Thermochemistry
Thermochemistry is the study of the heat
released (-H) or absorbed (+H) by chemical
and physical changes.
Thermochemistry
Some terminology
Temperature is defined as the degree of hotness/centigrade scale, °C; kelvin, K/
Heat is a form of energy
Heat capacity is the quantity of heat required to raise the temperature of a substance by 1°C
C = mass x specific heat
Thermochemistry
Specific heat is the heat capacity of one gram of a substance.
Specific heat of water: 4.184 J/(g°C)
Calorimeter is a device used to measure the heat transferred in chemical reactions.
Thermodynamic Systems - Definitions
Isolated System: No matteror energy cross systemboundaries. No work can bedone on the system.
Open System: Free exchangeacross system boundaries.
Closed System: Energy can beexchanged but matter cannot.
Adiabatic System: Special casewhere no heat can be exchangedbut work can be done on thesystem (e.g. PV work).
Thermodynamic Systems - Definitions
Surroundings: is the portion of the universe with which a system interacts
Exothermic reaction: is a chemical reaction in which heat is liberated (-H)
Endothermic reaction: is a chemical reaction in which heat is absorbed (+H)
Thermochemical equations
e.g.
C6H6(l) + 15,O’(g) = 6CO2(g) + 3H2O(l) -H
H = Hproducts – Hreactants
H = heat of reaction
H = enthalpy (heat content P=k)
H is given at 25°C and standard atmospheric pressure
First Law of Thermodynamics
Basic concepts:
1. Work and heat are both forms of energy
2. One form of energy can be converted into another form
3. Energy cannot be created or destroyed
First law of thermodynamics is the law of conservation of
energy
The total energy of the univers is a constant
Internal energy: E
In an isolated system E = constant
not known, cannot be calculated
E = Ef – Ei (f = final) (i= initial)
can be measured
q = + heat absorbed by the system
q = - heat evolved by the system
w = + work done by the system
w = - work done on the system
w = PV(on constant temperature)
E = q-PV
q = E + PV
E = q - w q = heat
W = work
Definition of Enthalpy
We can define a new state variable (one where the path to its current state does not affect its value) called enthalpy:
H = Ei + PV
Enthalpy = Internal Energy + PV
Heat contentq = H
Second Law of Thermodynamics One statement defining the second law is that a
spontaneous natural processes tend to even out the energy gradients in a isolated system.
Can be quantified based on the entropy of the system, S, such that S is at a maximum when energy is most uniform. Can also be viewed as a measure of disorder.
S = Sfinal - Sinitial > 0
The law states that a system will always undergo a spontaneous change in such a way as to increase the entropy
S total = S system + S surroundings
Entropy change: S is
If the reaction is exothermic:
TS – H > 0
The reaction is spontaneous if H < TS
S =q
t
Stotal = S - HT
Change in Entropy
Ssteam > Sliquid water > Sice
Relative Entropy Example:
Third Law Entropies:All crystals become increasingly orderedas absolute zero isapproached (0 K =-273.15°C) and at0 K all atoms are fixedin space so that entropyis zero.
ISOLATED SYSTEM
Spontaneous Reaction Direction
The change in enthalpy for any chemical reaction is constant, whether the reaction occurs in one step or in several steps.
Law of Hess
law of constant heat summation