Chapter 5 Chemical Reactions

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Chapter 5Chemical Reactions

• Douglass High School

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Section 5.1Nature of Chemical Reactions

• OBJECTIVES
• Recognize signs that a chemical Rx is taking
  place
• Explain chemical changes in terms of structure
  and motion of atoms and molecules
• Describe the difference between exothermic and
  endothermic Rx

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Chemical Reactions Change Substances

• When a chemical Rx takes place the results are
  different from the products you started with
• When substances undergo chemical changes to form
  brand new substances
• You can tell a Rx is taking place if
• You see bubbles
• You see a solid form
• You release energy in the form of heat or light

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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
• 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

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Energy and Reactions

• Energy is used to break the bonds of the
  reactants
• Activation energy
• Once the bonds are broken, energy is released
  when new bonds are formed
• It is always conserved (1st Law)
• Rx that release energy are exothermic
• Rx that absorb energy are endothermic

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Section 5.2Reaction Types

• OBJECTIVES
• Distinguish among five general types of Rx
• Predict the products of some Rx
• Describe Rx that transfer or share electrons

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1 - Synthesis Reactions

• Combine/put together reactants
• 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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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
• 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

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3 - 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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4 - 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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4 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 ?
• We can tell whether a reaction will happen
• Some chemicals are more active than others
• More active replaces less active

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4 - Single Replacement

• Nonmetals can replace other nonmetals
• Limited to F2 , Cl2 , Br2 , I2 (halogens)
• Higher replaces lower
• F2 HCl ?
• Br2 KCl ?

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5 - Double Replacement

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

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5 - 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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How to recognize which type

• Look at the reactants
• E E Synthesis
• C Decomposition
• C O2 Combustion
• 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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Section 8.3Balancing Chemical Equations

• OBJECTIVES
• Demonstrate how to balance chemical equations
• Interpret equations to determine number of moles
• Explain how the Law of Definite Proportions
  allows for predictions of amounts
• Identify mole ratios
• Calculate relative masses of reactants and
  products

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Symbols in equations

• Arrow separates the reactants from the products
• Read reacts to form
• The plus sign and

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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
• You can NOT change the chemical formulas
• Must match up the number of atoms on the left and
  right side of the reaction arrow

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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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Balanced Equation

• Atoms cant be created or destroyed
• 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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Write and Balance (Synthesis)

• 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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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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Law of Definite Proportions

• A compound always contains the same elements in
  the same proportions, regardless of how the
  compound is made or how much of the compound is
  formed
• Water is always water H2O
• Mole ratio is derived from the coefficients
• The coefficient tells you have many moles you
  need of that compound in the Rx

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Section 5.4Rates of Change

• Objectives
• Describe the factors affecting Rx rates
• Explain the effect of a catalyst on a chemical Rx
• Explain chemical equilibrium in terms of equal
  forward and reverse Rx rates
• Apply Le Chateliers principle

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Factors affecting Rates of Change

• For any Rx to occur particles must collide with
  one another
• What factors will speed up those collisions?
• Higher temperature
• Large surface area
• Concentrated solutions
• Higher pressure
• Size and shape of reactants
• Small molecules react faster
• Catalysts

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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
• Enzymes are very specific
• Most stop working above 45oC

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Equilibrium Systems

• indicates a reversible reaction
• These are Rx that dont continue until completion
• Wood burns until it is done (completed)
• Rx at equilibrium have the same rate going
  forward and backward
• Carbonation in a soda bottle
• This is a balanced state of RATES

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Le Chateliers Principle

• If a change is made to a system in chemical
  equilibrium, the equilibrium shifts to oppose the
  change until a new equilibrium is reached
• Every action has an equal and opposite reaction

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The End.