chemical reactions. l section 1: objectives –identify the parts of a chemical equation –learn...
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Chemical Reactions
Section 1: Objectives
–Identify the parts of a chemical equation
–Learn how to write a chemical equation
–Learn how to balance a chemical equation.
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.
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
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
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.
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)
(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
■ shows that heat is supplied to the reaction
Symbols in equations
■ Additional symbols on page 246 of textbook.
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)
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.
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.
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:
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
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
Section 2Types of Chemical Reactions
OBJECTIVES:
–Describe the five general types of reactions.
–Predict the products of the five general types of reactions.
Type of Reactions
Chemical reactions are classified into five general types
Combination (synthesis)
Decomposition
Single Replacement
Double Replacement
Combustion
Combination (Synthesis)
Two or more elements or simple compounds combine to form (synthesize) one product
A + B AB
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)
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
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)
Decomposition
One substance is broken down (split) into two or more elements.
These reactions usually require heat
AB A + B
Decomposition
1) Decomposition of binary compounds
2 H2O 2 H2 (g) + O2 (g) (electrolysis)
electricity
2 HgO 2 Hg + O2
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
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
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
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.
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!
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.
Single Replacement
2) Replacement of hydrogen in water by a metal.
Na + H2O NaOH + H2 (g)
Unbalanced!
Group 1 metals react vigorously with water.
Single Replacement
2) Replacement of hydrogen in water by a metal.
2 Na + 2 H2O 2 NaOH + H2 (g)
Balanced!
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.
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
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.
Double Replacement
2) Formation of a gas.
CaS + 2 HCl H2S (gas) + CaCl2
balanced
Calcium sulfide reacts to form hydrogen sulfide gas.
Double Replacement
3) Formation of a water.
HCl + NaOH NaCl + H2O (liquid)
balanced
Usually an acid-base reaction
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
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
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.
Learning Check
Balance the combustion equation
___C5H12 + ___O2 ___CO2 + ___H2O
Solution
Balance the combustion equation
1 C5H12 + 8 O2 5 CO2 + 6 H2O
End
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
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
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
Gain/Loss of Hydrogen
In organic and biological reactions
oxidation = Loss of H
reduction = Gain of H
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-
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-
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
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