodynamics

Thermodynamics helps us understand whether a reaction is energetically favorable or not. It primarily deals with parameters like Gibbs Free Energy (ΔG), enthalpy (ΔH), and entropy (ΔS). For our reaction, the significantly negative ΔG indicates a spontaneous and favorable reaction from a thermodynamic standpoint.

However, it's crucial to remember that thermodynamics does not give us information about the speed of the reaction. Even if a reaction is thermodynamically favorable, other factors need to be considered to determine if or how fast it will proceed.

reaction kinetics

Reaction kinetics is the study of the speed or rate at which chemical reactions occur. Unlike thermodynamics, which tells us if a reaction is favorable, kinetics answers the question of how quickly the reaction proceeds.

In our example, glucose and oxygen have a very slow reaction rate under normal conditions. This slow rate is due to a phenomenon known as activation energy. Therefore, despite the reaction being exergonic, kinetics control if and when it will happen.

activation energy

Every reaction needs a certain amount of energy to get started. This is called the activation energy (Ea). Activation energy is the barrier that reactants must overcome to transform into products.

Even though the reaction between glucose and oxygen is thermodynamically favorable, it has a high activation energy. This high barrier prevents the reaction from occurring rapidly or spontaneously under normal conditions. Hence, glucose remains stable in an oxygen-containing atmosphere.