Chapter 5
Thermochemistry
Terms I
• Thermochemistry
– Study of
–
• Kinetic Energy
– Energy of
• Potential Energy
– Energy of
Units of E
• JOULE
• Calorie vs. calorie
– 1 Cal =
– 1 cal is the amount of heat necessary to
raise the temperature of 1 g of water 1oC
• 1 cal =
J
Terms II
• Force
–A
• Work
– Energy used to move an object
–W=
• Energy
– Capacity to do
First Law of Thermodynamics
• Energy
•ΔE =
Equation II
•ΔE =
• q = heat in or heat out of system
• w = work done on or by the system
Practice
• Calculate the change in the internal
energy of the system for a process in
which the system absorbs 140 J of heat
from the surroundings and does 85 J of
work on the surroundings.
Endothermic Reaction
• Process when a system
• Gets
• ΔH >
Exothermic Reaction
• Process when a system
• Gets
• ΔH <
State Function
• Describes the condition of a system
• Does not matter how the system got
there
• For example…
ENTHALPY
• Heat…
–
• Represented by
• Is a
Enthalpy Equation
• ΔH = Hfinal – Hinitial = qp
• P indicates constant pressure
Enthalpy of Reaction
• ΔHrxn = H(products) – H(reactants)
• 2H2O (g)  2H2 (g) + O2 (g) ΔH =
• 2H2 (g) + O2(g)  2H2O(g) ΔH =
Return
Rules for Enthalpy and RXNS
1. Enthalpy is an extensive property
2. Enthalpy for a reaction
3. Enthalpy change depends on state of
reactants and products
Practice
• 2Mg(s) + O2(g)  2MgO(s) ΔH=-1204 kJ
• Is this
• How much heat is transferred when 2.40 g of
Mg are reacted?
• How many grams of MgO are produced
during an enthalpy change of 96.0 kJ?
Continued
• 2Mg(s) + O2(g)  2MgO(s) ΔH=-1204 kJ
• How many kJ of heat are absorbed when
7.50 g of MgO(s) are decomposed into
magnesium and oxygen at constant
pressure?
Calorimetry
• The study of
• Heat capacity (C):
– Temperature change of an object as it
– Amount of heat required to
– Greater heat capacity means…
Calorimetry
• Molar heat capacity (Cmolar)
• Specific heat capacity (Cp)
–
• Equation:
Calorimetry
• What is the heat required to raise
400.00 g of water by 34.50oC?
• What is the heat lost when 200.00 g of
iron changes from 115.50 oC to 22.00
oC?
Calorimetry
• Large beds of rocks are used in some
solar heated homes to store heat.
Assume that the specific heat of rock is
0.082 J/g-K. Calculate the quantity of
heat absorbed by 50.0 kg of rocks if
their temperature increases by 12.0 oC
Flashback…
• Law of conservation of energy (First law
of thermodynamics):
–
–
Calorimetry
• You have heated a 55.00 g piece of
iron, Cp = 0.385 J/g-K, to 200.00 oC.
You then put the iron into water in a
calorimeter. There are 300.00 g of
water at 22.00 oC. What is the final
temperature of the mixture?
Calorimetry
• In the calorimetry lab you will be mixing
an acid and a base and studying the
temperature changes.
• You mix 35.00 mL each of 1 M HCl and
1 M NaOH in a calorimeter. The
temperature increases from 21.0 to
27.5 oC. What is the enthalpy change
for the reaction in kJ/mol HCl?
Hess’s Law
Enthalpy of Formation
• Energy change for the formation
• Symbolized by Hf
• Tables are for standard conditions
– 1 atm
– 25oC
• Appendix C
Enthalpy of Formation
• Units for Hof are in
• Magnitude depends on state
• 2H2(g) + O2(g)  2 H2O(l) Hof = -285.8
• 2H2(g) + O2(g)  2 H2O(g) Hof = 241.8
Enthalpy of Formation
• By definition: the standard enthalpy of
formation of the most stable form of
ANY element is 0 kJ/mol
• Hof C(s) = 0
• Hof H2(g) = 0
Enthalpy of RXN
• Horxn = Σn Hof (products) - Σm Hof
(reactants)