Hess Law Chemistry Problems - Enthalpy Change - Constant Heat of Summation

Summary

This video focuses on solving problems related to Hess's Law, a principle in thermodynamics. It explains how to determine entropy changes in chemical reactions by manipulating and combining multiple reactions. The process involves reversing, multiplying, and adding reactions to find the overall entropy change for a new reaction. The video walks through multiple examples, demonstrating how to adjust reactions to match the desired equation and then summing the entropy changes accordingly. Key concepts include how to modify reactions and apply Hess's Law for accurate predictions in thermodynamic calculations.

Takeaways

• 😀 Hess's law allows us to calculate the enthalpy change of a reaction by adding the enthalpy changes of multiple reactions.
• 😀 If a reaction is multiplied by a factor, the enthalpy change must also be multiplied by the same factor.
• 😀 If a reaction is reversed, the sign of the enthalpy change also changes.
• 😀 When combining reactions, cancel substances that appear on both sides to simplify the final equation.
• 😀 To calculate enthalpy changes for a desired reaction, modify the given reactions by adjusting coefficients or reversing them as needed.
• 😀 In practice problems, focus on substances that appear in only one equation at a time to simplify the process.
• 😀 If you need to adjust a reaction, you may multiply it by a factor or reverse it, and this will change the enthalpy change accordingly.
• 😀 The enthalpy change for a combined reaction is the sum of the adjusted enthalpy changes from the individual reactions.
• 😀 When working with multiple reactions, avoid focusing on substances that appear in more than one reaction, as they can cancel out or confuse the process.
• 😀 Hess's law can be applied to a variety of reactions, including decompositions and combinations of compounds, to predict enthalpy changes effectively.

Q & A

What is the main idea behind Hess's Law?

The main idea behind Hess's Law is that the entropy change for a reaction can be found by adding the entropy changes of individual reactions that lead to the overall reaction.

How do you modify the entropy change when you multiply a reaction by a factor?

When you multiply a reaction by a factor, you must also multiply the entropy change by the same factor. For example, if you multiply the reaction by 2, the entropy change is multiplied by 2.

What happens to the sign of the entropy change when you reverse a reaction?

When you reverse a reaction, the sign of the entropy change is reversed. A positive entropy change becomes negative, and vice versa.

How do you use Hess's Law to calculate the entropy change for a new reaction?

To calculate the entropy change for a new reaction using Hess's Law, you adjust the given reactions so that when added, they match the new reaction. The entropy change of the new reaction is the sum of the modified entropy changes of the individual reactions.

What should you focus on when adjusting reactions for Hess's Law?

When adjusting reactions for Hess's Law, focus on substances that are only present in one of the reactions, and avoid focusing on substances that appear in both the reactant and product sides of multiple reactions.

How do you handle a situation where a substance appears on both sides of multiple reactions?

In such cases, avoid focusing on those substances because they may cancel out during the addition of reactions, making it harder to track the overall entropy change.

In a practice problem, how do you handle reactions that need to be reversed?

When a reaction needs to be reversed, you reverse both the chemical equation and the sign of the entropy change. The magnitude of the entropy change remains the same, but its sign changes.

How can you calculate the total entropy change when combining multiple reactions?

After modifying the reactions, add their individual entropy changes together. The total entropy change for the new reaction is simply the sum of the adjusted entropy changes.

What should you do if a substance appears in the new reaction but not in the given reactions?

If a substance appears in the new reaction but not in the given reactions, focus on the substances that can be adjusted to match the required amount in the new reaction. You may need to multiply or divide the reactions accordingly.

What is the entropy change for the reaction where acetylene decomposes into elemental carbon and hydrogen gas?

The entropy change for this reaction is -226 J, as calculated from the modified reactions provided in the problem.