Thermochemistry Spring Semester Final Exam Review
Jan 01, 2016
Thermochemistry
Spring Semester Final Exam Review
THERMOCHEMISTRY
The study of heat released or required by chemical reactions
Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt)
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + energy
What is Energy?
Energy
Kinetic energy (KE)
Potential energy (PE)
Energy due to motion
Stored energy
Total Energy = Kinetic Energy + Potential Energy
E = KE + PE
Temperature measures the average
Kinetic energy & potential energy are interchangeable
Heat is the total energy of a system:
Kinetic energy + potential energy
Systems & Surroundings
In thermodynamics, the world is divided into a system and its surroundings
A system is the part of the world we want to study (e.g. a reaction mixture in a flask)
The surroundings consist of everything else outside the system
EXOTHERMIC & ENDOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction) that releases heat to the surroundings.
A release of heat corresponds to a decrease in enthalpy
Exothermic process: H < 0 (at constant pressure)
Burning fossil fuels is an exothermic reaction
Exothermic Reactions
Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat from the surroundings.
An input of heat corresponds to an increase in enthalpy
Endothermic process: H > 0 (at constant pressure)
Photosynthesis is an endothermic reaction (requires energy input from sun)
Endothermic Reactions
Endothermic or Exothermic?
exothermic
endothermic
exothermic
endothermic
endothermic
Heating Curves
A plot of temperature vs. time that represents the process in which energy is added at a constant rate
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Heating Curves Animation
Tem
pera
ture
(o C
)
40
20
0
-20
-40
-60
-80
-100
120
100
80
60
140
Time
Melting - PE
Solid - KE
Liquid - KE
Boiling - PE
Gas - KE
Heating Curves
A plot of temperature vs. time that represents the process in which energy is added at a
constant rate
The standard enthalpy of reaction (DH0 ) is the enthalpy of a reaction carried out at 1 atm.
rxn
aA + bB cC + dD
DH0rxn dDH0 (D)fcDH0 (C)f= [ + ] - bDH0 (B)faDH0 (A)f[ + ]
DH0rxn nDH0 (products)f= S mDH0 (reactants)fS-
Calculate the heat of combustion of methane, CH4
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
H◦f CH4 (g) = -74.86 kJ/mol
H◦f O2(g) = 0 kJ/mol
H◦f CO2(g) = -393.5 kJ/mol
H◦f H2O(g) = -241.8 kJ/mol
15
pg. 316
Example Problem
2 mol(-241.8 kJ/mol) = -483.6 kJ
Step #1: multiply the H◦f H2O(g) by 2 since there are two moles of water in the products .
Calculate the heat of combustion of methane, CH4
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
H◦f CH4 (g) = -74.86 kJ
H◦f O2(g) = 0 kJ/
H◦f CO2(g) = -393.5 kJ
H◦ fH2O(g) = -483.6 kJ
16
pg. 316
Example Problem
Step #2: sum up all the H◦ f. :
H◦f = [-393.5 kJ + (-483.6 kJ)]- [-74.86 kJ + (0 kJ )]
H◦f = -802.2 kJ
Hrxn = Hf(products) - Hf(reactants)
Calculations Involving Specific Heat
TmCq
C = Specific Heat Capacityq = Heat lost or gainedT = Temperature
changeTf - Ti
ORTm
qC
Choose all that apply...
C(s) + 2 S(g) CS2(l) H = 89.3 kJ
Which of the following are true?
A) This reaction is exothermic
B) It could also be writtenC(s) + 2 S(g) + 89.3 kJ CS2(l)
C) The products have higher energy than the reactants
D) It would make the water in the calorimeter colder