Worked example: Determining the effect of temperature on thermodynamic favorability | Khan Academy

TL;DR

Learn how to calculate the standard change in free energy for a chemical reaction at different temperatures.

Transcript

• [Lecturer] Let's do a work example where we calculate the standard change in free energy, delta G naught, for a chemical reaction. And for our reaction, let's look at the synthesis of ammonia gas from nitrogen gas and hydrogen gas. At 25 degrees Celsius, delta H naught for this reaction is equal to negative 92.2 kilojoules per mole of reaction. S... Read More

Key Insights

• 🇬🇸 The value of delta G naught for a chemical reaction is dependent on delta H naught, delta S naught, and temperature.
• 😘 At low temperatures, a negative delta H naught dominates, resulting in a negative delta G naught and a thermodynamically favorable reaction.
• 🥺 At high temperatures, a positive delta S naught dominates, leading to a positive delta G naught and a thermodynamically unfavorable reaction.
• 🇬🇸 The crossover temperature is the point where delta G naught is zero, indicating a balance between delta H naught and delta S naught.

Questions & Answers

Q: What is the significance of delta H naught being negative and delta S naught being negative?

When both delta H naught and delta S naught are negative, the thermodynamic favorability of the reaction depends on the temperature. If the temperature is low enough, like 298 K, the negative value of delta H naught outweighs the negative value of delta S naught, resulting in a negative delta G naught and a thermodynamically favorable reaction.

Q: How can delta S naught be converted from joules to kilojoules per kelvin mole of reaction?

To convert delta S naught from joules to kilojoules per kelvin mole of reaction, you can move the decimal place three places to the left. For example, if delta S naught is -0.1987 kJ/K mol, you would move the decimal to get -0.1987 kJ per K mol.

Q: What does it mean when delta G naught is positive?

A positive delta G naught indicates that the forward reaction is thermodynamically unfavorable. In the case of the synthesis of ammonia, at a higher temperature of 1000 K, delta G naught is positive (106.5 kJ/mol), signifying that the reverse reaction is favorable.

Q: How is the crossover point temperature determined?

The crossover point temperature, where delta G naught is zero, can be calculated by setting delta G naught equal to zero and solving for the temperature. In the case of this reaction, the temperature at the crossover point is 464 K.

Summary & Key Takeaways

• The video explains how to calculate the standard change in free energy (delta G naught) for the synthesis of ammonia gas using nitrogen gas and hydrogen gas.

• At 25 degrees Celsius, delta H naught is negative (-92.2 kJ/mol) and delta S naught is also negative (-198.7 J/K mol), indicating an exothermic reaction.

• By using the equation delta G naught = delta H naught - T delta S naught, where T is the temperature in Kelvin, the value for delta G naught can be calculated. At 298 K, delta G naught is negative (-33.0 kJ/mol), indicating a thermodynamically favorable forward reaction.