Question

Comment on the following statement: Exothermic reactions are spontaneous, but endothermic reactions are nonspontaneous.

Short answer

Spontaneity depends on Gibbs free energy, not just the reaction being exothermic or endothermic.

Step-by-step solution

Clarify Terms

Begin by defining key terms. An **exothermic reaction** releases energy to the surroundings, usually in the form of heat, making the enthalpy change negative. An **endothermic reaction** absorbs energy, making the enthalpy change positive. **Spontaneous reactions** occur without any external input of energy and are usually driven by an increase in entropy and decrease in free energy.

Analyze Exothermic Reactions

Consider the statement that exothermic reactions are spontaneous. While many exothermic reactions are spontaneous (because they release energy, lowering the system's energy), spontaneity is not guaranteed solely by being exothermic. The overall change in Gibbs free energy (\( \Delta G \)) must be negative for the reaction to be spontaneous, which also considers entropy changes.

Analyze Endothermic Reactions

Consider the statement that endothermic reactions are nonspontaneous. Some endothermic reactions are indeed spontaneous if they are accompanied by a significant increase in entropy. The Gibbs free energy (\( \Delta G \)) can be negative if the temperature multiplied by the change in entropy (\( T \Delta S \)) is larger in magnitude than the enthalpy change (\( \Delta H \)).

Link to Gibbs Free Energy

Explain the importance of Gibbs free energy in determining spontaneity. The formula \( \Delta G = \Delta H - T\Delta S \) captures the balance between enthalpy and entropy. A reaction is spontaneous if \( \Delta G < 0 \). This means both exothermic and endothermic reactions can be spontaneous, depending on the entropy change and the temperature.

Conclusion

Summarize the findings. The spontaneity of a reaction is not determined solely by being exothermic or endothermic. Instead, it depends on the Gibbs free energy change, which takes into account both enthalpy and entropy changes.

Key concepts

Gibbs Free Energy

In chemical thermodynamics, Gibbs free energy is a central concept. It helps in predicting whether a reaction will occur spontaneously. The Gibbs free energy change, denoted by \( \Delta G \), is calculated using the formula: \[ \Delta G = \Delta H - T\Delta S \] where:

• \( \Delta H \) is the change in enthalpy (heat content).
• \( T \) is the temperature in Kelvin.
• \( \Delta S \) is the change in entropy (disorder).

The reaction is considered spontaneous if \( \Delta G < 0 \). This means the reaction is capable of proceeding without needing input energy from the outside, under certain conditions. Thus, both enthalpy and entropy changes play a significant role in this determination.

Exothermic Reactions

Exothermic reactions are fascinating because they release energy into their surroundings. This energy release usually comes in the form of heat. When this happens, the system's enthalpy decreases, making \( \Delta H \) negative.
Common examples include combustion reactions, like burning wood or gasoline, where heat is released.While many people assume all exothermic reactions are spontaneous, this is not always true. The spontaneity of these reactions also depends on the Gibbs free energy. It's possible for an exothermic reaction to be non-spontaneous if it results in a significant decrease in entropy.

Endothermic Reactions

In contrast to exothermic reactions, endothermic reactions absorb energy from their surroundings. This absorption usually manifests as heat being taken in from the environment, resulting in a positive \( \Delta H \). An example is the process of photosynthesis, where plants absorb sunlight.
Often, endothermic reactions are assumed to be non-spontaneous. However, under the right conditions, they can indeed be spontaneous—especially when the entropy increase is large enough to make \( \Delta G \) negative.
Temperature also plays a critical role here. At high temperatures, the entropy term \( T\Delta S \) can dominate the equation, leading to a spontaneous reaction despite positive \( \Delta H \).

Spontaneity

Spontaneity in chemical reactions indicates the potential for a reaction to proceed without external aid. It's a misconception that all exothermic reactions are spontaneous, while endothermic ones aren't.
The Gibbs free energy change, \( \Delta G \), is crucial for assessing a reaction's spontaneity:

• If \( \Delta G < 0 \), the reaction is spontaneous.
• If \( \Delta G > 0 \), the reaction is non-spontaneous.
• If \( \Delta G = 0 \), the system is in equilibrium.

In essence, whether a reaction is spontaneous depends on the delicate balance between changes in enthalpy and entropy, as well as the temperature at which the reaction occurs.

Entropy

Entropy is a measure of the disorder or randomness in a system. Natural processes tend to move towards a state of higher entropy. A simple way to think about it is the tendency of energy to spread out or become more disordered over time.
In thermodynamics, entropy change (\( \Delta S \)) plays a crucial role in determining the spontaneity of a reaction, as embodied in the Gibbs free energy formula.For example, when ice melts into water, the molecules in the liquid state have more freedom and higher entropy than in the solid state.Significant changes in entropy can heavily influence whether an endothermic reaction is spontaneous. When \( \Delta S \) is large enough, and at sufficiently high temperatures, it might drive a reaction to be spontaneous even if it absorbs energy from its surroundings.