Chapter 8 Chemical Reactions

1
Chapter 8Chemical Reactions
2
Section 8.1Describing Chemical Change

• OBJECTIVES
• Write equations describing chemical reactions,
  using appropriate symbols

3
Section 8.1Describing Chemical Change

• OBJECTIVES
• Write balanced chemical equations, when given the
  names or formulas of the reactants and products
  in a chemical reaction.

4
All chemical reactions

• have two parts
• Reactants - the substances you start with
• Products- the substances you end up with
• The reactants turn into the products.
• Reactants Products

5
In a chemical reaction

• The way atoms are joined is changed
• Atoms arent created of destroyed.
• Can be described several ways
• 1. In a sentence
• Copper reacts with chlorine to form copper (II)
  chloride.
• 2. In a word equation
• Copper chlorine copper (II) chloride

6
Symbols in equations-p.206

• the arrow separates the reactants from the
  products
• Read reacts to form
• The plus sign and
• (s) after the formula solid
• (g) after the formula gas
• (l) after the formula liquid

7
Symbols used in equations

• (aq) after the formula - dissolved in water, an
  aqueous solution.
• used after a product indicates a gas (same as
  (g))
• used after a product indicates a solid (same as
  (s))

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Symbols used in equations

• indicates a reversible reaction
  (more later)
• shows that
  heat is supplied to the reaction
• is used to indicate a catalyst
  is supplied, in this case, platinum.

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What is a catalyst?

• A substance that speeds up a reaction, without
  being changed or used up by the reaction.
• Enzymes are biological or protein catalysts.

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Skeleton Equation

• Uses formulas and symbols to describe a reaction
• doesnt indicate how many.
• All chemical equations are sentences that
  describe reactions.

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Convert these to equations

• Solid iron (III) sulfide reacts with gaseous
  hydrogen chloride to form iron (III) 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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Now, read these

• 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
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Balanced Equation

• Atoms cant 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

• C O2 CO2
• This equation is already balanced
• What if it isnt?

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O

C
C
O
O

• C O2 CO
• We need one more oxygen in the products.
• Cant change the formula, because it describes
  what it is (carbon monoxide in this example)

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C
O

O

C
O
C
O

• Must be used to make another CO
• But where did the other C come from?

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C
C
O

O

O
C
O
C

• Must have started with two C
• 2 C O2 2 CO

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Rules for balancing

• Assemble, 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) - save H and
  O until LAST!
• Check to make sure it is balanced.

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Never

• Never change a subscript to balance an equation.
• If you change the formula you are describing a
  different reaction.
• H2O is a different compound than H2O2
• Never put a coefficient in the middle of a
  formula
• 2 NaCl is okay, Na2Cl is not.

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Example
H2
H2O
O2

Make a table to keep track of where you are at
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Example
H2
H2O
O2

R
P
H
2
2
O
2
1
Need twice as much O in the product
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Example
H2
H2O
O2

2
R
P
H
2
2
O
2
1
Changes the O
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Example
H2
H2O
O2

2
R
P
H
2
2
O
2
1
2
Also changes the H
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Example
H2
H2O
O2

2
R
P
H
2
2
4
O
2
1
2
Need twice as much H in the reactant
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Example
H2
H2O
O2

2
2
R
P
H
2
2
4
O
2
1
2
Recount
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Example
H2
H2O
O2

2
2
R
P
H
2
2
4
4
O
2
1
2
The equation is balanced, has the same number of
each kind of atom on both sides
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Example
H2
H2O
O2

2
2
R
P
H
2
2
4
4
O
2
1
2
This is the answer
Not this
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Balancing Examples

• _AgNO3 _Cu _Cu(NO3)2 _Ag
• _Mg _N2 _Mg3N2
• _P _O2 _P4O10
• _Na _H2O _H2 _NaOH
• _CH4 _O2 _CO2 _H2O

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Section 8.2Types of Chemical Reactions

• OBJECTIVES
• Identify a reaction as combination,
  decomposition, single-replacement,
  double-replacement, or combustion

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Section 8.2Types of Chemical Reactions

• OBJECTIVES
• Predict the products of combination,
  decomposition, single-replacement,
  double-replacement, and combustion reactions.

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Types of Reactions

• There are millions of reactions.
• Cant remember them all
• Fall into several categories.
• We will learn 5 major types.
• Will be able to predict the products.
• For some, we will be able to predict whether they
  will happen at all.
• Will recognize them by the reactants

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1 - Combination Reactions

• Combine - put together
• 2 substances combine to make one compound.
• Ca O2 CaO
• SO3 H2O H2SO4
• We can predict the products if they are two
  elements.
• Mg N2 __?__

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Write and balance

• Ca Cl2
• Fe O2 iron (II) oxide
• Al O2
• Remember that the first step is to write the
  correct formulas
• Then balance by using coefficients only

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2 - Decomposition Reactions

• decompose fall apart
• one reactant falls apart into two or more
  elements or compounds.
• NaCl Na Cl2
• CaCO3 CaO CO2
• Note that energy is usually required to decompose

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2 - Decomposition Reactions

• Can predict the products if it is a binary
  compound
• Made up of only two elements
• Falls apart into its elements
• H2O
• HgO

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2 - Decomposition Reactions

• If the compound has more than two elements you
  must be given one of the products
• The other product will be from the missing pieces
• NiCO3 CO2 ?
• H2CO3(aq) CO2 ?

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3 - Single Replacement

• One element replaces another
• Reactants must be an element and a compound.
• Products will be a different element and a
  different compound.
• Na KCl K NaCl
• F2 LiCl LiF Cl2

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3 Single Replacement

• Metals replace other metals (and hydrogen)
• K AlN
• Zn HCl
• Think of water as HOH
• Metals replace one of the H, combine with
  hydroxide.
• Na HOH

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3 Single Replacement

• We can tell whether a reaction will happen
• Some chemicals are more active than others
• More active replaces less active
• There is a list on page 217 - called the Activity
  Series of Metals
• Higher on the list replaces lower.

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3 Single Replacement

• Note the concerning Hydrogen
• H can be replaced in acids by everything higher
• Li, K, Ba, Ca, Na replace H from acids and
  water
• Fe CuSO4
• Pb KCl
• Al HCl

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3 - Single Replacement

• What does it mean that Hg and Ag are on the
  bottom of the list?
• Nonmetals can replace other nonmetals
• Limited to F2 , Cl2 , Br2 , I2 (halogens)
• Higher replaces lower.
• F2 HCl
• Br2 KCl

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4 - Double Replacement

• Two things replace each other.
• Reactants must be two ionic compounds or acids.
• Usually in aqueous solution
• NaOH FeCl3
• The positive ions change place.
• NaOH FeCl3 Fe3 OH- Na1 Cl-1
• NaOH FeCl3 Fe(OH)3 NaCl

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4 - Double Replacement

• Has certain driving forces
• Will only happen if one of the products
• doesnt dissolve in water and forms a solid (a
  precipitate), or
• is a gas that bubbles out, or
• is a covalent compound (usually water).

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Complete and balance

• assume all of the following reactions take place
• CaCl2 NaOH
• CuCl2 K2S
• KOH Fe(NO3)3
• (NH4)2SO4 BaF2

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How to recognize which type

• Look at the reactants
• E E Combination
• C Decomposition
• E C Single replacement
• C C Double replacement

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Examples

• H2 O2
• H2O
• Zn H2SO4
• HgO
• KBr Cl2
• AgNO3 NaCl
• Mg(OH)2 H2SO3

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5 - Combustion

• Means add oxygen
• A compound composed of only C, H, and maybe O is
  reacted with oxygen
• If the combustion is complete, the products will
  be CO2 and H2O.
• If the combustion is incomplete, the products
  will be CO (possibly just C) and H2O.

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Examples

• C4H10 O2 (assume complete)
• C4H10 O2 (incomplete)
• C6H12O6 O2 (complete)
• C8H8 O2 (incomplete)

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An equation...

• Describes a reaction
• Must be balanced in order to follow the Law of
  Conservation of Mass
• Can only be balanced by changing the
  coefficients.
• Has special symbols to indicate physical state,
  and if a catalyst or energy is required.

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Reactions

• Come in 5 major types.
• Can tell what type they are by the reactants.
• Single Replacement happens based on the activity
  series
• Double Replacement happens if the product is a
  solid, water, or a gas.

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Section 8.3Reactions in Aqueous Solution

• OBJECTIVES
• Write and balance net ionic equations.

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Section 8.3Reactions in Aqueous Solution

• OBJECTIVES
• Use solubility rules to predict the precipitate
  formed in double-replacement reactions.

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Net Ionic Equations

• Many reactions occur in water- that is, in
  aqueous solution
• Many ionic compounds dissociate, or separate,
  into cations and anions when dissolved in water
• Now we can write a complete ionic equation

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Net Ionic Equations

• Example
• AgNO3 NaCl ? AgCl NaNO3
• 1. this is the full equation
• 2. now write it as an ionic equation
• 3. can be simplified by eliminating ions not
  directly involved (spectator ions) net ionic
  equation

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Predicting the Precipitate

• Insoluble salt a precipitate - note Figure
  8.13, p.227
• General rules Table 8.3, p. 227, Reference p.7
  (back of textbook), and in Lab manual p.338
• Sample problem 8-11, p.228