Intro to Chapter 5 Quiz (5.1, 5.2 and 5.3)

5.4 hess’s law Learning Objective:

Think of this situation: Person A takes the stairs to get to the 5th floor, while person B takes the elevator. At the end, both of them has reached the same floor, even though the path they took to get there was different.

Hess’s Law The enthalpy change for the conversion of reactants to products is the same whether the conversion occurs in one step or several steps.

Why do we need Hess’s Law? Hess’s Law is very useful for studying energy changes in chemical reactions that cannot be analyzed using calorimetry. Some reactions are too fast or dangerous, while others are too slow. So different reactions are taken that give the same results to calculate the enthalpy change.

Rules for Enthalpy Change To use Hess’s Law to calculate enthalpy changes for chemical reactions, one must apply the two rules: If you reverse a chemical reaction, you must also reverse the sign of ΔH For example: 𝑋𝑒+2 𝐹 2 → 𝑋𝑒𝐹 4 ΔH = -251 kJ If we reverse it, 𝑋𝑒𝐹 4  𝑋𝑒+2 𝐹 2 ΔH = 251 kJ 2. The magnitude of ΔH is directly proportional to the number of moles of reactants and products in a reaction. If the coefficients in a balanced equation are multiplied by a factor, the value for ΔH is multiplied by the same factor. For example, the enthalpy change for 1 mol of xenon gas and 2 moles of fluorine gas is -251 kJ. If we double the number of moles, the enthalpy change becomes -502 kJ.

Example Sample Problem 1 Copy down the problem from the board!

Task Do the following questions: sample problem 2 on page 316 practice questions 1-3 on page 317 questions 4, 5, 6, 7 and 8 on page 318