An Overview of Chemical Reactions

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Chapter 9
An Overview of Chemical Reactions
2
Reactants
Products
Substances that are about to chemically react.
Substances that are formed from a chemical
reaction.
3
Chemical Equation
A representation in which reactants are written
before an arrow and products are written after
the arrow.
4
CO2
O2
C
Reactants
Products
5
Coefficients
Integers used to indicate the ratio by which
reactants react and products form.
6
H2
O2
2 H2O
2
1

Reactants
Products
7
Balancing Guidelines

• Balance one element at a
• time. Start with compounds.

2) If you incidentally unbalance an element
leave it alone.
3) Make successive passes.
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(not balanced)
Al2O3
C
CO2
Al


Al
Al
C
Al
C
O
O
O
O
O
9
2
Al2O3
C
CO2
Al


Al
Al
Al
C
C
O
O
O
O
O
Al
10
2
3
2
Al2O3
C
CO2
Al


C
O
O
Al
Al
Al
O
O
O
C
C
O
O
Al
C
O
O
Al
Al
O
O
O
11
2
3
2
3
Al2O3
C
CO2
Al


C
O
O
C
Al
Al
Al
O
O
O
C
C
O
O
Al
C
C
O
O
Al
Al
O
O
O
12
Balanced
4
3
2
3
Al2O3
C
CO2
Al


C
O
O
C
Al
Al
Al
O
O
O
C
C
O
O
Al
Al
C
C
O
O
Al
Al
Al
O
O
O
13
not balanced
MgCl2
SiCl4
Mg

Si

14
balanced
MgCl2
2
2
1
1
SiCl4
Mg

Si

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9.2 Chemists Use Relative Masses to Count
Atoms and Molecules
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1 kg
1 kg
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Different atoms have different masses.
Their relative masses are given in the periodic
table.
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Formula mass
The sum of the atomic masses of the elements in a
chemical formula.
(molecular mass)
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Formula mass of O2
x
2
32 amu

Oxygen (16 amu)
Formula mass of NO
Oxygen 16 amu
NO 30 amu
Nitrogen 14 amu
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If you want equal numbers, you dont necessarily
measure equal masses
Instead, you measure in proportion to the
relative masses given in the periodic table.
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Avogadros Number
The atomic mass of an element expressed in grams
is equal to one mole of that element.
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He
4.003
6.02 x 1023 He atoms
4.003 g He
1 mole of He
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Pb
207.2
6.02 x 1023 Pb atoms
207.2 g of Pb

1 mole of Pb
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The formula mass of a compound expressed in grams
is equal to one mole of that compound.
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H2O
(18 amu)
6.02 x 1023 H2O molecules
18 g of H2O

1 mole of H2O
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Molar mass
The mass of one mole of a substance.
Pb 207.2 grams/mole
H2O 18 grams/mole
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C
O2
CO2
1
1
1

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1
1 mole
1 mole
1 mole
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H2
O2
H2O
2
2
1

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A neon sign contains 6.348 g of neon. How many
moles of neon are in the sign? How many atoms of
neon are in the sign?
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What mass of NO is produced when 56 g of nitrogen
(N2) react with 64 g of oxygen (O2) in the
unbalanced reaction N2 O2 ? NO
38
9.3 Reaction Rate Is Influenced by Concentration
and Temperature
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Reaction rate
How quickly the concentration of products
increases or the reactants decrease.
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The rate of a reaction is dependent upon the
collisions among reactant molecules.
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Increasing concentration increases the number of
collisions per second
and, therefore, increases the rate of reaction.
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Less concentrated
More concentrated
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Not all collisions lead to products
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Activation Energy, Ea
The minimum energy required to overcome the
initial breaking of bonds in reactants.
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Energy
Reaction progress
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Catalyst
A substance that increases the rate of a chemical
reaction by lowering the activation energy.
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Exothermic
A reaction in which there is a net release of
energy.
Endothermic
A reaction in which there is a net absorption of
energy.
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Selected Bond Energies
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endothermic
exothermic
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2 H2
O2
2 H2O

-464 kJ/mol
-464 kJ/mol
436 kJ/mol
498 kJ/mol
-464 kJ/mol
-464 kJ/mol
436 kJ/mol
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2 H2
O2
2 H2O

-464 kJ/mol
-464 kJ/mol
498 kJ/mol
436 kJ/mol
-464 kJ/mol
-464 kJ/mol
436 kJ/mol
1370 kJ
-1856 kJ
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2 H2
O2
2 H2O

Energy absorbed
1370 kJ
Energy released
-1856 kJ
Net energy of reaction
-486 kJ
Exothermic
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E
2 H2
O2
2 H2O


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Energy
Reaction progress
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An endothermic reaction involves a net absorption
of energy.
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N2
O2
2 NO

-631 kJ/mol
946 kJ/mol
498 kJ/mol
-631 kJ/mol
1444 kJ
-1262 kJ
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N2
O2
2 NO

Endothermic
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E
N2
O2
2 NO


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Energy
Reaction progress
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H2 Cl2 ? 2 HCl Is the reaction
endothermic or exothermic? Write the chemical
equation showing the energy term. Draw the
reaction profile.
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Entropy
Term used to describe the natural spreading of
energy
Because energy always spreads, entropy is always
increasing
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Energy tends to disperse
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Entropy
Chemical reactions that increase entropy can
occur on their own
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Two considerations
Energy within substances
Energy of reaction
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1,700 J
6 J/K
298K
72 J/K
carbon
salt
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Standard Entropies
Gases gt Liquids gt Solids
Complex gt Simple
Mixtures gt Pure
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H (g) H (g) ? H2 (g)
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2 H2
O2
2 H2O

(gas)
(gas)
(liq)
So why does this reaction occur so readily?
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Two considerations
Energy within substances
Energy of reaction
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Do the products possess more entropy than the
reactants?
No
Is the reaction exothermic?
No
Yes
Products Favored
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Do the products possess more entropy than the
reactants?
Yes
Is the reaction exothermic?
Yes
Reactants Favored Requires input of Energy
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Do the products possess more entropy than the
reactants?
Is the reaction exothermic?
No
Products Favored (If heat of rxn is large and
increase in entropy small)
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Do the products possess more entropy than the
reactants?
Is the reaction exothermic?
Products Favored (At higher temperatures)
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Laws of Thermodynamics
Energy is conserved
1.
Entropy is always increasing
2.