Download Chapter 9 An overview of chemical reaction

Chapter 9 An overview of
chemical reaction
Chemical reactions happen in nature to
produce natural substance, and even
living organism.
Chemical reactions have been used by us to
produce millions of substance that do not
exist in nature.
Chemical reactions are the central part of
the chemical sciences.
9.1 Chemical reactions are represented by
chemical equations
During a chemical reaction, one or more new compounds
are formed as a result of the rearrangement of atoms.
The chemical reaction can be represented by a
chemical equation ( 化 学 反 应 式 ), which shows the
substances about to react, called reactants (反应物), to
the left of an arrow that points to the newly formed
substances, called products (产物).
Law of mass conservation ( 质 量 守 恒 定 律 ):
matter is neither created nor destroyed during a
chemical reaction. The finding of oxygen
resulted from this law.
The chemical equation must be balanced (平衡).
9.2 Chemists use relative masses to
count atoms and molecules
How to measure a specific number of atoms and
molecules?
By relative masses or molar mass (摩尔质量).
2H2
+
2 moles
Which is
4 grams
Which is
12.04×1023molecules
O2
1 mole
Which is
32 grams
Which is
6.02×1023molecules
=
H2O
2 moles
which is
36 grams
which is
12.04×1023molecules
9.3 Reaction rate (反应速率) is
influenced by concentration and
temperature
How fast is a given reaction? Kinetics (动力学)
Reaction rate can be determined by how fast the
product generates or the reactant disappears.
time
Low concentration of
products, high
concentration of
reactants
Reactants
Products
high concentration of
products, low
concentration of
reactants
The factors influencing the reaction rate:
(1). How often do the reactant molecules meet?
Concentration!
(2). How often do the encountered reactant molecules lead
to a reaction? High enough kinetic energy to break the
chemical bonds in the reactants, temperature!
(3). ….
Fig 9.7 during a reaction, reactant molecules collide with each other
Reaction rate
• mA + nB
lC
• Rate = k[A]m[B]n
• Arrhenius Equation: k = Ae-Ea/RT
– Where Ea is the activation energy, A is
frequency factor, which is related to the
frequency of collisions and the probability that
the collisions are favorably oriented for
reaction.
Broken bonds are a necessary first step in most of
chemical reactions. The energy required for this
in initial breaking of bonds can be viewed as an
energy barrier. The minimum energy required to
overcome this energy barrier is known as the
activation energy (活化能).
At any given temperature, there is a wide
distribution of kinetic energies in reactant
molecules. When the temperature of
reactions is increased, the number of
reactant molecules having sufficient
energy to pass over the barrier also
increases, leading to increased reaction
speed.
Discussion topics: reaction selectivity
Sometimes, electromagnetic fields, such as
sunlight, may also lead to reactions. Question:
What is the advantage and disadvantage of
application of electromagnetic fields in
chemical reactions?
Direction of collision on reaction selectivity?
Synthesis of protein: group protection
• Amino acids are linked through peptide bonds (肽键). A
group of amino acids linked together through peptide
bonds is called peptide. Peptides containing more than
ten amino acids are generally called polypeptides.
9.4 Catalysts (催化剂) increase the rate of chemical
reactions
• A third way to increase the rate of a reaction is to
add a catalyst, which is any substance that increases
the rate of a chemical reaction by lowing its
activation energy. The catalyst may participate as a
reactant, but it is then regenerated as a product.
• Cl + O3
ClO + O2
• ClO + O3
Cl + O2
• One chlorine atom in the ozone layer can
catalyze the transformation of 100,000
ozone molecules to oxygen molecules in
one or two years.
• Catalytic converter
• To convert nitrogen monoxide (NO) to nitrogen and
oxygen, carbon monoxide to carbon dioxide. In 1960, a
typical car emitted about 11 grams of uncombusted fuel,
4 grams of nitrogen oxide (NOx) , and 84 grams of
carbon monoxide per mile traveled. In 2000, emitted less
than 0.5 gram of uncombusted fuel, 0.5 gram of nitrogen
oxide (NOx) , and 3 grams of carbon monoxide.
Discussion: Homogeneous reaction (catalysis)
and heterogeneous reaction (catalysis)
• Advantage and disadvantage?
• Example: protein synthesis
9.5 Chemical reaction can be either
exothermic (放热的) or endothermic (吸热
的)
For the chemical reactions taking place in burning wood, there is a net
release of energy. For those taking place in a photosynthetic plant,
there is a net absorption of energy.
•
•
•
•
•
•
•
•
•
•
The energy absorbed as a bond breaks or released as one forms is called
bond energy (键能). Each chemical bond has its own characteristic bond
energy.
2H2 + O2
2H2O
Type of bond Number of moles
Bond energy
Total energy
H-H
2
+436 kJ/mole
+872 kJ.mole
O-O
1
+498 kJ/mole
498 kJ/mole
Total energy absorbed
+1370 kJ
Type of bond Number of moles
Bond energy
Total energy
H-O
4
-464 kJ/mole
- 1856 kJ/mole
Total energy released
- 1856 kJ
Net energy of reaction = energy absorbed + energy released = - 486 kJ
Bond
Bond energy(KJ/mol)
Bond
Bond energy(KJ/mol)
H-H
436
O-O
138
H-C
414
Cl-Cl
243
H-N
389
N-N
159
H-O
464
N=O
631
H-F
569
O=O
498
H-Cl
431
O=C
803
H-S
339
N=N
946
C-C
347
C=C
837
Table 9.1
selected
bond
energies
In an exothermic reaction, the product molecules are at a lower
potential energy than the reactant molecules. The net amount of energy
released by the reaction is equal to the difference in potential energies
of the reactants and products