Chapter 18
18.1

The heat given off by the
corrosion reaction of an
iron-magnesium alloy with
salt water can produce a
hot meal. The rate of
reaction is increased by
adding salt water, so heat
is produced rapidly. You
will learn some ways in
which the rate of a reaction
can be increased.
18.1

Collision Theory
◦ How is the rate of a chemical change expressed?
 In chemistry, the rate of chemical change, or the
reaction rate, is usually expressed as the amount of
reactant changing per unit time.
18.1
 Rates of chemical reactions are often measured as a
change in the number of moles during an interval of
time.
18.1
According to collision theory, atoms, ions, and
molecules must collide to react.
A reaction will occur if:
1. The colliding particles have enough kinetic energy.
2. The particles collide with the correct orientation
Remember: energy is absorbed to break bonds and
released when bonds form
18.1
 Effective Collision
18.1
 Ineffective Collision

When substances react, the following
sequence of events occurs:
◦ Reactant(s) particles collide
◦ If with sufficient energy and orientation, existing
bonds break
◦ Particles rearrange
◦ New bonds form product(s)

Potential energy changes associated with
these steps can be illustrated graphically:
18.1
Exothermic Potential Energy Diagram
18.1
◦ The minimum energy needed by colliding reactant
particles is called the activation energy.
◦ The activated complex is an unstable arrangement
of atoms that forms momentarily at the peak of the
activation-energy barrier.
◦ The difference in energy between the reactants and
products is called the heat of reaction, ∆H
∆H = Hprod – Hreact
Exothermic reactions have a -∆H
Endothermic reactions have a +∆H
Endothermic Potential Energy Diagram
18.1

Factors Affecting Reaction Rates
◦ What four factors influence the rate of a chemical
reaction?

The rate of a chemical reaction depends upon temperature,
concentration, surface area, and the use of a catalyst.
18.1
Temperature
• Storing foods in a refrigerator keeps them fresh
longer. Low temperatures slow microbial action.
18.1
Concentration
a. In air, a lighted splint glows and soon goes out.
b. When placed in pure oxygen (higher oxygen
concentration), the splint bursts into flame.
18.1
Particle Size
 The minute size of the reactant particles (grain
dust), and the mixture of the grain dust with
oxygen in the air caused the reaction to be
explosive, destroying the grain elevator.
18.1
Catalysts provide an alternate pathway that requires
less activation energy
What is the
effect on ∆H with
a catalyst?
18.2

In the early 1900s, German
chemists refined the process
of making ammonia from
elemental nitrogen and
hydrogen.This process
allows the manufacture of
nitrogen fertilizers. You will
learn how reaction
conditions can influence the
yield of a chemical reaction.
18.2
◦ A reversible reaction is one in which the conversion
of reactants to products and the conversion of
products to reactants occur simultaneously.
 In a closed system, reversible reactions can attain
equilibrium.
1. At chemical equilibrium, the rate of the forward reaction
equals the rate of the reverse reaction.
2. At chemical equilibrium, no net change occurs in the
concentrations of reactants and products.
18.2
 If the rate of the shoppers
going up the escalator is
equal to the rate of the
shoppers going down, then
the number of shoppers on
each floor remains constant,
and there is an equilibrium.
18.2
SO2 and O2
react to give
SO3
SO3
decomposes
to SO2 and O2
At equilibrium, all three types of molecules are present.
18.2
 What is EQUAL at chemical equilibrium?
 What is CONSTANT at chemical equilibrium?
18.2
◦ The French chemist Le Châtelier proposed what has
come to be called Le Châtelier’s principle: If a stress
is applied to a system in dynamic equilibrium, the
system changes in a way that relieves the stress.

What three stresses can cause a change in the equilibrium
position of a chemical system?
◦ Stresses that upset the equilibrium of a chemical
system include changes in the concentration of
reactants or products, changes in temperature, and
changes in pressure.
◦ Concentration
 Rapid breathing during and after vigorous exercise
helps reestablish the body’s correct CO2:H2CO3
equilibrium, keeping the acid concentration in the
blood within a safe range.
◦ Temperature
 Dinitrogen tetroxide is a colorless gas; nitrogen
dioxide is a brown gas. The flask on the left is in a
dish of hot water; the flask on the right is in ice.
 2NO2(g) ⇆ N2O4(g) + heat
 Pressure – ONLY AFFECTS EQUILIBRIA WITH GASES
 Pressure affects a mixture of nitrogen, hydrogen, and
ammonia at equilibrium
 N2(g) + 3H2(g) ⇆ 2NH3(g)
18.4

Inside a pile of oily rags or a
stack of hay that has not been
thoroughly dried,
decomposition causes heat to
build up. When heat cannot
escape, the temperature can
become high enough to cause
a fire. You will learn about the
conditions that will produce a
spontaneous chemical
reaction.
18.4
◦ Entropy (S) is a measure of the disorder
(randomness) of a system.
18.4
 For a given substance, the entropy of the gas is greater
than the entropy of the liquid or the solid. Similarly,
the entropy of the liquid is greater than that of the
solid.
18.4
 Entropy increases when a substance is divided into
parts.
18.4
 Entropy tends to increase in chemical reactions in which
the total number of product molecules is greater than the
total number of reactant molecules.
18.4
 Entropy tends to increase when temperature
increases. As the temperature increases, the
molecules move faster and faster, which increases
the disorder.

Enthalpy is a measure of the heat content
(∆H) of a substance.
◦ Endothermic reactions make products with
increased enthalpy.
◦ Exothermic reactions make products with decreased
enthalpy.
◦ A spontaneous reaction occurs naturally and favors
the formation of products at the specified
conditions.
18.4
Spontaneous reactions tend to
1. create products with low enthalpy (energy), and
2. create products with high entropy (disorder;
randomness).
18.4