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Chapter 8. Chemical Reactions. Indications of a Chemical Reaction. Evolution of heat and light Production of a gas Formation of a precipitate Color change. All chemical reactions. have two parts Reactants - the substances you start with Products - the substances you end up with

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Chapter 8 l.jpg

Chapter 8

Chemical Reactions


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Indications of a Chemical Reaction

  • Evolution of heat and light

  • Production of a gas

  • Formation of a precipitate

  • Color change


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All chemical reactions

  • have two parts

  • Reactants - the substances you start with

  • Products- the substances you end up with

  • Reactants yield Products


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Characteristics of Chemical Equations

  • Represent the known facts

  • Contain correct formulas and symbols

  • Satisfy the law of conservation of mass (they must be balanced using coefficients)


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Word and Formula Equations

  • Copper reacts with oxygen to form copper (II) oxide.

  • word equation

  • Copper + oxygen ® copper (II) oxide

  • formula equation

  • Cu + O2® CuO

  • balanced chemical equation

  • 2Cu + O2® 2CuO


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Symbols Used In Chemical Equations

  •  yields

  • + and

  • (s) or (cr) solid

  • (g) gas

  • (l) liquid

  • (aq) aqueous

    solution


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Symbols Used In Chemical Equations

  • ­ gaseous product

  • ¯ precipitate

  • reversible reaction

  • reactants are heated

  • A catalyst used to change the rate of the reaction


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What is a catalyst?

  • A substance that speeds up a reaction without being changed by the reaction.

  • Enzymes are biological or protein catalysts.


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What is a reversible reaction?

  • A chemical reaction in which the products reform the original reactants.


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Write the formula equation

  • Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form solid iron (II) chloride and hydrogen sulfide gas.

  • Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.


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Write the word equations

  • Fe(s) + O2(g) ® Fe2O3(s)

  • Cu(s) + AgNO3(aq) ® Ag(s) + Cu(NO3)2(aq)

  • NO2 (g) N2(g) + O2(g)


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Balancing Chemical Equations

A step by step approach


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Balanced Equation

  • Atoms can’t be created or destroyed

  • All the atoms we start with we must end up with

  • A balanced equation has the same number of each element on both sides of the equation.


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®

O

+

C

C

O

O

O

  • Carbon + oxygen yields carbon dioxide

  • C + O2® CO2

  • This equation is already balanced


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®

O

+

C

C

O

O

  • Carbon + oxygen yields carbon monoxide

  • C + O2® CO

  • We need one more oxygen in the products.

  • Can’t change the formula


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C

O

®

O

+

C

O

  • Must be another CO

  • But where did the other C come from?

C

O


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C

C

O

®

O

+

O

  • Must have started with two C

  • 2 C + O2® 2 CO

C

O

C


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Rules for balancing

  • Write the correct formulas for all the reactants and products

  • Count the number of atoms of each type appearing on both sides

  • Balance the elements one at a time by adding coefficients (the numbers in front)

  • Balance polyatomic ions that appear on both sides of the equation as single units


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Always

  • Recount atoms as coefficients are added or changed

  • Reduce the coefficients to represent the smallest whole number ratio of reactants and products


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Never

  • Add, delete, or change a subscript to balance an equation.

  • Never put a coefficient in the middle of a formula


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Example

H2 +

O2

®

H2O

Make a table to keep track of atoms


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Example

H2 +

O2

®

H2O

2

H

2

2

O

1

Need twice as much O in the product


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Example

H2 +

O2

®

2

H2O

2

H

2

2

O

1

Changes the O


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Example

H2 +

O2

®

2

H2O

2

H

2

2

O

1

2

Also changes the H


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Example

H2 +

O2

®

2

H2O

2

H

2

4

2

O

1

2

Need twice as much H in the reactant


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Example

2

H2 +

O2

®

2

H2O

2

H

2

4

2

O

1

2

Recount


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Example

2

H2 +

O2

®

2

H2O

4

2

H

2

4

2

O

1

2

The equation is balanced, has the same

number of each kind of atom on both sides


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Example

2

H2 +

O2

®

2

H2O

4

2

H

2

4

2

O

1

2

This is the answer

Not this


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Examples

  • P. 252 – example problem 8-3

  • P. 252 – practice problems

  • P. 253 – example 8-4,8-5

  • P. 254 – practice problems

  • H.W. p. 254 Section Review

    P. 270 # 18,19, 22

    Website for balancing equations

    http://funbasedlearning.com/chemistry

    Classic Chembalancer


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Types of Reactions5 Basic Types(pages 256-264)

Predicting the Products


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Synthesis

  • the combination of 2 or more substances to form a compound

  • Two reactants one product

A + B  AB


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Synthesis

H2(g) + Cl2(g)  2 HCl(g)


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Synthesis

  • 2 elements, or compounds combine to make one compound.

  • Three types:

  • Reaction of elements + oxygen or sulfur

  • Reaction of metals + halogens

  • Reactions with oxides


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Synthesis

  • Examples (see pages 256-258)


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Decomposition

  • a compound breaks down into 2 or more simpler substances

  • one reactant two products

AB  A + B


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Decomposition

  • one reactant breaks apart into two or more elements or compounds.

  • Usually requires heat or electricity

  • A catalyst may be used to speed up the reaction

  • The decomposition of a substance by electricity is called electrolysis


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Decomposition

  • Can predict the products easily if it is a binary compound

  • 2H2O 2H2 + O2

  • 2NaCl 2Na + Cl2


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Decomposition

2 H2O(l)  2 H2(g) + O2(g)


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Decomposition

  • decomposition of metal carbonates

  • decomposition of metal hydroxides

  • decomposition of metal chlorates

  • decomposition of acids.


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Decomposition

  • Examples (see pp. 259-260)

    h.w. handout


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Single Replacement

  • one element replaces another in a compound

    • metal replaces metal (+) and hydrogen

    • nonmetal replaces nonmetal (-)

A + BC  B + AC


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Single Replacement

Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)


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Single Replacement

  • Examples: (see p. 261-262)


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Double Replacement

  • ions in two compounds “change partners”

  • cation of one compound combines with anion of the other

AB + CD  AD + CB


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Double Replacement

Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)


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Double Replacement

  • Formation of a precipitate, gas, or water

  • Examples ( see pages 262-263)

  • H.w. handout


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Combustion

  • the burning of any substance in O2 to produce a large amount of energy in the form of heat and light

CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)


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Combustion

  • Combustion of a hydrocarbon with oxygen

  • the products will be CO2 and H2O.

    (may get C and CO under less than perfect conditions)

    Balance C, H, then O


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Examples

  • C2H6 + O2®

  • C3H8 + O2®

  • C4H10 + O2®

  • C7H16 + O2®


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Homework

  • P. 269 # 1-11

  • P. 264 # 1-4

  • P. 270 # 20, 21

  • P. 271 # 25-29, 33

  • P. 272 # 44, 48