1 Chapter 7 Chemical Reactions
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Chapter 7 Chemical Reactions
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Section 7.1Describing Chemical Change
OBJECTIVES:
–Write equations describing chemical reactions, using appropriate symbols
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Section 7.1Describing Chemical Change
OBJECTIVES:
–Write balanced chemical equations, when given the names or formulas of the reactants and products in a chemical reaction.
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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. Reactants Products
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In a chemical reaction The way atoms are joined is changed Atoms aren’t created of destroyed. Can be described several ways:
1. In a sentence
Copper reacts with silver nitrate to form silver and copper (II) nitrate .
2. In a word equation
Copper + silver nitrate silver + copper (II) nitrate
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Cu( ) + AgNO3( ) Ag( ) + Cu(NO3)2( )
reactants products
Or a skeleton equation
Or a balanced equation
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Symbols in equations-p.144 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
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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.
heat ,
Pt
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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 doesn’t 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)
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NO2 (g) N2(g) + O2(g) Pt
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Balancing Chemical Equations
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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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C + O2 CO2
This equation is already balanced What if it isn’t?
C + OO COO
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C + O2 CO We need one more oxygen in the products. Can’t change the formula, because it
describes what it is (carbon monoxide in this example)
C + O COO
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Must be used to make another CO But where did the other C come from?
C +O
C
OO
OC
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Must have started with two C 2 C + O2 2 CO
C
+O
C
OO
OC
C
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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!
Element Carbon Hydrogen Oxygen Check to make sure it is balanced.
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Never change a subscript to balance an equation.– If you change the formula (subscripts) 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 + H2OO2
Make a table to keep track of where you are at
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Example
H2 + H2OO2
Need twice as much O in the product
R PH
O
2
2
2
1
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Example
H2 + H2OO2
Changes the O
R PH
O
2
2
2
1
2
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Example
H2 + H2OO2
Also changes the H
R PH
O
2
2
2
1
2
2
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Example
H2 + H2OO2
Need twice as much H in the reactant
R PH
O
2
2
2
1
2
2
4
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Example
H2 + H2OO2
Recount
R PH
O
2
2
2
1
2
2
4
2
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Example
H2 + H2OO2
The equation is balanced, has the same number of each kind of atom on both sides
R PH
O
2
2
2
1
2
2
4
2
4
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Example
H2 + H2OO2
This is the answer
R PH
O
2
2
2
1
2
2
4
2
4
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 7.2Types of Chemical Reactions
OBJECTIVES:
–Identify a reaction as combination, decomposition, single-replacement, double-replacement, or combustion
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Section 7.2Types 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. Can’t 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 (synthesis) 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
electricity
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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
electricity
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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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The Activity Series of the Metals
Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead Hydrogen Hydrogen Bismuth Copper Mercury Silver Platinum Gold
Metals can replace other metals provided that they are above the metal that they are trying to replace.
Ex. Metals above hydrogen can replace hydrogen in acids.
Cu + AgNO3
Zn + NaCl
Copy list to your periodic table
CuNO3 + Ag
No reaction
???
???
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The Activity Series of the Halogens
Fluorine Chlorine Bromine Iodine
Halogens can replace other halogens in compounds, provided that they are above the halogen that they are trying to replace.
2NaCl(s) + F2(g) 2NaF(s) + Cl2(g)
MgCl2(s) + Br2(g) ???No Reaction
???
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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 155 - 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 Fe+3 OH- + Na+1 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:
–doesn’t 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:
Combination
Element + Element Compound (s,l or g) (s,l or g) (s,l or g)
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Decomposition
Compound Element + Element
(s,l or g) (s,l or g) (s,l or g)
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Single Displacement
Element + Compound Element + Compound
(s,l or g) (aq) (s,l or g) (aq or s)
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Compound + Compound Compound + Compound
(aq) (aq) (aq or s) (aq or s)
Double Displacement
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Combustion
CxHy + O2 (g) CO2(g) + H2O(g)
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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 7.3Reactions in Aqueous Solution
OBJECTIVES:
–Write and balance net ionic equations.
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Section 7.3Reactions 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(aq) + NaCl(aq) AgCl(s)+ NaNO3(aq)
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 7.13,
p.156 General rules: Table 7.3, p. 161, Reference
p.708 (back of textbook) Sample problem 7-9, p.156