Hybrid and Electric Automobiles
The fuel cell shown here is
based on the combination
reaction:
H2 + O2  H2O
Definitions
Thermodynamics  The study of energy and it’s
transformations.
Thermochemistry  The thermodynamics of
chemical reactions.
Energy  The capacity to do work or transfer heat.
Work  The energy required to move an object
against an opposing force.
W = F  d
Heat  Derived from the movements of atoms and
molecules (including vibrations and rotations).
Types of Energy
Kinetic Energy  The energy of motion, energy in
action.
Ek = (1/2)mv2
Potential Energy  The energy of position, stored
energy.
Work & Chemical Reactions  For the vast
majority of chemical reactions there are two types of
work that can happen.
– Mechanical Work – done by creating/destroying a
gas (i.e. automobile cylinder, air bag, etc.)
– Electrical Work – A redox reaction flowing
through an external circuit (i.e. battery, fuel cell)
1st Law of Thermodynamics
Energy is neither
created nor destroyed
(conservation of
energy)
Internal Energy = heat + work
DE
= q + w
Enthalpy
Enthalpy (H)  The change in enthalpy,
DH, is defined as the heat gained or lost
by the system under constant pressure.
DH = qp
Where qp = the heat flow at constant
pressure
Properties of Enthalpy
1. Enthalpy is a state function.
2. Enthalpy is an extensive property.
3. Enthalpy is reversible. If we reverse
the reaction the magnitude of DH
remains the same but the sign changes.
4. The enthalpy change depends upon the
states of the reactants and products.
Constant-Volume (Bomb) Calorimetry
Commonly used for
studying reactions,
especially combustion
reactions.
Because pressure isn’t
constant, the bomb
calorimeter measures
DE rather than DH
Constant-Pressure Calorimetry
We’ll use this kind of
calorimeter in lab
1. Calibrate the calorimeter
in order to determine its
specific heat.
2. The calorimeter minimizes
heat transfer into or out of
the system.
3. By measuring the change in
the temperature of the
solution, we can calculate the
heat given off in a reaction.
Energy Stored in Carbs/Sugar
The body breaks carbohydrates down into glucose (blood
sugar), C6H12O6, which is then combusted in our bodies
C6H12O6 (s) + 6O2(g)  6CO2(g) + 6H2O(l) DH° = -2803 kJ
In terms of energy released per gram of glucose one mole
of glucose has a mass of 180 g so that
-2803 kJ/(180 g/mol) = 16 kJ/g
Fuel values for foods are typically reported in Calories,
which is stands for Kilocalories or kcal
1 Cal = 1000 cal
1 cal = 4.184 J
1 kcal = 4.184 kJ
So for glucose 16 kJ/g x (1 kcal/4.18 kJ) = 3.7 kcal/g
Energy Stored in Fats
If we take a typical fat, such as tristearin, C57H110O6 the
reaction with oxygen in our cells is:
2C57H110O6 (s) + 163 O2(g)  114CO2(g) + 110H2O(l) DH° = -75,520 kJ
In terms of energy released per gram one mole of
tristearin has a mass of 891 g so that
-75,520 kJ/(2891 g/mol) = 42 kJ/g
In terms of kcal (Calories) tristearin would be
42 kJ/g x (1 kcal/4.18 kJ) = 10 kcal/g
In general carbohydrates and proteins have an average
fuel value of 4 kcal/g and fats are 9 kcal/g. This is
what you see as Calories on food labels.
Energy Stored in Fuels
Octane (2,3,5 trimethyl pentane, DHº = -255 kJ/mol)
2 C8H18 (l) + 25 O2(g)  16 CO2(g) + 18 H2O(g) DH° = -10,138 kJ
-10,138 kJ/(2114 g/mol) = -44 kJ/g
Ethanol (DHº = -278 kJ/mol)
C2H5OH (l) + 3 O2(g)  2 CO2(g) + 3 H2O(g) DH° = -1234 kJ
-1234 kJ/(31 g/mol) = -40 kJ/g
Methane (DHº = -75 kJ/mol)
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) DH° = -802 kJ
-802 kJ/(16 g/mol) = -50 kJ/g
Hydrogen (DHº = 0 kJ/mol)
2 H2 (g) + O2(g)  2 H2O(g) DH° = -484 kJ
-484 kJ/(22 g/mol) = -121 kJ/g