Thermochemistry
Energy Review
• Energy – the capacity to do work or transfer heat.
• Work – the energy used to move an object against
a force.
• Heat – energy used to cause the temperature of
an object to rise.
• Kinetic energy – energy of motion.
• Potential energy - stored energy based on the
position of the particles.
• Chemistry is all about electrostatic energy.
Electrostatic Interactions
• 𝐸𝑒𝑙 =
𝑘𝑞1𝑞2
𝑑
• q = charge.
• Like charges repel, opposite charges attract.
• The closer they are the stronger the force of
attraction/ repulsion.
• Energy is required to move the particles against
the force.
• These interactions create the chemical potential
energy stored in bonds.
• Changes in their kinetic energy is thermal energy.
Other Definitions and Relationships
• Units of energy are Joules and calories.
• 4.184J = 1 cal; 1000cal = 1Cal
• The calorie is the amount if energy required to raise
one gram of water one degree Celsius.
• Law of Conservation of Energy. (1st Law of Thermo)
• The system is the part that we are studying.
• The surroundings is everything outside of the system.
• Closed systems exchange energy but not matter.
• Isolated systems don’t exchange anything.
Work vs. Heat
• Work is the energy required to put a force on an
object to move it.
• Force is any push or pull on an object.
• Heat is energy transferred from a hot object to a
cold one. (Endo and exothermic)
• Internal energy of the system is the total of all
potential and kinetic energies of the system.
• Rather than focus on total energy, we focus on
changes in energy.
• ∆E = q + w
State Functions
• State functions depend only on the present state of
the system.
• The path used to get to that state is irrelevant.
• Energy is a state function, so any changes in energy
are independent of path as well.
• Energy changes are state functions, but whether that
change was through heat and work is not.
• Enthalpy changes are state functions as well which is
why Hess’ Law works.
• Also explains why enthalpy changes can be found
through Enthalpy of Formation calculations.
• The reaction mechanism has no effect.
Hess’ Law
N2(g) + 2O2(g)  2NO2(g) H = 68 kJ/mol
Can be determined by:
N2(g) + O2(g)  2NO(g)
 H = 180 kJ/mol
2NO(g) + O2(g)  2NO2(g)
 H =- 112 kJ/mol
N2(g) + 2O2(g)  2NO2(g)
 H = 68 kJ/mol
Enthalpy of Formation
• Amount of energy required to make one mole of a
substance from elements.
• ∆Hof = standard enthalpy of formations.
• All ∆Hof for elements = 0 kJ/mol if they are under
standard conditions.
• ∆H = ∆Hof (products) - ∆Hof (reactants)
• These values will be provided when needed.
• Since it is kJ/mole, then the ∆Hof values must be
multiplied by the coefficients in the balanced
equation to get energy.
Enthalpy
• Enthalpy is the total energy of the system plus the
product of pressure and volume.
• H = E + PV
• PV is pressure-volume work
• When the pressure is constant, w = -P∆V
• ∆H = ∆E + P∆V and ∆E = q + w
• Since w = -P∆V
• Then for constant pressure systems, ∆H = q, or
simply ∆H = change in heat.
Calorimetry
• Calorimetry is a way of measuring heat flow.
• We measure the temperature change of a
substance to determine the heat flow.
• We use water since it is easy to measure to
determine the energy change of something we
can’t measure.
• Heat capacity is the amount of energy an object
needs to raise its temperature 1oC
• Specific heat capacity is the amount of energy per
gram.
Calorimetry
• ∆H = m Cp ∆T
• This equation can be used if the substance is not
reacting or changing phase.
• 2 types of situations
– A substance at a different temperature than water is
put in the water and left.
– A reaction is taking place in water
• In both cases, ∆H(H2O) = - ∆H(system)
• Reactions in solutions will require solution to be
treated as water.
Types of Calorimetry
• Constant pressure – done in a container that
will change volume, or done in an open air
container.
• Constant volume – Bomb Calorimeter.
– Seal reaction and water in a container.
– Since V doesn’t change, no work is done, so ∆E = q
– Calculate the heat capacity of the container.
– ∆E = ∆H essentially.