Consider the following hypothetical reactions: A → B ΔH = +30 kJ B → C ΔH = +60 kJ (b) Construct an enthalpy diagram for substances A, B, and C, and show how Hess's law applies.

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Identify the enthalpy changes for each reaction: A → B has ΔH = +30 kJ and B → C has ΔH = +60 kJ.

Understand that Hess's Law states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in.

To construct the enthalpy diagram, plot the enthalpy on the y-axis and the reaction progress on the x-axis.

Start with substance A at a certain enthalpy level, then draw an upward arrow to represent the enthalpy change of +30 kJ to reach substance B.

From substance B, draw another upward arrow to represent the enthalpy change of +60 kJ to reach substance C. The total enthalpy change from A to C should be the sum of the individual changes, illustrating Hess's Law.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy (ΔH)

Enthalpy is a thermodynamic quantity that represents the total heat content of a system. It is often expressed in kilojoules (kJ) and indicates the energy change during a chemical reaction at constant pressure. A positive ΔH value signifies an endothermic reaction, where heat is absorbed, while a negative ΔH indicates an exothermic reaction, where heat is released.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to complete the reaction. This principle allows for the calculation of enthalpy changes for complex reactions by summing the enthalpy changes of individual steps, making it a powerful tool in thermochemistry for determining reaction energies.

Enthalpy Diagram

An enthalpy diagram visually represents the enthalpy changes of reactants and products in a chemical reaction. It typically shows the relative energy levels of the substances involved, with the vertical axis representing enthalpy and the horizontal axis representing the progress of the reaction. This diagram helps illustrate the energy changes and the application of Hess's Law by showing how the total enthalpy change can be derived from the individual steps.

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