How to Calculate Enthalpy Changes Using Hess's Law

TL;DR

To calculate the enthalpy change of a reaction using Hess's Law, manipulate existing reactions by reversing and adjusting their coefficients. Focus on unique reactants and products that only appear once. Combine these adjusted reactions to find the total enthalpy change for the desired reaction.

Transcript

so this video is going to be all about hess's law and how we can calculate the enthalpy change of a reaction using it so consider this reaction nitrogen gas reacts with hydrogen gas to produce ammonia and the enthalpy change for that reaction is going to be negative 92 kilojoules and then we have another reaction water decomposes to hydrogen gas an... Read More

Key Insights

• 👻 Hess's Law allows us to calculate the enthalpy change of a reaction by manipulating and combining other reactions.
• ❓ Focusing on reactants and products that appear only once simplifies the calculations.
• 💱 Reversing reactions changes the sign of the enthalpy change, and coefficients are applied to both the reactions and the enthalpy changes.

Questions & Answers

Q: What is Hess's Law?

Hess's Law states that the enthalpy change of a reaction can be calculated by manipulating and combining the enthalpy changes of other reactions.

Q: How do we choose which species to focus on in the reactions?

We focus on the reactants and products that appear only once in the reactions, as they simplify the calculations. Species found in multiple reactions should be avoided.

Q: How do we adjust the reactions to get the desired reaction?

We may need to reverse reactions and adjust coefficients to ensure the desired reactants and products are on the correct side. Reversing a reaction changes the sign of the enthalpy change, and coefficients must also be applied to the enthalpy change.

Q: How do we calculate the enthalpy change of the desired reaction?

We add the adjusted reactions together and sum up the enthalpy changes. The resulting enthalpy change is the enthalpy change of the desired reaction.

Summary & Key Takeaways

• Hess's Law allows us to calculate the enthalpy change of a reaction by combining the enthalpy changes of other reactions.

• We focus on the reactants and products that appear only once in the reactions to simplify the calculations.

• By adjusting and combining the reactions, we can determine the enthalpy change of the desired reaction.