6.9 energy changes in chemical reactions

7/30/2019 6.9 Energy Changes in Chemical Reactions

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General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc. 1

Chapter 6 Chemical Reactions and

Quantities

6.9

Energy Changes in

Chemical Reactions


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Reaction Conditions

Reaction conditions for a chemical reaction require

collisions between reacting molecules

collisions with sufficient energy to break the bonds in

the reactants the breaking of bonds between atoms of the

reactants

the forming of new bonds to give products


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Chemical Reactions

In the reaction H2(g) + I2(g) 2HI(g),

the reactants H2 and I2 collide

the bonds of H2 and I2 break

the bonds for HI form

H2 + I2 collision bonds break HI

new bonds form


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Activation Energy

Activation energy

is the minimum energy required upon collision

for a reaction to take place


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Heat of Reaction

The heat of reaction

is the amount of heat

absorbed or released during

a reaction is the difference in the

energy of the reactants and

the products

Has the symbol

HH = Hproducts Hreactants


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Exothermic Reactions

In an exothermic reaction,

the energy of the products is less

than the energy of the reactants

heat of reaction is released heat is a product

C(s) + O2(g) CO2(g) + 394 kJ

H=394 kJ/mole (heat released)


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Endothermic Reactions

In an endothermic reaction,

heat is absorbed

the energy of the products is

greater than the energy of thereactants

heat is a reactant

N2(g) + O2(g) + 180 kJ 2NO(g)

H= 180 kJ (heat added)


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Summary

Reaction Energy Heat Sign of

Type Change in Reaction H

Endothermic Heat absorbed Reactant side +

Exothermic Heat released Product side


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Learning Check

Identify each of the following reactions as:

EX) exothermic or EN) endothermic

A. N2(g) + 3H2(g) 2NH3(g) + 22 kcal

B. CaCO3(s) + 133 kcal CaO(s) + CO2(g)

C. 2SO2(g) + O2(g) 2SO3(g) + heat


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Solution

EX A. N2(g) + 3H2(g) 2NH3(g) +22 kcal

EN B. CaCO3(s) +133 kcal CaO(s) + CO2(g)

EX C. 2SO2(g) + O2(g) 2SO3(g) + heat


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General, Organic, and Biological Chemistry Copyright 2010 Pearson Education, Inc.

Guide to Calculations Using Heat

of Reaction ( H)

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Heat Calculations for Reactions

In the reaction

N2(g) + O2(g) 2NO(g),H= 180 kJ.

If 15.0 g of NO are produced, how many kJ wereabsorbed?

STEP 1 Given: 15.0 g of NO produced

H = 180 kJ/2 moles of NO

Need: kJ absorbed

STEP 2 Plan: g of NO moles of NO kJ


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Heat Calculations for Reactions

(continued)

STEP 3 Write the conversion factors:

2 moles NO = 180 kJ

180 kJ and 2 moles NO

2 moles NO 180 kJ1 mole NO = 30.0 g of NO

1 mole NO and 30.0 g NO

30.0 g NO 1 mole NO

STEP 4 Set up the problem to calculate kJ:

15.0 g NO x 1 mole NO x 180 kJ = 45 kJ

30.0 g NO 2 moles NO


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Learning Check

How many grams of O2 reacted if 306 kcal are released

in the following reaction?

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + 213 kcal

1) 91.9 g of O2

2) 46.0 g of O2

3) 2.87 g of O2


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Solution

1) 91.9 g of O2

STEP 1 Given: 306 kcal released

H = 213 kcal /2 moles of O2

Need: g of O2 reactedSTEP 2 Plan: kcal moles of O2 g of O2

STEP 3 Write the conversion factors:

2 moles of O2 = 213 kcal

213 kcal and 2 moles O2

2 moles O2 213 kcal


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Solution (continued)

STEP 3 (continued)

1 mole of O2 = 32.0 g of O2

32.0 g O2 and 1 mole O2

1 mole O2 32.0 g O2

STEP 4 Set up the problem to calculate g of O2:

306 kcal x 2 moles O2 x 32.0 g O2 = 91.9 g of O2 (1)

213 kcal 1 mole O2

6.9 energy changes in chemical reactions

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