Chapter 7 Chemical Reactions - Wikispaces7... · · 2014-01-0710 Learning Check Identify the...

1

Chapter 7 Chemical

Reactions

7.1 Chemical Reactions

Copyright © 2008 by Pearson Education, Inc.

Publishing as Benjamin Cummings

2

Physical Change

In a physical change,

• The identity and

composition of the

substance do not

change.

• The state can

change or the

material can be torn

into smaller pieces.



3

Chemical Change

In a chemical change,

• Reacting

substances form

new substances

with different

compositions and

properties.

• A chemical

reaction takes

place.



4

Some Examples of Chemical

and Physical Changes

Table 7.1



5

Classify each of the following as a

1) physical change or 2) chemical

change

A. ____Burning a candle.

B. ____Ice melting on the street.

C. ____Toasting a marshmallow.

D. ____Cutting a pizza.

E. ____Polishing a silver bowl.

Learning Check

6

Classify each of the following as a

1) physical change or 2) chemical

change

A. 2 Burning a candle.

B. 1 Ice melting on the street.

C. 2 Toasting a marshmallow.

D. 1 Cutting a pizza.

E. 2 Polishing a silver bowl.

Solution

7

Chemical Reaction

In a chemical reaction

• A chemical change

produces one or

more new

substances.

• There is a change

in the composition

of one or more

substances.



8

Chemical Reaction

In a chemical reaction,

• Old bonds are broken

and new bonds are

formed.

• Atoms in the reactants

are rearranged to

form one or more

different substances.

• Fe and O2 form rust

(Fe2O3).



9

Evidence of a Chemical

Reaction

• Physical

changes that

can be seen

are evidence

of a chemical

reaction.

Table 7.2



10

Learning Check

Identify the visible evidence of a

chemical reaction in each of the

following:

1. Methane gas in an outdoor heater

burns with a blue flame.

2. Bleach removes stains from a shirt.

3. Bubbles of CO2 are released when

baking soda is mixed with vinegar.

11

Solution

Identify the visible evidence of a chemical reaction in each of the following:

1. Methane gas in an outdoor heater burns with a blue flame.

Heat and a flame

2. Bleach removes stains from a shirt.

Color change (color removed)

3. Bubbles of CO2 are released when baking soda is mixed with vinegar.

Formation of a gas (bubbles)

12

Chapter 7 Chemical

Reactions

7.2 Chemical Equations



13

Chemical Equations

A chemical equation

• Gives the chemical formulas of the reactants

on the left of an arrow and the products on the

right.

Reactants Product

C(s)

O2(g) CO2 (g)

Copyright © 2008 by Pearson Education,

Inc. Publishing as Benjamin Cummings

14

Chemical Equations are

Balanced In a balanced chemical reaction

• Atoms are not gained or lost.



15

Chemical Equations are

Balanced

In a balanced chemical reaction

• The number of reactant atoms is equal to the

number of product atoms.



16

Symbols Used in Equations

Symbols are used in

equations to show

• The states of the

reactants.

• The states of the

products.

• The reaction

conditions.



Table 7.2

17

A Balanced Chemical Equation

In a balanced chemical equation,

• There must be the same number of each type

of atom on the reactant and on the product

sides of a balanced equation.

• Numbers called coefficients are used in front of

one or more formulas.

Al + S Al2S3 Not Balanced

coefficients

2Al + 3S Al2S3 Balanced

2Al = 2Al

3S = 3S

18

Learning Check

State the number of atoms of each element on the

reactant and on the product sides of the equations:

A. P4(s) + 6Br2(l) → 4 PBr3(g)

B. 2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)

19

Solution

State the number of atoms of each element on the

reactant and on the product sides of the equations:

A. P4(s) + 6Br2(l) → 4PBr3(g)

4 P 4 P

12 Br 12 Br

B. 2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)

2 Al 2 Al

2 Fe 2 Fe

3 O 3 O

20

Learning Check

Determine if each equation is balanced or not.

A. Na(s) + N2(g) → NaN3(s)

B. C2H4(g) + H2O(l) → C2H5OH(l)

21

Solution

Determine if each equation is balanced or not.

A. Na(s) + N2(g) → NaN3(s)

No. 2N on reactant side, 3N on product side.

B. C2H4(g) + H2O(l) → C2H5OH(l)

Yes. 2 C = 2 C

6 H = 6 H

1 O = 1 O

22

Chapter 7

Chemical Reactions

7.3

Balancing a Chemical Equation



23



24

STEP 1 Write the equation with the correct formulas.

NH3(g) + O2(g) NO(g) + H2O(g)

STEP 2 Determine if the equation is balanced.

No, not all atoms are balanced.

STEP 3 Balance with coefficients in front of formulas.

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) STEP 4 Check that atoms of each element are equal in reactants and products.

4 N (4 x 1N) = 4 N (4 x 1N)

12 H (4 x 3H) = 12 H (6 x 2H)

10 O (5 x 2O) = 10 O (4O + 6O)

Balancing Chemical Equations

25

To balance the following equation,

Fe3O4(s) + H2(g) Fe(s) + H2O(l)

• Work on one element at a time.

• Use only coefficients in front of formulas.

• Do not change any subscripts.

Fe: Fe3O4(s) + H2(g) 3Fe(s) + H2O(l)

O: Fe3O4(s) + H2(g) 3Fe(s) + 4H2O(l)

H: Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

Steps in Balancing an Equation

26

Check the balance of atoms in the following:

Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

A. Number of H atoms in products.

1) 2 2) 4 3) 8

B. Number of O atoms in reactants.

1) 2 2) 4 3) 8

C. Number of Fe atoms in reactants.

1) 1 2) 3 3) 4

Learning Check

27

Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

A. Number of H atoms in products.

3) 8 (4H2O)

B. Number of O atoms in reactants.

2) 4 (Fe3O4)

C. Number of Fe atoms in reactants.

2) 3 (Fe3O4)

Solution

28

Equation for A Chemical

Reaction



29

Checking a Balanced Equation

Reactants Products

1 C atom = 1 C atom

4 H atoms = 4 H atoms

4 O atoms = 4 O atoms



30

Balance each equation and list the coefficients in the balanced equation going from reactants to products:

A. __Mg(s) + __N2(g) __Mg3N2(s)

1) 1, 3, 2 2) 3, 1, 2 3) 3, 1, 1

B. __Al(s) + __Cl2(g) __AlCl3(s)

1) 3, 3, 2 2) 1, 3, 1 3) 2, 3, 2

Learning Check

31

A. 3) 3, 1, 1

3Mg(s) + 1N2(g) 1Mg3N2(s)

B. 3) 2, 3, 2

2Al(s) + 3Cl2(g) 2AlCl3(s)

Solution

32

Equations with Polyatomic Ions



33

Balancing with Polyatomic Ions

MgCl2(aq) + Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)

• Balance PO43- as a unit

MgCl2(aq) + 2Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)

2PO43- = 2PO4

3-

• Balance Mg and Cl

3MgCl2(aq) + 2Na3PO4(aq) 6NaCl(aq) + Mg3(PO4)2(s)

3 Mg2+ = 3 Mg2+

6 Na+ = 6 Na+

6 Cl- = 6 Cl-

34

Balance and list the coefficients from reactants to products:

A. __Fe2O3(s) + __C(s) __Fe(s) + __CO2(g)

1) 2, 3, 2,3 2) 2, 3, 4, 3 3) 1, 1, 2, 3

B. __Al(s) + __FeO(s) __Fe(s) + __Al2O3(s)

1) 2, 3, 3, 1 2) 2, 1, 1, 1 3) 3, 3, 3, 1

C. __Al(s) + __H2SO4(aq) __Al2(SO4)3(aq) + __H2(g)

1) 3, 2, 1, 2 2) 2, 3, 1, 3 3) 2, 3, 2, 3

Learning Check

35

A. 2) 2, 3, 4, 3

2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g)

B. 1) 2, 3, 3, 1

2Al(s) + 3FeO(s) 3Fe(s) + 1Al2O3(s)

C. 2) 2, 3, 1, 3

2Al(s) + 3H2SO4(aq) 1Al2(SO4)3(aq) + 3H2(g)

Solution

36

Chapter 7 Chemical

Reactions

7.4

Types of Reactions



37

Type of Reactions

Chemical reactions can be classified as

• Combination reactions.

• Decomposition reactions.

• Single Replacement reactions.

• Double Replacement reactions.

• Combustion reactions.

38

Combination

In a combination reaction,

• Two or more elements form one product.

• Or simple compounds combine to form one

product.

+

2Mg(s) + O2(g) 2MgO(s)

2Na(s) + Cl2(g) 2NaCl(s)

SO3(g) + H2O(l) H2SO4(aq)

A B A B

39

Formation of MgO

Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

40

Decomposition

In a decomposition reaction,

• One substance splits into two or more simpler

substances by adding heat.

2HgO(s) 2Hg(l) + O2(g)

2KClO3(s) 2KCl(s) + 3O2(g)



41

Decomposition of HgO



42

Learning Check

Classify the following reactions as

1) combination or 2) decomposition:

___A. H2(g) + Br2(g) 2HBr(l)

___B. Al2(CO3)3(s) Al2O3(s) + 3CO2(g)

___C. 4Al(s) + 3C(s) Al4C3(s)

43

Solution


1) combination or 2) decomposition:

1 A. H2(g) + Br2(g) 2HBr(l)

2 B. Al2(CO3)3(s) Al2O3(s) + 3CO2(g)

1 C. 4Al(s) + 3C(s) Al4C3(s)

44

Single Replacement

In a single replacement reaction,

• One element takes the place of a different

element in a reacting compound.

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)



45

Zn and HCl is a Single

Replacement Reaction


46

Double Replacement

In a double replacement reaction,

• Two elements in the reactants exchange places.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)


47

Example of a Double

Replacement



48

Learning Check


1) single replacement 2) double replacement

__A. 2Al(s) + 3H2SO4(aq) Al2(SO4)3(s) + 3H2(g)

__B. Na2SO4(aq) + 2AgNO3(aq) Ag2SO4(s) + 2NaNO3(aq)

__C. 3C(s) + Fe2O3(s) 2Fe(s) + 3CO(g)

49

Solution



1 A. 2Al(s) + 3H2SO4(aq) Al2(SO4)3(s) + 3H2(g)

2 B. Na2SO4(aq) + 2AgNO3(aq) Ag2SO4(s) + 2NaNO3(aq)

1 C. 3C(s) + Fe2O3(s) 2Fe(s) + 3CO(g)

50

In a combustion reaction,

• A compound such as carbon reacts with oxygen, O2.

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

CH4(g) + 2O2(g) CO2 (g) + 2H2O(g)

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

• Fuels burned in oxygen produce CO2, H2O, and energy.

Combustion

51

Example of a Combustion

Reaction



52

Learning Check

Balance the combustion equation

___C5H12 + ___O2 ___CO2 + ___H2O

53

Solution

Balance the combustion equation

1 C5H12 + 8 O2 5 CO2 + 6 H2O

54

Summary of Reaction Types


Table 7.4

55

Learning Check

Identify each reaction as: 1) combination

2) decomposition 3) combustion


A. 3Ba(s) + N2(g) Ba3N2(s)

B. 2Ag(s) + H2S(aq) Ag2S(s) + H2(g)

C. SiO2(s) + 4HF(aq) SiF4(s) + 2H2O(l)

D. PbCl2(aq) + K2SO4(aq) 2KCl(aq) + PbSO4(s)

E. K2CO3(s) K2O(aq) + CO2(g)

F. C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

56

Solution

Identify each reaction as: 1) combination

2) decomposition 3) combustion


1 A. 3Ba(s) + N2(g) Ba3N2(s)

4 B. 2Ag(s) + H2S(aq) Ag2S(s) + H2(g)

5 C. SiO2(s) + 4HF(aq) SiF4(s) + 2H2O(l)

5 D. PbCl2(aq) + K2SO4(aq) 2KCl(aq) + PbSO4(s)

2 E. K2CO3(s) K2O(aq) + CO2(g)

3 F. C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(g)

57

Chapter 7

Chemical Reactions

7.5

Energy in Chemical Reactions



58

Heat of Reaction

The heat of reaction,

• Is the amount of heat absorbed or released during a reaction at constant pressure.

• Is the difference in the energy of the reactants and the products.

• Is shown as the symbol ΔH.

ΔH = Hproducts − Hreactants

59

Endothermic Reactions

In an endothermic reaction,

• Heat is absorbed.

• The sign of ΔH is +.

• The energy of the products is

greater than the energy of the

reactants.

• Heat is a reactant.

N2(g) + O2 (g) + 181 kJ 2NO(g)

ΔH = +181 kJ (heat added)



60

Exothermic Reactions

In an exothermic reaction,

• Heat is released.

• The sign of ΔH is -.

• The energy of the products is less than the energy of the reactants.

• Heat is a product.

C(s) + 2H2(g) CH4(g) + 75 kJ

ΔH = - 75 kJ (heat given off)



61

Summary

Reaction Energy Heat Heat of

Change Reaction

Endothermic Heat absorbed Reactant +ΔH

Exothermic Heat released Product ─ΔH

62

Learning Check

Identify each reaction as

1) exothermic or 2) endothermic.

A. N2 + 3H2 2NH3 + 92 kJ

B. CaCO3 + 556 kJ CaO + CO2

C. 2SO2 + O2 2SO3 + heat

63

Solution

Identify each reaction as

1) exothermic or 2) endothermic.

(1) A. N2 + 3H2 2NH3 + 92 kJ

(2) B. CaCO3 + 556 kJ CaO + CO2

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

64

Heat Calculations for

Reactions

In the reaction

N2(g) + O2 (g) 2NO(g) ΔH = 181 kJ

181 kJ are absorbed when 1 mol N2 and 1 mol O2

react to produce 2 mol NO.

N2(g) + O2 (g) + 181 kJ 2NO(g)

This can be written as conversion factors.

181 kJ 181 kJ 181 kJ

1 mol N2 1 mol O2 2 mol NO

65

Heat Calculations for

Reactions

N2(g) + O2 (g) + 181 kJ 2NO(g)

If 15.0 g NO are produced, how many kJ were

absorbed?

15.0 g NO x 1 mol NO x 181 kJ = 45.2 kJ

30.01 g NO 2 mol NO

66

Learning Check

How many grams of O2 react if 1280 kJ are released

in the following reaction?

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ

67

Solution

How many grams of O2 react if 1280 kJ are released

in the following reaction?

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ

1280 kJ x 2 mol O2 x 32.00 g O2 = 92. g O2

890 kJ 1 mol O2

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