chemical reactions. l section 1: objectives –identify the parts of a chemical equation –learn...
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Chemical Reactions
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Section 1: Objectives
–Identify the parts of a chemical equation
–Learn how to write a chemical equation
–Learn how to balance a chemical equation.
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A chemical reaction is the process by which one or more substances are changed into one or more different substances.
Chemical reactions are described by chemical equations.
A chemical equations represents, with symbols and formulas, the identities and relative amounts of the starting material and products.
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Indications of a Chemical Reaction
1) Evolution of heat and light.
2) Production of a gas.
3) Formation of a precipitate (solid).
(a solid appears after two liquids are mixed)
4) Color change
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All chemical reactions…
have two parts:1. Reactants = the substances you
start with (left side of arrow)2. Products = the substances you end
up with (right side of arrow)
The reactants will turn into the products.
Reactants Products
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In a chemical reaction
Atoms aren’t created or destroyed (according to the Law of Conservation of Mass)
A reaction can be described several ways:
#1. In a sentence every item is a word:
Copper reacts with chlorine to form
copper (II) chloride.
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Symbols in equations
the arrow (→) separates the reactants from the products (arrow points to products)
– Read as: “reacts to form” or yields
(s) after the formula = solid: Fe(s)
(g) after the formula = gas: CO2(g)
(l) after the formula = liquid: H2O(l)
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(aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) is a salt water solution
↑ used after a product indicates a gas has been produced: H2↑
used after a product indicates a solid has been produced: PbI2↓
Symbols in equations
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■ shows that heat is supplied to the reaction
Symbols in equations
■ Additional symbols on page 246 of textbook.
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Formula Equations:
Represents the reactants and products by symbols and formulas:
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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Balanced Chemical Equations Atoms can’t be created or destroyed in
an ordinary reaction:
–All the atoms we start with we must end up with (meaning: balanced!)
A balanced equation has the same number of each element on both sides of the equation.
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Rules for balancing:1) Assemble the correct formulas for all the
reactants and products, using “+” and “→”2) Count the number of atoms of each type
appearing on both sides3) Balance the elements one at a time by
adding coefficients (the numbers in front) (order of balancing on next slide)
4) Double-Check to make sure it is balanced.
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1) Metals
2) Non-metals
3) save balancing the H and O until LAST!
(hint: I prefer to save O until the very last)
Rules for balancing:
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Never change a subscript to balance an equation (You can only change coefficients)
– If you change the subscript (formula) you are describing a different chemical.
– H2O is a different compound than H2O2
Never put a coefficient in the middle of a formula; they must go only in the front
2NaCl is okay, but Na2Cl is not.
Rules
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Practice Balancing Examples
_AgNO3 + _Cu _Cu(NO3)2 + _Ag
_Mg + _N2 _Mg3N2
_P + _O2 _P4O10
_Na + _H2O _H2 + _NaOH
_CH4 + _O2 _CO2 + _H2O
2 2
3
4 5
2 22
2 2
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Section 2Types of Chemical Reactions
OBJECTIVES:
–Describe the five general types of reactions.
–Predict the products of the five general types of reactions.
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Type of Reactions
Chemical reactions are classified into five general types
Combination (synthesis)
Decomposition
Single Replacement
Double Replacement
Combustion
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Combination (Synthesis)
Two or more elements or simple compounds combine to form (synthesize) one product
A + B AB
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Combination (Synthesis)
1) Reactions of elements with oxygen and sulfur.
The groups 1 and 2 metals react with oxygen to form oxides.
2 Mg + O2 2 MgO (group 2)
4 Li + O2 2 Li2O (group 1)
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Combination (Synthesis)
1) Reactions with oxygen and sulfur.
Sulfur which is right below oxygen reacts in a similar manner
8 Ba + S8 8 BaS
16 Rb + S8 8 Rb2S
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Combination (Synthesis)
2) Reactions of metals with halogens
2 Na + Cl2 2 NaCl (Group 1)
2 K + I2 2 KI (Group 1)Mg + F2 MgF2 (Group 2)
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Decomposition
One substance is broken down (split) into two or more elements.
These reactions usually require heat
AB A + B
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Decomposition
1) Decomposition of binary compounds
2 H2O 2 H2 (g) + O2 (g) (electrolysis)
electricity
2 HgO 2 Hg + O2
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Decomposition
2) Decomposition of metal carbonates
CaCO3 CaO + CO2
K2CO3 K2O + CO2
3) Decomposition of metal hydroxides
Ca(OH)2 CaO + H2O
2NaOH Na2O + H2O
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Learning Check
Classify the following reactions as
1) combination or 2) decomposition:
___A. H2 + Br2 2HBr
___B. Al2(CO3)3 Al2O3 + 3CO2
___C. 4 Al + 3C Al4C3
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Solution
Classify the following reactions as
1) combination or 2) decomposition:
_1_A. H2 + Br2 2HBr
_2_B. Al2(CO3)3 Al2O3 + 3CO2
_1_C. 4 Al + 3C Al4C3
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Single Replacement
One element replaces a similar element in a compound.
A + BX AX + B
Zn + 2HCl ZnCl2 + H2
Fe + CuSO4 FeSO4 + Cu
Many single replacement reactions take place in an aqueous solution.
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Single Replacement
1) Replacement of a metal by another metal.
Refer to activity series table to determine if the reaction will occur.
2 Al + 3 Pb(NO3)2 3 Pb + 2 Al(NO3)3
Balanced!
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Single Replacement
1) Replacement of a metal by another metal.
Refer to activity series table to determine if the reaction will occur.
Pb + Al(NO3)3 No Reaction
Pb is below Al on the activity series chart.
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Single Replacement
2) Replacement of hydrogen in water by a metal.
Na + H2O NaOH + H2 (g)
Unbalanced!
Group 1 metals react vigorously with water.
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Single Replacement
2) Replacement of hydrogen in water by a metal.
2 Na + 2 H2O 2 NaOH + H2 (g)
Balanced!
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Single Replacement
3) Replacement of hydrogen in an acid by a metal.
Mg + 2 HCl MgCl2 + H2 (g)
balanced!
Group 1 and 2 metals can react with acids to produce hydrogen gas.
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Double Replacement
Two elements in reactants exchange places.
AX + BY AY + BX
A and B are metals
X and Y are nonmetals
One new product is usually a precipitate or gas
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Double Replacement
1) Formation of a precipitate.
MgI2 + Pb(NO3)2 Mg(NO3)2 + PbI2 (s)
balanced
Exchange of metals in the reaction.
PbI2 is a solid that precipitates out of the reaction.
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Double Replacement
2) Formation of a gas.
CaS + 2 HCl H2S (gas) + CaCl2
balanced
Calcium sulfide reacts to form hydrogen sulfide gas.
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Double Replacement
3) Formation of a water.
HCl + NaOH NaCl + H2O (liquid)
balanced
Usually an acid-base reaction
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Learning Check
Classify the following reactions as
1) single replacement
2) double replacement
__A) 2Al + 3H2SO4 Al2(SO4)3 + 3H2
__B) Na2SO4 + 2AgNO3 Ag2SO4 + 2NaNO3
__C) 3C + Fe2O3 2Fe + 3CO
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Solution
Classify the following reactions as
1) single replacement
2) double replacement
1_A) 2Al + 3H2SO4 Al2(SO4)3 + 3H2
2_B) Na2SO4 + 2AgNO3 Ag2SO4 + 2NaNO3
1_C) 3C + Fe2O3 2Fe + 3CO
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Combustion
A reaction in which a compound (often carbon)
reacts with oxygen.
C + O2 CO2
CH4 + 2 O2 CO2 + 2H2O
C3H8 + 5 O2 3 CO2 + 4 H2O
C6H12O6 + 6 O2 6 CO2 + 6 H2O
Reaction releases energy in the form of heat or light.
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Learning Check
Balance the combustion equation
___C5H12 + ___O2 ___CO2 + ___H2O
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Solution
Balance the combustion equation
1 C5H12 + 8 O2 5 CO2 + 6 H2O
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End
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Oxidation and Reduction
Reactions that involve a loss or gain of electrons
Occurs in many of the 4 types of reactions and combustion
Important in food metabolism, batteries, rusting of metals
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Requirements for Oxidization-Reduction
Electrons are transferred
Two processes occur
Oxidation = Loss of electrons (LEO)
Zn Zn2+ + 2e-
Reduction = Gain of electrons (GER)
Cu2+ + 2e- Cu
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Balanced Red-Ox Equations
Combine the oxidation and reduction reactions to make
Loss of electrons = Gain of electrons
Zn + Cu2+ + 2e- Zn2+ + 2e- + Cu
Zn + Cu2+ Zn2+ + Cu
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Gain/Loss of Hydrogen
In organic and biological reactions
oxidation = Loss of H
reduction = Gain of H
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Learning Check R3
Identify the following as an 1) oxidation or a reduction process:
__A. Sn Sn4+ + 4e-
__B. Fe3+ + 1e- Fe2+
__C. Cl2 + 2e- 2Cl-
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Solution R3
Identify the following as an 1) oxidation or a reduction process:
1_ A. Sn Sn4+ + 4e-
2_ B. Fe3+ + 1e- Fe2+
2_ C. Cl2 + 2e- 2Cl-
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Learning Check R4
In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction
Ag+ + Cl- Ag + Cl
A. Which reactant is oxidized
1) Ag+ 2) Cl- 3) Ag
B. Which reactant is reduced?
1) Ag+ 2) Cl- 3) Cl
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Solution R4
In light-sensitive sunglasses, UV light initiates an oxidation-reduction reaction
Ag+ + Cl- Ag + Cl
A. Which reactant is oxidized
2) Cl- Cl- Cl + e-
B. Which reactant is reduced?
1) Ag+ Ag+ + e- Ag