GenChem2 Lesson 8: Spontaneous Change

Summary

In this General Chemistry 2 lesson, Alex Pascal explains the concepts of spontaneous and non-spontaneous reactions. He clarifies that spontaneity refers to whether a reaction occurs naturally, not its speed, using examples like cooling chocolate, combustion, and rusting. The lesson highlights the relationship between enthalpy and spontaneity, noting that exothermic reactions are generally spontaneous, while some endothermic reactions, like melting ice, can also be spontaneous. Pascal introduces the second law of thermodynamics and the concept of entropy, emphasizing that energy transfer increases disorder. The session concludes with criteria for spontaneity and how equilibrium constants relate to reaction favorability.

Takeaways

• 😀 Spontaneous processes are reactions that proceed on their own without external intervention, such as hot chocolate cooling down.
• 😀 Non-spontaneous processes require external energy to occur, like heating a cold cup of chocolate to make it hot again.
• 😀 The speed of a reaction does not determine spontaneity; some spontaneous reactions can be very slow, like graphite turning into diamonds.
• 😀 Combustion reactions, like burning paper or alcohol, are examples of spontaneous processes.
• 😀 All exothermic reactions, which release heat (negative enthalpy), are spontaneous reactions.
• 😀 Not all spontaneous reactions are exothermic; endothermic reactions, like ice melting, can also be spontaneous.
• 😀 The second law of thermodynamics states that energy transformations lead to more disorder, or entropy, in the universe.
• 😀 Entropy represents the measure of disorder and is associated with energy that can no longer perform work.
• 😀 Spontaneous reactions generally favor the formation of products, while non-spontaneous reactions favor the reactants.
• 😀 For spontaneous reactions, the equilibrium constant (K) is usually greater than 1, indicating product formation is favored.
• 😀 Non-spontaneous reactions typically have a positive enthalpy (absorb heat) and negative entropy, making them less likely to occur without external influence.
• 😀 Understanding spontaneity involves considering both enthalpy (ΔH) and entropy (ΔS) of a system, not just reaction speed.

Q & A

What is the definition of a spontaneous process?

A spontaneous process is a reaction or process that occurs naturally on its own, without any external intervention or effort to drive it.

How does a non-spontaneous process differ from a spontaneous one?

A non-spontaneous process does not occur naturally and requires external energy or intervention to proceed, unlike a spontaneous process which happens on its own.

Does spontaneity indicate how fast a reaction occurs?

No, spontaneity only indicates whether a reaction will occur, not the speed. Spontaneous reactions can be very fast, like combustion, or extremely slow, like graphite turning into diamonds.

What is the relationship between exothermic reactions and spontaneity?

Most exothermic reactions (releasing heat, ΔH < 0) are spontaneous, as they naturally favor the formation of products due to energy release.

Can endothermic reactions be spontaneous?

Yes, some endothermic reactions (absorbing heat, ΔH > 0) can still be spontaneous if they lead to an increase in disorder (entropy) in the system, such as ice melting at room temperature.

What does the second law of thermodynamics say about spontaneous processes?

The second law of thermodynamics states that as energy is transferred or transformed, more of it becomes wasted, and isolated systems naturally tend toward a more disordered state, which is associated with spontaneous processes.

How is entropy related to spontaneity?

Entropy measures the disorder or randomness in a system. Spontaneous reactions generally increase entropy (ΔS > 0), meaning the system becomes more disordered over time.

Provide examples of spontaneous processes.

Examples include: a hot cup of coffee cooling down, combustion of paper or wood, rusting of iron, and melting of ice at room temperature.

Provide examples of non-spontaneous processes.

Examples include: a cold cup of chocolate heating up on its own, reversing combustion, and reversing rust formation without energy input.

How is the equilibrium constant (K) related to spontaneity?

For spontaneous reactions, the equilibrium constant K is usually greater than 1, indicating that the reaction favors product formation. For non-spontaneous reactions, K is less than 1, favoring reactants.

What is the difference between negative enthalpy and positive entropy in spontaneous reactions?

Negative enthalpy (ΔH < 0) indicates that the reaction releases heat, while positive entropy (ΔS > 0) indicates that the system becomes more disordered. Both factors contribute to making a reaction spontaneous.

Why does the rusting of iron take so long if it is spontaneous?

Spontaneity does not dictate reaction rate. Rusting is spontaneous but very slow due to kinetic barriers and low reaction speed, demonstrating that spontaneous reactions can vary greatly in how fast they occur.