chapter
17: REACTION ENERGY AND REACTION
KINETICS
Reaction
Energy – Thermodynamics
Energy
is transferred in two ways:
indirectly (through empty space
like radiant energy) or directly (through particles contacting each
other). Energy is transferred between a
system and it's surroundings. In
chemistry the system can be a compound and it's energy changes or a chemical
reaction. Energy is transferred because
of a difference in energy, from areas of high energy to areas of low energy
until the two areas are the same energy.
Heat
is energy transferred due a difference in temperature. Heat is transferred from areas of high
temperature to areas of low temperature.
(Temperature is a measure of the average kinetic energy of the
particles.) The symbol for heat is
q. Heat is measured in Joules (J) or
kiloJoules (kJ). This unit is similar to a more familiar unit, the calorie.
A
calorimeter is an instrument used to measure energy.
Specific
heat is the energy needed to change the temperature of one gram of a substance
one degree Celsius. The symbol for
specific heat is Cp. The
unit for specific heat is J/g.ºC or J/g.K. The specific heat value for water when in
the liquid phase is 4.184 J/g.ºC, it is different for the solid and
vapor phases. The Cp value for most solids and liquids remains
fairly constant. The Cp value for gases varies with the temperature,
pressure and volume of the gas. Cp values can be found in Table17-1 on page 513
and on Table A-8 on the handout. All
elements and compounds have their own Cp value.
The
amount of heat lost or gained depends on three things: 1) how much of the substance is
present, 2) the magnitude of the
temperature change, and 3) the type of substance. These are reflected in the algebraic
equation that expresses this concept: 
. Here m stands for the mass of the substance
(the amount), ∆ T (read delta T) stands for the change in temperature,
and Cp is the specific heat
value. 
,
where Ti is the initial temperature and Tf is the final
temperature. If a substance is releasing
heat ∆T will be negative and so will q.
If a substance is absorbing heat ∆T will be positive and so will
q.
Since
energy is neither created nor destroyed it must be transferred between two
objects, therefore the heat that is released or lost by an object must be
absorbed or gained by some other object.
If two objects are in the same system and one loses heat then the other
must gain heat. The amount of heat lost
must be equal in magnitude, but opposite in sign, to the amount of heat
gained. Heat is exchanged until the
entire system comes to one final temperature.
The final temperature of everything is the same (both objects end up at
the same temperature).
Energy
also accompanies all phase changes.
During a phase change, the temperature remains constant until all of the
substance has changed phase. The amount
of energy that is required for a substance to melt is called the heat of fusion
(∆Hfus). The amount of
energy that is required for a substance to vaporize is called the heat of vaporization
(∆Hvap). The opposites
are called heat of crystallization and heat of condensation. The units for any of these is J/g or
kJ/mol.
Figure
1 , below, shows the relationship between temperature and energy change:
AÒ B
B Ò A
B B XX
B
Ò C
C Ò B
C XX C
C
Ò D D Ò C
D D XX
D
Ò E E Ò D
E XX E
E
Ò F F Ò E
fig 1 fig
2 fig
3
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Chemical
reactions always involve a change in energy.
Energy is stored in chemical bonds.
A chemical reaction involves the breaking of the chemical bonds in the
reactant(s) and the forming of new chemical bonds in the product(s).
Reactants
are written on the left side of an equation.
They are the starting materials, the substance(s) that enter into a
chemical reaction. Products are written
on the right side of the equation. They
are the ending materials, the new substance(s) formed in a chemical reaction.
Thermodynamics
is the study of the interaction of energy and matter, flow of energy between
substances.
Standard
thermodynamic conditions are room temperature, 25 ºC, and standard atmospheric
pressure, 1.00 atm or 101.3 kPa.
Activation
energy is the minimum amount of energy needed to start a chemical
reaction. (Energy needed to form the
activated complex.) The activated
complex is the species formed when reactants collide with enough energy to meet
the activation energy requirement.
Energy
diagrams, figures 2 & 3, above, show how energy changes as a chemical
reaction proceeds:
A
Ò D Ò
B
Ò E Ò
C
Ò
A
state function is a thermodynamic quantity: the quantity is determined by the
initial and final conditions only and not on how it got there (path
independent). Examples are:
temperature, pressure, volume, enthalpy, entropy, internal energy, and free energy.
Internal
energy is the energy of the system in relation to it's surroundings. It is represented by U. The algebraic equation is: 
;
where q is the heat flow between the system and it's surroundings and 
, and w is the work between the system and
it's surroundings and 
. Heat can be released to the surroundings
(-q) or absorbed from the surroundings (+q); work can be done on the system by
the surroundings (+w) or by the system on the surroundings (-w). If the system is in isolation there is no
interaction between the system and the surroundings so ∆U = 0. Internal energy is a state function.
Isothermal
implies a constant temperature.
Isobaric implies a constant pressure.
Enthalpy
is the energy or heat content of a system.
It is the energy needed to break the chemical bonds in the reactants and
form new chemical bonds in the products.
It is represented by H. The
algebraic equation is: 
. (
is
formation from the elements, 
is at standard thermodynamic conditions) Nature tends towards low energy. The ∆H of elements in their
"normal" state is zero (the "normal" state of oxygen is O2(g), of sodium is Na(s), etc).
Enthalpy is a state function and is measured in kJ. Values for 
can be found in you textbook.
An
endothermic reaction: 1) absorbs more
energy than it releases; 2) tends to be
non-spontaneous at room temperature; 3)
Hproducts > Hreactants; and 4) ∆H > 0.
An
exothermic reaction: 1) releases more
energy than it absorbs; 2) tends to be
spontaneous at room temperature; 3) Hproducts
< Hreactants; and 4) ∆H < 0
A
thermodynamically stable compound does not decompose spontaneously at room
temperature because it's ∆Hf > 0 (it would take energy to decompose it). A kinetically stable compound will decompose spontaneously at
room temperature but does so very slowly as to be almost imperceptible.
Entropy
is the amount of disorder in a system.
It is represented by S. Values
for 
can be found in Table A-8 on the
handout. The algebraic equation
is: 
.
Nature
tends towards disorder. ∆S < 0
is a decrease in disorder and tends to be non-spontaneous.
∆S > 0 is an increase in
disorder and tends to be spontaneous.
Examples of systems increasing in entropy are when: going from solid to liquid or gas; going from liquid to gas; there are more molecules formed; a mixture is formed; or
the volume of gas increases.
Entropy is a state function and is measured in J/K.
Free
energy is the energy available to do work.
In chemistry it is the chemical potential energy of a system. It is represented by G. Values for 
can be found in Table A-8 on the
handout. It can be determined two ways
using two different algebraic equations:
, or
by looking at the interaction between energy and disorder, 
(T is temperature and must be in
Kelvin). When ∆G > 0 there is
an increase in free energy and the reaction is non-spontaneous. When ∆G < 0 there is a decrease in
free energy and the reaction is spontaneous.
∆G can also be zero. When
this happens the system is said to be in equilibrium. All reactions tend towards having ∆G = 0. Free energy is a state function and is
measured in kJ.
Reactions
tend to be spontaneous when ∆H < 0, when ∆S > 0 and when
∆G < 0.
Hess's
Law states that the net heat of reaction ∆Hnet is the sum of
the heats of reactions of the series of equations that add up to the overall
reaction. 
When dealing with Hess's Law there are two
major conditions that need to be considered:
1) Are major compounds on the correct side? If not, the equation must be switched around
the –>.
That makes
a) products become reactants and reactants become products,
b) endothermic becomes exothermic and exothermic becomes
endothermic, and
c) sign of ∆H is reversed; and
2) Are the number of moles in the series equation the same
as in the net equation? If not, the series
equation must be multiplied by some
number to equal the moles in the net equation.
The ∆H must also be multiplied by this number.
chapter
17: REACTION ENERGY AND REACTION
KINETICS
Reaction Kinetics – Rate of
Reaction
Reactants are the substances that enter
into a chemical reaction, written on the left side of the equation.
Products are the substances that are formed
from a chemical reaction, written on the right side of the equation.
Spontaneous reaction: when Gibbs free energy ∆G < 0; this occurs when ∆H < 0 and ∆S > 0, or at high
temperatures when ∆H > 0 and ∆S > 0, or at low temperatures
when ∆H < 0 and ∆S < 0.
Two ways for a substance to be stable:
1.
thermodynamically - ∆G > 0 , not a spontaneous reaction
2.
kinetically - ∆G < 0 but happens at an extremely slow rate as
to be imperceptible
What happens in a chemical reaction? breaking of the bonds in the reactants and
the forming of new bonds in the products
1.
the simplest reactions go to completion - the reactants, or at least one
of them, are completely used up; when
this occurs the reaction stops
2.
most reaction don't go to completion - reaction proceeds forward with
the reactant(s) making product(s) and then at some point the reaction reverses
so the product(s) begins to make the reactant(s)
Reaction rate is a measure of the speed
of the transformation of reactants to products in a chemical reaction. It can be measured by measuring the rate of
the disappearance of reactant(s) or the rate of the appearance of the
product(s). It is measured in
concentration units per time, mol/L/sec.
Energy diagram
1.
activation energy - minimum energy needed for a reaction to take
place, energy needed to form the
activated complex
2.
activated complex - species formed when reactants collide with enough
energy to meet the activation energy requirement
3.
forward versus reverse reaction
4.
reactants, products, ∆H
5.
catalyzed reaction
The Collision Theory explains what needs
to happen for a chemical reaction to take place à must have energetic collisions:
1.
reactants must collide
2.
the energy of the collisions must meet activation energy requirement
3.
the reactants must have the correct spatial orientation with respect to
each other when they collide
Factors Affecting the Reaction Rate, the
Mechanism Involved (why it is affected), and the Effect on the Reaction Rate
(how it is affected)
1.
nature of reactants
A. ionic reactions (double displacement and some acid-base
reactions) - don't involve electron transfer
and the rearrangement of the electron clouds; less energy is needed for the reaction to take place; the reactions happen rapidly, fast reaction
rate
B. covalent reactions - involves bond rearrangement or electron
transfer, the electron clouds must be rearranged; this requires more energy than the transfer of ions in ionic
reactions; the reactions are slower
2.
concentration
A. usually measured in mol/L;
a square bracket around a substance’s chemical formula refers to the
concentration in mol/L - [substance]
B. if you increase the number of molecules there are to react; more collisions will occur; the reaction rate is increased
3.
temperature
A. increasing the temperature increases the kinetic energy (speed)
of the molecules; there is a greater chance for more collisions; the rate is increased
B. when you increase the temperature each molecule has more
energy; there is a greater chance they
can satisfy the activation energy requirement upon collision; the rate is increased;
C. the increase in energy is more important in increasing the
reaction rate than the effect of increasing the number of collisions
4.
catalyst - a substance that changes the rate of a chemical reaction
without being consumed by the reaction
A. positive - increases the rate by forming an activated complex
which requires a lower activation energy
1.
heterogeneous - also called a contact catalyst
a. the catalyst is in a different phase then
the reactants
b. it provides a surface to react on and works
by adsorbing one of the reactants upon its surface; this weakens the bonds of the adsorbed substance and less energy
is needed for the reaction to take place;
reaction rate is increased
c. the surface can hold the reactant in a more
favorable position for a reaction to occur;
this puts the reactant in the correct spatial orientation; reaction rate is increased
2.
homogeneous
a. the catalyst is in the same phase as the
reactants
b. the catalyst enters into the reaction by
forming an intermediary species; the
intermediary has a lower activation energy requirement therefore less energy is
needed for the reaction to take place;
reaction rate is increased
c. the catalyst is released from the
intermediary and returned unchanged
B. negative - called an inhibitor or antioxidant; this slows down a reaction by tying up one of the reactants or the
catalyst to prevent it from reacting
5.
surface area - like concentration - if the reaction occurs at an
interface, increasing the surface area gives more area for the reaction to
occur upon; more collisions occur; the rate is increased
6.
pressure - only for gases - like concentration - if the pressure on
gases is increased it decreases the volume which increases the density
(concentration); more collisions
occur; the rate is increased
The Rate Expression
1.
rate is directly dependent upon concentration
2.
rate = k[reactant]
3.
k is the specific rate constant, it depends on the size and kind of
molecules and is specific for a given temperature (there is a different k for every temperature)
4.
the rate expression can only be determined experimentally
Reaction Mechanism
1.
reactions actually occur in a series of steps
2.
the series of steps are called the mechanism
3.
the slowest step in the mechanism is called the rate determining step
A. it determines how fast the overall reaction will occur
B. it determines the rate expression
4.
the mechanism and the rate determining step can only be determined
experimentally