thermochemistry spring semester final exam review
TRANSCRIPT
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