Gibbs free energy example | Thermodynamics | Chemistry | Khan Academy

TL;DR

This video explains how to determine the spontaneity of a chemical reaction by calculating the change in Gibbs free energy using the equation ΔG = ΔH - TΔS.

Transcript

I have this reaction here where if I had a mole of methane, and I react that with two moles of oxygen, I'll produce a mole of carbon dioxide and two moles of water. And what we want to answer in this video is whether this reaction is spontaneous. And we learned in the last video that to answer that question, we have to turn to Gibbs free energy, or... Read More

Key Insights

• 🥶 The change in Gibbs free energy (ΔG) is a reliable indicator of the spontaneity of a chemical reaction.
• ❎ A negative ΔH suggests that the reaction is exothermic and releases energy.
• 💱 The change in entropy (ΔS) can contribute to the spontaneity of a reaction, with positive ΔS favoring spontaneity.
• 🖐️ The temperature (T) also plays a role in determining the spontaneity of a reaction.
• 🦖 The calculation of ΔG requires knowledge of ΔH, ΔS, and T, which can be obtained from literature or databases.
• 🇦🇪 The units of ΔG, ΔH, and ΔS must be consistent (e.g., all in joules or all in kilojoules) for accurate calculations.
• ✋ The spontaneity of a reaction may change at different temperatures, with high temperatures potentially favoring non-spontaneity.

Questions & Answers

Q: How is the change in Gibbs free energy (ΔG) calculated?

ΔG is calculated by subtracting the enthalpy change (ΔH) from the temperature (T) multiplied by the change in entropy (ΔS).

Q: What factors determine if a reaction is spontaneous?

A reaction is spontaneous if the change in Gibbs free energy (ΔG) is less than zero.

Q: Why is the reaction in the video considered exothermic?

The reaction is considered exothermic because it releases energy, as indicated by a negative enthalpy change (ΔH).

Q: Does the change in entropy (ΔS) affect the spontaneity of a reaction?

Yes, the change in entropy (ΔS) can affect the spontaneity of a reaction. If ΔS is positive, it promotes spontaneity, while a negative ΔS opposes spontaneity.

Summary & Key Takeaways

• In order to determine if a reaction is spontaneous, the change in Gibbs free energy (ΔG) must be calculated.

• ΔG is calculated by subtracting the enthalpy change (ΔH) from the temperature (T) multiplied by the change in entropy (ΔS).

• If ΔG is less than zero, the reaction is spontaneous.