Lesson Links. In this lesson, you will cover the topics of using Hess's Law to find the heat of reaction and the standard heat of formation.

Transcription

1 17.4 Lesson 17.4 Calculating Heats of Reaction Objectives Identify two ways that the heat of reaction can be determined when it cannot be directly measured. Lesson Links Ch. 17 Lab 35: Heats of Reaction Ch. 17 Lab Practicals: Thermochemistry: Specific Heat 17.4 Lesson Overview (PowerPoint file) Ch. 17 Core TR: Section 4 Review 17.4 Chemistry Tutorial: Calculating the Standard Heat of Reaction Chapter 17 Online Student Edition Study WB Chapter 17 Lesson 4 Overview/Materials Overview In this lesson, you will cover the topics of using Hess's Law to find the heat of reaction and the standard heat of formation. Classroom Materials Standard 1 Block 0.5 Standard Heats of Formation: candle, metric ruler, aluminum foil, balance, safety matches, clock or watch, temperature probe (optional) Standard There are no items. Chemistry & You Engage Instruct students to read the Chemistry & You feature and look at the accompanying Chapter 17 Online Student Edition page 1 of 6

2 photograph of a diamond on p Ask Given that a diamond is too valuable to be destroyed, is there a way to determine the heat of reaction without actually performing the reaction? (Yes. The heat of reaction might be determined indirectly by studying the enthalpy changes for related reactions. These reactions could involve less valuable forms of the structures found in graphite.) Activate Prior Knowledge Engage Have students recall how the overall enthalpy change for converting one mole of ice to water vapor can be calculated by summing the enthalpy changes for each step in the process. A similar strategy can be used for determining the enthalpy change for a chemical reaction. Hess's Law Explain 17.4 Kinetic Art: Hess's Law Provide the following analogy for enthalpy change: Figure Figure A group of hikers is attempting to reach the summit of a mountain. Two routes are possible: one is a short, steep path that goes directly to the top; the other is a long path that is less steep but involves a number of changes in direction. Although the distances of the two routes are different, the final altitude is the same. Explain that the change in altitude is analogous to the enthalpy change for a reaction. The distances traveled by the hikers on each path are analogous to the number of steps required to form a given set of products. No matter how one goes from a given set of reactants to a given set of products, in one step or in several, the overall enthalpy change is the same. Describe that this is the process used to calculate the heat of reaction when it cannot be directly measured. page 2 of 6

3 Display 17.4 Kinetic Art: Hess's Law on PearsonChem.com and then have the students work through the questions on their own or with a partner. Have students examine Figure Explain Figure and Figure and read the captions aloud. Provide students with their own copy of the diagrams. Encourage students to take notes directly on the diagrams. Assign students to compare the diagrams and to create a summary paragraph explaining what they have learned. Ask What is the enthalpy change of the reverse reaction-from graphite to diamond? Why? (ΔH = 1.9 kj, because the enthalpy change for the reverse reaction is equal to the negative ΔH of the forward reaction. The negative of 1.9 kj is 1.9 kj.) Ask How can you tell from Figure that the formation of CO is exothermic? (The arrow points downward and ΔH is negative.) Standard Heats of Formation Explain Display Table Define standard heat of formation as the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states at 25 C and kpa. Point out that the standard heat of formation of a free element in its standard state is defined as 0. Demonstrate how to use standard heats of formation to calculate heats of reaction at standard conditions using Sample Problem 17.8 on p Have the students work through 17.4 Chemistry Tutorial: Calculating the Standard Heat of Reaction on PearsonChem.com for additional practice. Chapter 17 Online Student Edition 17.4 Chemistry Tutorial: Calculating the Standard Heat of Reaction Table 17.4 Figure Have students study Figure Ask Does water have a lower or higher enthalpy than the elements from which it is formed? (lower) Ask On what other basis can you account for your answer? (It takes energy to decompose water.) page 3 of 6

4 Standard Heats of Formation Extend In this thermite reaction, iron (III) oxide is reduced to metallic iron by aluminum. Fe2O3(s) + 2Al(s) 2Fe(s) + Al2O3(s) ΔH= kj The reaction reaches a temperature of about 3000 C. Ask students to research the resulting products of this reaction and describe the condition of each. (mpfe=1530 C; mpsoln=2030 C). (The products are iron and aluminum oxide, and are molten.) Standard Heats of Formation Explore Have the students work together to complete Ch. 17 Probeware Lab: Small- Scale Lab: Heat of Combustion of a Candle to practice executing an open ended experiment. Have them design extension experiments as directed in the lab. Materials: candle, metric ruler, aluminum foil, balance, safety matches, clock or watch, temperature probe (optional) Assess and Remediate Evaluate Have students explain two ways that scientists can determine the heat of a reaction indirectly. Encourage students to use examples, symbols, and/or diagrams in their explanations. (1. Add two or more thermochemical equations to give the desired equation. 2. Find the difference between the standard heats of formation for all reactants and the standard heats of formation for all products in the reaction.) Chapter 17 Online Student Edition Table 17.4 Then, have the students complete the 17.4 Lesson Check. page 4 of 6

5 Remediate Write an overall reaction on the board, using compounds found in Table Challenge students to identify the reaction steps, if any, needed to account for the enthalpy change. Then, have the students use Hess's law of heat summation and the standard heats of formation to indirectly determine the enthalpy change for the reaction. Chapter 17 Review Choose from the chapter review and assessment resources for Chapter 17: Thermochemistry listed at the right. Ch. 17 Problem Set Ch. 17 Core TR: Practice Problems Ch. 17 Core TR: Interpreting Graphics Ch. 17 Core TR: Vocabulary Review Ch. 17 Core TR: Chapter Quiz Ch. 17 Core TR: Chapter Test A Ch. 17 Core TR: Chapter Test B Differentiated Instruction Special Needs Students When assessing students, have them give a thumbs up or a nod when they want to keep an enthalpy value the same, and a thumbs down or a head shake to indicate that they want to change the sign on the enthalpy value. Study WB Chapter 17 Lesson 4 Ch. 17 Core TR: Section 4 Review Advanced Students Tell students to prepare a 10 minute lesson on Hess's Law directed to struggling students. Tell students to use an analogy, such as moving a ball down a football field or soccer field, to visually explain the concept. If appropriate, have them present their lesson to small groups of students. Struggling Students If students are having trouble visualizing the page 5 of 6

6 application of Hess's Law, give them the following simplified equations Show students how 2 and 3 can be cancelled from both sides of the equations. The two equations can then be added together to produce Focus on ELL Frontload the Lesson Ask each student to draw a map from their house to the school using two different routes. Have each student present the two routes and explain the pros and cons of each route. Relate this to the two methods of determining the heats of reaction in the lesson. Study WB Chapter 17 Lesson 4 Ch. 17 Core TR: Section 4 Review My Notes Homework page 6 of 6