Hess's law

Welcome to the sixth episode of the Thermochemistry course! Building upon our previous discussions of thermochemistry, enthalpy, entropy, heat capacity, and calorimetry, this episode focuses on Hess's Law. Hess's Law is a fundamental principle that allows us to calculate the enthalpy change for a reaction that may be difficult or impossible to measure directly. We will explore the concept that enthalpy is a state function, the core principle behind Hess's law, and learn how to manipulate and combine known enthalpy changes of other reactions to determine the enthalpy change of the target reaction. This episode is essential for mastering thermochemical calculations and predicting the energy changes in complex chemical processes. It will form the basis of your understanding for future lessons, such as Gibbs Free Energy.

Check your understanding

These are the same multiple-choice questions you will see in the Quiz section after you listen to the episode. Use them here to preview or review the answers.

What is the main principle of Hess's Law?

  1. The enthalpy change of a reaction depends on the pathway taken.
  2. The enthalpy change of a reaction is independent of the pathway taken.
  3. Entropy always decreases in a chemical reaction.
  4. Heat capacity is always constant.
  5. Calorimetry is inaccurate.
  6. Entropy is a state function.

What is a state function?

  1. A property that depends on the path taken to reach a specific state.
  2. A property that depends only on the current state of the system.
  3. A property that is always constant.
  4. A property that can only be measured in a calorimeter.
  5. A property related only to gases.
  6. A measurement of disorder.

If you reverse a chemical reaction, what happens to its enthalpy change (ΔH)?

  1. It remains the same.
  2. It becomes zero.
  3. Its sign changes.
  4. Its magnitude doubles.
  5. Its magnitude halves.
  6. It becomes undefined.

If you multiply the coefficients of a chemical reaction by a factor of 2, what happens to its ΔH?

  1. It remains the same.
  2. It is divided by 2.
  3. It is multiplied by 2.
  4. It is squared.
  5. It becomes zero.
  6. It's sign changes.

How can Hess's Law be used to calculate the enthalpy change of a reaction?

  1. By measuring the heat capacity of the reactants and products.
  2. By using a calorimeter to directly measure the heat flow.
  3. By combining known enthalpy changes of other reactions.
  4. By measuring the temperature change during the reaction.
  5. By calculating the entropy change.
  6. By using the Gibbs Free energy equation

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