Thermochemistry
Thermochemistry is the name given to the study of the energy
changes that accompany chemical reactions and changes in the
physical states of matter.
Write the balanced equation for the
combustion of methane, CH4
Thermochemistry is the study of
the energy and heat associated
with chemical reactions and/or Heat is released!
physical transformations
Systems
Type of system affects interactions of the system(area
of study) with its surroundings(area outside the area
of study).
ISOLATED
CLOSED
OPEN
Energy
• Energy is the capacity to do work
• Work is performed when a force acts through a
distance
Work= force x distance
= [m(kg) x a (m s-2)] x d (m)
= kg m2s-2
=unit of energy called the Joule
1 J = 1 kg m2s-2
• Potential energy has the potential to do work
when released
• Kinetic energy is the energy of a moving
object
Energy of molecules
• Kinetic energy – energy of a moving object
– Translational energy of molecules or atoms
– Movement within the molecule
(vibrational)
EK = 1/2mv2
• Potential energy
– Stored as energy within bonds of a
molecule or intermolecular bonds
Energy can neither be created or destroyed, only transferred
between different bodies or to surroundings
Through a chemical or physical change, energy can change its form:
kinetic energy, potential, nuclear, electric, heat, light, etc.
Heat
Heat is energy transferred between a system and its
surroundings as a result of a temperature difference.
Heat “flows” from the hotter body to the colder one until the
average molecular kinetic energies of the two bodies becomes
the same (the temperatures are equal)
Amount of heat needed to cause a certain
temperature change depends on:
oAmount of the substance
oNature of the substance
oTemperature change
Heat capacity
Heat capacity is the quantity of heat
required to change the temperature of
the system by one degree.
If the system is a mole of substance →
molar heat capacity
If the system is a gram of substance →
specific heat capacity
The heat gained by a system is lost from the surroundings
and vice versa (law of conservation of energy)
Q system + Q surrounding = 0
Q system = -Q surrounding
Heat of reaction, Qrxn
The heat of reaction is the quantity of heat exchanged
between a system and its surroundings when a chemical
reaction occurs within the system at a constant temperature
Exothermic Endothermic
Bomb Calorimeter
Qcalor = heat capacity of calorimeter x ΔT
Qrxn = -Qcalor
“Coffee-Cup” Caloriometer
Work
• Chemical reactions can
also be accompanied by
work (in addition to
heat effects)
• An expanding gas from
a chemical reaction can
push up a piston and
thus do work
w = -Pext x ΔV
Internal Energy
• Total energy; kinetic energy and potential energy
• The internal energy of a system can be changed
by heating the system or by doing work on it
• If the system is isolated, its internal energy is
constant, ΔU = 0
ΔU = Q+W
The internal energy of a system is a
state function
State Function
• To describe a systems state, we indicate temperature,
pressure, kinds and amounts of substances present.
• The value of a function of the state depends on the
state of the system, not on the path it took to arrive
there
Heat and work are path-dependent functions
– Their values depend on the path followed when a
system undergoes a change
Initial state
Final state
ΔU = Qrxn + W
At constant volume (like in bomb calorimeter):
ΔU = QV
What is heat of reaction in container open to atmosphere at
constant pressure?
ΔU = QV and ΔU =QP + W
ΔU = QP -P ΔV
QP = ΔU +P ΔV
Enthalpy (H)
Enthalpy is a the total energy of a system including the internal
energy and the energy required to make room for it by
displacing its environment and establishing its volume and
pressure.
H = U + pV
ΔH = ΔU +P ΔV
Enthalpy Change (ΔH)
Enthalpy is the heat of reaction at constant pressure
ΔH = Qp
Summary
Heat capacity (c) is the quantity of heat (Q) required to change the
temperature of the system by one degree. The total amount of heat
required to depends on the amount (m) and temperature change (ΔT).
Internal Energy (U) is the total kinetic and potential energy of a system.
The change in internal energy (ΔU) is the sum of the heat (Q) and work (W)
which changed the system.
ΔU = Q + W
When the volume is constant then the change in internal energy is equal to
Qv (heat at a constant volume).
ΔU = Qv
When the pressure is constant the heat of reaction, Qp (heat at a constant
pressure) is called the change in enthalpy (ΔH)
ΔH = Qp
Enthalpy Change and
Change in State of Matter
Heat is required to change the state of matter of a substance
Heat of fusion: the enthalpy change (ΔH) needed to melt a
substance.
ΔHfus. for H2O is 334J/g
Heat of vaporization: the enthalpy change (ΔH) needed to
vaporize (boil) a substance.
ΔHvap. for H2O is 2260J/g
Enthalpy is a state function
ΔH > 0 (positive) Enthalpy has increased (enthalpy
of products higher than reactants)
Heat is absorbed and the reaction is ENDOthermic
ΔH < 0 (negative) Enthalpy decreased (enthalpy of
products lower than reactants)
Heat is released and the reaction is EXOthermic
Sign convention
Work done on system Work done by system
W>0 (positive) W<0 (negative)
Heat absorbed system Heat released by system
Q > 0 (positive) Q<0 (negative)
SYSTEM Enthalpy of system
Enthalpy of system
decreased
increased
ΔH <0 (negative)
ΔH >0 (positive)
Energy emitted by
Energy absorbed by
system
system
E <0 (negative)
E >0 (positive)
Hess’s Law
• Enthalpy is a state function, so it is not dependant upon the path taken to
get from reagents to products
• If a process occurs in stages or steps (even hypothetically) the enthalpy
change for the overall process is the sum of the enthalpy changes for the
individual steps
Hess’s Law
• ΔH is proportional to amount
A + B → C ΔH = 20kJ
2A + 2B → 2C ΔH = 40kJ
• ΔH changes sign when a process is reversed
A + B → C ΔH = 20kJ
C → A + B ΔH = -20kJ
Hess’ Law
Standard enthalpy of formation, ΔH°f
The enthalpy change occurring in the formation of one mole of
substance in the standard state from the reference forms of the
elements in their standard states.
Standard state of a solid or liquid is
the pure element at a pressure of
105 Pa. The standard state of a gas is
for the pure gas behaving as an ideal
gas at 105 Pa.
Standard enthalpy of reaction
ΔHreaction= ΔHf (products)- ΔHf (reagents)
Bond Enthalpy
The Bond Enthalpy is the energy required to break a chemical
bond. It is usually expressed in units of kJ/mol, measured at 298 K.
ΔH = ∑ ΔH(bonds broken) - ∑ ΔH(bonds formed)
