Hess's Law Problems & Enthalpy Change - Chemistry | Summary and Q&A

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September 23, 2017
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The Organic Chemistry Tutor
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Hess's Law Problems & Enthalpy Change - Chemistry

TL;DR

Learn how to use Hess's Law to calculate the enthalpy change of a reaction by manipulating and combining the enthalpy changes of other reactions.

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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.

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