Download 4.02 Chemical Reactions

4.02 Chemical Reactions
The Law of Conservation of Mass
Dr. Fred Omega Garces
Chemistry 111
Miramar College
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Chemical Reactions
January 10
Making Substances
Chemical Reactions; the heart of chemistry is the chemical reaction
which is responsible for making new substances. If the reaction is new,
then it is similar to inventing a new product.
Synthesis of: Nylon:
Aspirin: Salicylic acid + acetic anhydride
O
OH
C
O
H
H 3C
+
C
O
O
C H3
O
C
g Aspirin (acetylsalicylic acid)
OH
C
O
C
O
O
O
C H3
H
O
C
C H3
Acetylsalicyclic acid
(Aspirin)
Glucose:
Air Bag: 2Na3N (s)
2
+ 3O(s) _
CO2 + H2O
+ hν g C6H12O6
3Na2O (s) + N2 (g)
Chemical Reactions
January 10
The Law of Conservation of Mass
Chemical reactions are based on the "Law of conservation of mass".
Σ Atoms reacting
Before reaction
i) 12 H2 + 6 N2
=
g
i)
24 H’s + 12 N’s
g
ii)
C3H8
g
+
5 O2
+
3
Σ Atoms produced
After reaction
8 NH3
+
2N2 (excess)
24 H’s + 12 N’s
3 CO2
+
4H2O
+
Chemical Reactions
January 10
Reaction Process
9 H2(g) + 6 N2 (g)
g
6 NH3 (g) + 3 N 2
Atoms in reactant
=
Atoms in product
(excess N2)
N
Note: 18 H and 6 N on each side of the
chemical equation !!
The net molecular equation is:
2
N2 H2
H2
H2
H2
N2
H2
H2
H2
N2
H2
H2
N
N2
2 NH 3
2 NH 3
2
2 NH 3
9 H2 (g) + 3 N2 (g) g
6 NH3 (g)
Symbol
Meaning
+
and or plus
a
reacts to form or yields
(s)
Solid
(l)
liquid
(g)
gas
(aq)
aqueous (water) solution
Δ
Heat added
Pt
Catalyst (in this case, platinum)
""→
!!
!→
4
(g)
Chemical Reactions
January 10
Evidence of a Chemical Change
Chemical change requires that reactant bonds are broken and product bonds form.
The chemical identity of the starting material is altered. This can be
detected by making physical observations during the reaction.
Evidence:
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1. 
Gas formation: Combustion of ethanol.
2. 
Precipitation or solid formation: BaCl2 and Potassium chromate.
3. 
Temperature change: NaOH in water.
4. 
Color change: Pigments or reactions.
Chemical Reactions
January 10
Chemical Reactions
Reactants
P4 (s) + 6 Cl2 (g) →
Products
4 PCl3 (s)
1 molecule 6 molecules → 4 molecules
1.00 mole 6.00 moles → 4.00 moles
Δ
(g)
6
(g)
Chemical Reactions
(g)
(l)
January 10
Generic Form
aA + bB → c C + dD
Where
A, B, C and D are specie (compounds) that are involve in the reaction.
a, b, c and d are coefficient that tell how many of each species react.
# of atoms (reacted) = # of atoms (produced)
+
C3H8
(g)
Species:
+
+
5 O2 (g)
g
3 CO2 (g)
+
4 H2O
(g)
C3H8 (propane), O2 (oxygen), CO2 (carbon dioxide), H2O (water)
Number of species: 1 (propane), 5 O2 (oxygen), 3 (carbon dioxide) and 4 (water)
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Chemical Reactions
January 10
Balancing Process
Consider the following reaction:
Mg
magnesium
To balance this equation:
Mg
Mg
2 Mg
+ O2 g MgO2
+ O2 g 2 MgO
+ O2 g 2 MgO
+
O2
Oxygen from air
g
MgO
Product
i) No!! (never since MgO2 is different from MgO)
ii) Oxygen balanced, but Mg is not balanced
ii) Correct, all species are balanced
Note: Once the formula for the substance is
established, (i.e., the chemicals involved in the reaction
are determined) the subscript can never be changed.
Only the coefficient can be altered.
*
MgO cannot be changed to MgO2 just to balance the
equation. Furthermore, MgO2 is not chemically sound.
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Chemical Reactions
January 10
Type of Reactions
1. Combination Reaction
A + B g AB
Example:
i) __ SO3 + __ H2O
ii) __ P + __ S
g
__ H2SO4
__ P2S5
g
iii) __ N2 + __ H2
4a. Double Displacement Reaction:
AB + CD g AD + CB
Example:
i) __ Ba(NO3)2 + __ H2S g __ BaS + __ HNO3
ii) __ MgCl2 + __ Na3PO4 g __ Mg3(PO4)2 + __ NaCl
g __ NH3
2. Decomposition Reaction
AB g A + B
Example:
i) __ H2CO3 g __ H2O
+
4b. Acid Base (Double Displacement) Reaction:
HA + MOH g H2O + MA
Example:
i) __ HNO3 + __ Ba(OH)2 g __ H2O + __ Ba(NO3)2
__ CO2
ii) __ HCl + __ NH4OH g
ii) __ KClO3 g __ KCl + __ O2
3. Single Displacement Reaction:
A + BC g AC + B
Example:
i) __ Zn + __ Cu(NO3)2 g __ Zn(NO3)2 + __ Cu
ii) __ Ca + __ H2O g __ Ca(OH)2 + __ H2
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__ H2O
+ __ NH4Cl
5. Combustion Reaction
CxHy + O2 g CO2 + H2O
Example:
i) __ C4H10 + __ O2 g __ CO2 + __ H2O
ii) __ C6H12O6 + __ O2 g
Chemical Reactions
__ CO2 + __ H2O
January 10
Balancing (Guidelines)
1
H2
+
2H
2
H2
O2
H2O
2O
2H 1O
H2 O2 ?
+
O2
2 H 2O
2H's, 2 O's
3 2 H2
+
4H's, 2O ' s
O2
2
4H's, 2 O's
4 2 H2
+
H2 O
4H's, 2O ' s
O2
2
H2 O
Chemical identified
oxygen is not balance
Cannot since identity will be change d
Change coefficient of product to two
waters (H2O)
Oxygen is balanced but hydrogen is not.
Coefficient must be adjust e d
Two placed in front of H2, to change H
atom to four
2 O's and 4 H's are on each side of the
balanced equation
The equation is now balanc e d
Summary : Balancing chemical reaction
•
•
•
•
Box the chemical and do not change the formula.
Assign coefficient of 1 to the most complicated molecule.
Balance elemental chemicals last. i.e., H2, N2, O2
Use fraction coefficient to balance odd number of atoms in the equation.
• Balance polyatomic cations / anions as a single unit if these ions occur in
reactant and product unchanged
(i.e., PO43- and SO42- )
• Convert fraction to integer.
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Chemical Reactions
January 10
Summary : Balancing chemical reaction
1• Box the chemicals so that you don’t change the formula.
2• Assign a coefficient of one to the most complicated molecular
specie. Ex#1 - 5
3• Balance elemental chemicals last. i.e., H2 N2 O2
Ex#2 & 3
4• Use fractional coefficients to balance an odd number of
atoms in the equation Ex#2 & 3
5• Convert the fraction to integer by multiplying through whole
equation by the denominator of the fraction Ex#2 & 3
6• Balance polyatomic cation / anion as a single unit if these ions
occurs in reactant and product unchanged (i.e., PO43- and
SO42- ). Ex#4 & 5
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Chemical Reactions
January 10
Example: Balancing Reactions
#1 Sodium oxide and water forms sodium hydroxide
#2
Ferric(III) Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide
#3 C5H12
#4
(g)
+ O2
g
(g)
H2SO4 (aq) + Ca(OH)2
#5 Ca3(PO4)2 (s) +
Na2O + H2O
g
Fe2S3 + 9/2 O2
(aq)
g H2O(l) + CaSO4
H2SO4 (aq) g CaSO4 (s) +
(s)
H3PO4 (aq)
2NaOH
g
Fe2O3 + 3SO2
C7H14 (g) + 21/2 O2 (g)
12
H2O(g) + CO2 (g)
g
convert to integer by multiplying by 2
7H2O(g) + 7CO2 (g)
Chemical Reactions
January 10
Example: Balancing Reactions
#1 Sodium oxide and water forms sodium hydroxide
__Na2O + __H2O
1 +
1
g
g
2
__NaOH
Sum Coef.
a) 3 b) 4
c) 5 d) 6
#2 Ferric(III) Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide
__Fe2S3 + __O2
Fe2S3 + 9/2 O2
1 +
13
9/2
g
1
g
g __Fe2O3 + __SO2
Fe2O3 + 3SO2
+
3
Chemical Reactions
Sum Coef.
a) 10 b) 14
c) 19 d) None
January 10
Example: Balancing Reactions
#3 C5H12
(g)
+ O2
__C5H12 (g) + __O2 (g)
6
g
1 +
8
#4
H2SO4 (aq) + Ca(OH)2
g
+
H2O(g) + CO2 (g)
g
(g)
__H2O(g)
5
Sum Coef.
a) 13 b) 20
c) 18 d) None
(aq)
g
__H2SO4 (aq) + __Ca(OH)2 (aq) g
1 +
14
1
g
2
+
__CO2 (g)
H2O(l) + CaSO4 (s)
__H2O(l)
+ __CaSO4 (s)
Sum Coef.
a) 4 b) 5
c) 10 d) None
1
Chemical Reactions
January 10
Example: Balancing Reactions
#1 Sodium oxide and water forms sodium hydroxide
#2
Ferric(III) Sulfide & molecular oxygen yields ferric(III) oxide & sulfur dioxide
#3 C7H14
#4
(g)
+ O2
H2SO4 (aq) + Ca(OH)2
#5 Ca3(PO4)2 (s) +
Na2O + H2O
g
Fe2S3 + 9/2 O2
H2O(g) + CO2 (g)
(aq)
g H2O(l) + CaSO4
H2SO4 (aq) g CaSO4 (s) +
(s)
H3PO4 (aq)
2NaOH
g
Fe2O3 + 3SO2
C7H14 (g) + 21/2 O2 (g)
15
g
(g)
g
convert to integer by multiplying by 2
7H2O(g) + 7CO2 (g)
Chemical Reactions
January 10
Type of Reactions (1): Combination
1. Combination Reaction -A Reaction in which two substances chemically
combine to form a third.
A + B
g
Example:
i) __ SO3 (g) + __ H2O
1:1:1
ii) __ P4
16:5:8
(s)
+ __ S8
(s)
iii) __ Al(s) + __ Br2 (l)
AB
(l)
g __ H2SO4 (aq)
g __ P2S5 (s)
g __ Al2Br6 (s)
2:3:1
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Chemical Reactions
January 10
Type of Reactions (2): Decomposition
2. Decomposition Reaction -A Reaction in which a single
compound breaks up into two or more other substances.
AB
g A + B
Example:
i) __ H2CO3 (aq) g __ H2O(l) +
__ CO2 (g)
1:1:1
ii) __ KClO3
(s)
g __ KCl(s) + __ O2 (g)
iii)2:2:3__ HgO(s) g __ Hg
(l)
+ __ O2 (g)
2:2:1
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Chemical Reactions
January 10
Type of Reactions (3): Single displacement
C. Single Displacement Reaction -A Reaction in which one element
reacts by replacing another element in a compound.
A + BC
g AC
+
B
Example:
i) __ Zn (s) + __ Cu(NO3)2 (aq)
ii) __ Ca
(s)
+ __ H2O
(l)
g __ Zn(NO3)2 (aq) + __ Cu
g __ Ca(OH)2 (aq) + __ H2 (aq)
iii) __ Cu(s) + __ AgNO3 (aq) g __ Cu(NO3)2 (aq) + __ Ag(s)
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Chemical Reactions
(s)
1:1:1:1
1:2:1:1
January 10
Type of Reactions (4a) Double Displacement
(Chp8)
4a. Double Displacement Reaction: - A Reaction in which two
compounds exchange partners to form two new compounds.
AB + CD
g
AD +
Example:
i) __ Ba(NO3)2 (aq) + __ H2S (aq) g
CB
__ BaS(s) + __ HNO3 (aq)
ii) __ MgCl2 (aq) + __ Na3PO4 (aq) g __ Mg3(PO4)2 (s) + __ NaCl (aq)
iii) __ Ba(NO3)2
19
(aq)+
__ K2CrO4 (aq) g __ BaCrO4 (s) + __ KNO3 (aq)
Chemical Reactions
1:1:1:2
3:2:1:6
January 10
Type of Reactions (4) Driving Force for Reactivity
Factors Towards Reactivity
1. Gas formation (Low molar mass covalent molecule)
__ZnS(s) + __ HCl
(aq)
g
__ZnCl2 (aq) +
__H2S
(g)
__HNO3(aq) + __NaCN(aq) g __ NaNO3 (aq) + __HCN
(g)
2. Weak Electrolyte or water formation (Neutralization)
__HI(aq) + __Al(OH)3 (s)
g __H2O
(l)
+ __AlI3
(aq)
__LiOH(aq) + __HNO3 (aq) g __LiNO3(aq) + __H2O (l)
3. Precipitation formation (Solubility Table)
__Pb(NO3)2
(aq)
+ __NaCl(aq) g __ NaNO3 (aq) + __PbCl2(s)
__MnS(s) + __ H2SO4
20
(aq)
g ????
Chemical Reactions
January 10
Type of Reactions (4b): Neutralization, a type of double
displacement
4b. Acid-Base (Neutralization Reaction): -A Reaction that occurs
between an acid and a base with the formation of an ionic compound
and water.
HA + MOH
g
H2 O
+
MA
Example:
i)
__ HNO3
(aq)
+ __ NaOH (aq)
ii) __ H3PO4 (aq) + __ NH3
21
(aq)
g
__ H2O
g __ H2O(l) +
Chemical Reactions
(l)
+ __ NaNO3 (aq)
1:1:1:1
__ (NH4)3PO4 (aq)
1:3:1:1
January 10
Type of Reactions (5) Combustion
5. Combustion Reaction -A Reaction of a substance with
either pure oxygen or oxygen in the air with the rapid
release of heat and the appearance of a flame.
CxHy + O2 g
Example:
i) __ C6H12O6 (l) + __ O2 (g)
CO2 + H2O
g __ CO2 (g) + __ H2O(l)
1:6:6:6
ii) __ C4H10 (l) + __ O2 (g) g
__ CO2 (g) + __ H2O
2:13:8:10
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Chemical Reactions
(l)
2:13:8:10
January 10
Summary
Understand the Basic idea of a Chemical Reaction
Write and Balance a Chemical Reaction
Recognize the different chemical reactions
1. Combination
2. Decomposition
3. Single Displacement
4. Double Displacement
5. Combustion
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Chemical Reactions
January 10