The total heat gained by the calorimeter + water
contents = 4.28 x 103 J + 6.89 x 103 J
= 1.117 x 104 J
So we can write the following equivalence statement:
0.569 g benzoic acid  1.117 x 104 J
The molar mass of benzoic acid = 122 g/mol
Therefore the amount of heat released when 1 mol of
benzoic is burned is:
121
= 1 mol benzoic acid x
1.117x104 J
(122g benzoic acid)(
)
1mol benzoic acid 0.569g benzoic acid
= 2.39 x 103 kJ
122
= 1 mol benzoic acid x
1.117x104 J
(122g benzoic acid)(
)
1mol benzoic acid 0.569g benzoic acid
= 2.39 x 103 kJ
In this case, since the volume is constant, we have
actually determined the change in internal energy
of benzoic acid to be -2.39 x 103 kJ/mol.
123
124
125
A short list of common “reaction” types
Heat of “Reaction”
Example
126
A short list of common “reaction” types
Heat of “Reaction”
Example
Enthalpy of solution: NH4NO3(s)
H2O
NH4+(aq) + NO3-(aq)
127
A short list of common “reaction” types
Heat of “Reaction”
Example
Enthalpy of solution: NH4NO3(s)
Enthalpy of dilution:
H2SO4(aq)
H2O
NH4+(aq) + NO3-(aq)
H2O
H2SO4(aq)
128
A short list of common “reaction” types
Heat of “Reaction”
Example
Enthalpy of solution: NH4NO3(s)
Enthalpy of dilution:
Enthalpy of fusion:
H2SO4(aq)
H2O(s)
H2O
NH4+(aq) + NO3-(aq)
H2O
H2SO4(aq)
H2O(l)
129
A short list of common “reaction” types
Heat of “Reaction”
Example
Enthalpy of solution: NH4NO3(s)
Enthalpy of dilution:
Enthalpy of fusion:
H2SO4(aq)
H2O(s)
Enthalpy of vaporization:
H2O
NH4+(aq) + NO3-(aq)
H2O
H2SO4(aq)
H2O(l)
H2O(l)
H2O(g)
130
A short list of common “reaction” types
Heat of “Reaction”
Example
Enthalpy of solution: NH4NO3(s)
Enthalpy of dilution:
Enthalpy of fusion:
H2SO4(aq)
H2O(s)
Enthalpy of vaporization:
H2O
NH4+(aq) + NO3-(aq)
H2O
H2SO4(aq)
H2O(l)
H2O(l)
H2O(g)
Enthalpy of reaction: MgCl2(s) + 2 Na(s)
2 NaCl(s)
+ Mg(s)
131
Applications of thermochemistry
1. BB
2. SHSC
132
Chemical Kinetics
133
Chemical Kinetics: The study of rates and
mechanisms of chemical reactions.
134
Chemical Kinetics: The study of rates and
mechanisms of chemical reactions.
The word rate means the change of a certain
quantity with time. In the present case, it will be a
change of concentration of a reactant or product
with time that will be of interest.
135
Two Key Questions
If a reaction goes, how fast?
136
Reaction Rate: A measure of how rapidly a reaction
occurs. It is the change of a reactant or product
concentration divided by the time interval required
for the change to occur.
137
Reaction Rate: A measure of how rapidly a reaction
occurs. It is the change of a reactant or product
concentration divided by the time interval required
for the change to occur.
Reaction Mechanism: The sequence of elementary
steps that lead to product formation.
138
Factors that affect the reaction rate.
139
Factors that affect the reaction rate.
1. Nature of the reactants: Elements and compounds
in general, have differing reactivities. That is,
different tendencies toward bond formation and
bond breaking.
140
Factors that affect the reaction rate.
1. Nature of the reactants: Elements and compounds
in general, have differing reactivities. That is,
different tendencies toward bond formation and
bond breaking.
2. The ability of the reactants to meet: The gas
phase and liquid phase allow the possibility for
reactants to intermingle on the molecular level.
141
Factors that affect the reaction rate.
1. Nature of the reactants: Elements and compounds
in general, have differing reactivities. That is,
different tendencies toward bond formation and
bond breaking.
2. The ability of the reactants to meet: The gas
phase and liquid phase allow the possibility for
reactants to intermingle on the molecular level.
The solid phase is generally a very poor medium
for chemical reactions.
142
The effect of surface area on reaction rate.
143
If all the reactants are in the same phase (e.g. all in
solution) the reaction is called a homogeneous
reaction.
144
If all the reactants are in the same phase (e.g. all in
solution) the reaction is called a homogeneous
reaction.
If all the reactants are not in the same phase (e.g. a
gas reacting with a solid surface) the reaction is
called a heterogeneous reaction.
145
If all the reactants are in the same phase (e.g. all in
solution) the reaction is called a homogeneous
reaction.
If all the reactants are not in the same phase (e.g. a
gas reacting with a solid surface) the reaction is
called a heterogeneous reaction.
Factors such as molecular shape have an extremely
important bearing on whether reactive centers in
different reactants can meet.
146
3. The concentration of the reactants. This is
obviously closely linked to number 2 above.
147
3. The concentration of the reactants. This is
obviously closely linked to number 2 above.
4. The temperature of the system.
148
3. The concentration of the reactants. This is
obviously closely linked to number 2 above.
4. The temperature of the system.
5. The presence of catalysts.
149
3. The concentration of the reactants. This is
obviously closely linked to number 2 above.
4. The temperature of the system.
5. The presence of catalysts.
A catalyst is defined as follows: A substance that
increases the rate of reaction without being used
up.
150
Note: This definition of a catalyst does not exclude
the possibility that the catalyst undergoes some
chemistry. If it does in some step, it has to be
regenerated in a sequent step in the overall
reaction scheme.
151
Rate of Reaction
The reaction rate is the change in concentration of
a particular reactant or product. The unit of
concentration most commonly employed is
mol/liter, that is the molar concentration unit. Two
common time units are seconds or minutes.
152
Rate of Reaction
The reaction rate is the change in concentration of
a particular reactant or product. The unit of
concentration most commonly employed is
mol/liter, that is the molar concentration unit. Two
common time units are seconds or minutes.
The most common unit of reaction rate is thus:
(mol/liter)/second = mol l-1 s-1 = Ms-1
153
Consider the reaction:
O
CH3C Cl + H2O
acetyl chloride
CH3CO2H + HCl
154
Consider the reaction:
O
CH3C Cl + H2O
CH3CO2H + HCl
acetyl chloride
The rate of the reaction can be defined as the
change of the reactant concentration over a
certain time interval.
155
Consider the reaction:
O
CH3C Cl + H2O
CH3CO2H + HCl
acetyl chloride
The rate of the reaction can be defined as the
change of the reactant concentration over a
certain time interval.
[CH3COCl]
rate  
time interval
156
Consider the reaction:
O
CH3C Cl + H2O
CH3CO2H + HCl
acetyl chloride
The rate of the reaction can be defined as the
change of the reactant concentration over a
certain time interval.
[CH3COCl]
rate  
time interval
That is:
[CH3COCl]
rate  
t
157
Delta notation:
[ X ]  [ X ] final  [ X ]initial
158
Delta notation:
[ X ]  [ X ] final  [ X ]initial
t  t final  tinitial
Often the initial time is taken as zero seconds.
159
Note that the negative sign in the definition is
to ensure that the rate is a positive quantity.
rate  
[CH3COCl]
t
160
Note that the negative sign in the definition is
to ensure that the rate is a positive quantity.
rate  
[CH3COCl]
t
The concentration of the CH3COCl is
decreasing, so [CH3COCl] is a negative
quantity, hence the negative sign is needed to
make the rate positive.
161
When the stoichiometric coefficients are not equal
to unity, they need to be explicitly taken into
account in the definition of the rate.
162
When the stoichiometric coefficients are not equal
to unity, they need to be explicitly taken into
account in the definition of the rate.
For the generic reaction:
aA + bB
cC + dD
163
When the stoichiometric coefficients are not equal
to unity, they need to be explicitly taken into
account in the definition of the rate.
For the generic reaction:
aA + bB
cC + dD
the rate is given by
rate   1 [A]   1 [B]  1 [C]  1 [D]
a t
b t c t
d t
164
Example:
2 HI(g)
H2(g) + I2(g)
165
Example:
2 HI(g)
H2(g) + I2(g)
The rate of reaction in terms of HI is
rate   1 [HI]
2 t
166
Kinetic data for the hydrolysis of acetyl chloride.
Time (sec)
[CH3COCl]
[CH3CO2H]
0
1.20
0
2.0
1.05
0.15
4.0
0.93
0.27
6.0
0.81
0.39
8.0
0.71
0.49
10.0
0.63
0.57
---------------------------------------------------------------From the above date, the rate over the first 4
second interval is:
167
rate = -
(0.93 – 1.20) M
4.0 s
= 0.068 Ms-1
168
rate = -
(0.93 – 1.20) M
4.0 s
= 0.068 Ms-1
If you calculate the rate over the same time interval
at later times you will find the rate of reaction is not
constant.
169
rate = -
(0.93 – 1.20) M
4.0 s
= 0.068 Ms-1
If you calculate the rate over the same time interval
at later times you will find the rate of reaction is not
constant.
For most reactions, the rate constant does not
remain constant as the reaction progresses.
170
rate = -
(0.93 – 1.20) M
4.0 s
= 0.068 Ms-1
If you calculate the rate over the same time interval
at later times you will find the rate of reaction is not
constant.
For most reactions, the rate constant does not
remain constant as the reaction progresses.
The above calculation of the rate constant is
unsatisfactory in the sense that it only gives us
average rates.
171
C2H4(g) + O3(g)
C2H4O(g) + O2(g)
172
In practice, we are interested mainly in the rate of a
reaction at a specific time, and not in the average
rate, which is arbitrary – because its value depends
on the time interval we choose.
173
In practice, we are interested mainly in the rate of a
reaction at a specific time, and not in the average
rate, which is arbitrary – because its value depends
on the time interval we choose.
We can eliminate this arbitrariness by calculating
the rates over smaller and smaller time intervals.
174
In practice, we are interested mainly in the rate of a
reaction at a specific time, and not in the average
rate, which is arbitrary – because its value depends
on the time interval we choose.
We can eliminate this arbitrariness by calculating
the rates over smaller and smaller time intervals.
When the time interval is made infinitesimally
small, the rate becomes the slope of the
concentration versus time plot (at a specific time).
175
The hydrolysis of acetyl chloride can also be studied
by monitoring the build-up of acetic acid with time.
In this case the rate is given by:
[CH3CO2H]
rate 
t
176
The hydrolysis of acetyl chloride can also be studied
by monitoring the build-up of acetic acid with time.
In this case the rate is given by:
[CH3CO2H]
rate 
t
Note that for a rate expressed in terms of a product
concentration, we do not need a minus sign on the
right-hand side of the equation.
177
The hydrolysis of acetyl chloride can also be studied
by monitoring the build-up of acetic acid with time.
In this case the rate is given by:
[CH3CO2H]
rate 
t
Note that for a rate expressed in terms of a product
concentration, we do not need a minus sign on the
right-hand side of the equation. In this case
[CH3CO2H] is a positive quantity (the concentration
of acetic acid is increasing as the time increases).
178
Rate Laws
179
Rate Laws
The rate of a reaction can be expressed in a second
way. For the hydrolysis of acetyl chloride, we can
write
rate  [CH3COCl]n
180