Lesson: Energy Changes in Reactions

In this lesson, we will learn how to calculate net change in bond energy during a reaction from the energies of bonds in the reactants and products.

Worksheet: 18 Questions

Q1:

Polytetrafluoroethylene (PTFE), a polymer widely used in lubricants and nonstick coatings, is produced from tetrafluoroethene:

C F F C F F

Tetrafluoroethene is synthesized via two reaction steps. In the first step, chloroform ( C H C l ) 3 reacts with hydrogen fluoride gas to form chlorodifluoromethane ( C H C l F ) 2 . In the second step, chlorodifluoromethane is converted to tetrafluoroethene. Both reactions generate hydrogen chloride gas as the only other product. The energies of selected bonds are shown in the table.

Bond H F H C l C H C F C C l C C C C C H
Bond Energy (kJ/mol) 565 428 411 485 327 346 602 411

Calculate the total change in bond energy when one mole of chloroform is converted to chlorodifluoromethane.

Calculate the total change in bond energy when one mole of chloroform is converted to tetrafluoroethene.

Q2:

Acrylonitrile ( C H N ) 3 3 is used in the production of synthetic rubbers and other polymeric materials. The main method for synthesizing acrylonitrile is to react propene gas ( C H ) 3 6 with ammonia and oxygen at high temperature. The bonds in these molecules have different energies, as indicated in the diagram. All labeled bond energies are in units of kilojoules per mole.

+ 6 02 4 11 3 46 6 4 59 2 + 4 94 3 + 3 86 2 6 02 4 11 2 8 87 O H H C C C H H H H H H O O H N H H C C C H N H H

Calculate the total change in bond energy for this reaction per mole of acrylonitrile produced.

Q3:

Ozone is a pungent pale blue gas involved in many important reactions in the atmosphere. It is generated reversibly from oxygen gas and reacts with nitric oxide ( N O ) to produce nitrogen dioxide ( N O ) 2 and nitrogen trioxide ( N O ) 3 . The bonds in these molecules have different energies, some of which are shown in the diagram. All bond energies are in units of kilojoules per mole.

3 79 6 26 4 66 4 94 N O N O O N O O O O O

For the reaction the total change in bond energy per mole of ozone reacted is 2 0 2 kJ/mol.

Calculate the total change in bond energy when one mole of nitrogen trioxide is produced from ozone and nitrogen dioxide.

Calculate the total change in bond energy when one mole of ozone is produced from oxygen gas.

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