CHEMICAL THERMODYNAMICS
NANIK DWI NURHAYATI,S.Si, M.Si
www.nanikdn.staff.uns.ac.id
(0271) 821585, 081556431053
Enthalpy and Enthalpy Change of Chemical
Reactions
• We have already investigated energy transfers during physical
transformations, what about chemical transformations –
reactions?
H = Hproducts – Hreactants
• An endothermic reaction (H > 0) is a reaction in which heat is
absorbed from the surroundings.
• An exothermic reaction (H < 0) is a reaction in which heat is
given off to the surroundings.
Thermochemical Equations
• Thermochemical Equation: a balanced chemical
equation (including phase labels) with the molar
enthalpy of reaction written directly after the equation
N2 (g) + 3 H2 (g)  2 NH3 (g); H = -91.8 kJ
a) Molar Interpretation: When 1 mol of nitrogen gas reacts
with 3 mol of hydrogen gas to form 2 mol of ammonia
gas, 91.8 kJ of energy is given off.
Heat Transfer
Heat Capacity (C): quantity of energy required to
increase the temperature of a sample by one
degree
C = q/T
The magnitude of the heat capacity depends on:
– Mass of the sample
– Composition of the sample
Calculate the heat capacity of an aluminum block that must
absorb 629 J of heat from its surroundings in order for its
temperature to rise from 22 C to 145 C.
Heat Capacity (C):
C =q/T
C = 629 J / (145 - 22 C)
= 629 J / 123 C
= 5.11 J/ C
Heat Capacity
When comparing the heat capacities of different
substances with different masses, it is more useful to
compare specific heat capacities.
Specific Heat capacities (c): quantity of energy needed
to increase the temperature of one gram of a
substance by one degree Celsius
• Molar Heat Capacity (cm): related to specific heat,
but for one mole of substance
Heat Capacity
When dealing with specific heat capacities (c):
Given: c = q/mT
Derive:
q = cmT
T = Tfinal – Tinitial = q/cm
m = q/cT
q = thermal heat
c = specific heat
m = mass
T = change in temperature
What will be the final temperature of a 5.00 g silver ring
at 37.0 C that gives off 25.0 J of heat to its surroundings
(c = 0.235 J/g C)?
T = Tfinal – Tinitial = q/cm
Tfinal – 37.0 C = -25.0 J / (0.235 J/g C)(5.00 g)
Tfinal – 37.0 C = -21.3 C
Tfinal = 37.0 C - 21.3 C
Tfinal = 15.7 C
148 J of heat are transferred to a a piece of glass (c =
0.84 J/gC), raising the temperature from 25.0 C to
49.4 C. What is the mass of the glass?
m = q/cT
m = (148 J)/(0.84 J/gC)(24.4 C)
m = 7.2 g
Phase Changes
• We just saw that energy transfers ALWAYS
accompany temperature changes.
• Energy transfers also accompany physical and
chemical changes, even when there is no change in
temperature.
• eg. Energy is always transferred into or out of a
system during a phase change.
Melting/Freezing
Heat of Fusion: quantity of thermal energy that must be
transferred to a solid as it melts (qfusion = - qfreezing)
Water:
Heat of fusion =
+333 J/g at 0 C.
Specific Heat (l) =
1.00 cal/gC
Specific Heat (s)
depends on T:
 0.5 cal/gC
near 0 C