Chemistry and Energy
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Transcript Chemistry and Energy
How are they related?
CHEMISTRY AND ENERGY
Energy Encountered Daily
Is Heat Used or Released?
Endothermic reactions used heat from the
surroundings
Sweating
Refrigeration
Exothermic heat releases heat to the
surroundings
Hot hands
Combustion
Exercise
Endothermic Reactions
Decrease in kinetic energy decrease in
temperature heat will transfer from the
environment to the system resulting in a
cooler environment
Absorbs heat from its surrounding.
The system gains heat
Positive value for q
H = q = 0
Hproducts Hreactants
Exothermic Reactions
Increase in kinetic energy increase in
temperature of system heat released to
the environment resulting in a hotter
environment
Releases heat to its surroundings
The system loses heat
Negative value for q
H = q = 0
Hproducts Hreactants
Enthalpy
Heat content for systems at constant
pressure
Symbol is H
Terms heat and enthalpy are used
interchangeably for this course
H = q = m C T
Heat moves from ________ to ___________.
Law of Conservation of
Energy
Energy is not lost or gained in a chemical
reaction
In a chemical reaction potential energy is
transferred to kinetic energy
Thermochemical Equations
An equation that includes the heat change
Example: write the thermochemical equation
for this reaction
CaO(s) + H2O(l) Ca(OH)2(s)
H = -65.2 kJ
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ
Stoichiometry and Thermochemistry
Tin metal can be extracted from its oxide
according to the following reaction:
SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ
Sn(s) + 4HNO3(aq)
How much energy will be required to
extract 59.5 grams of tin?
How to solve
1. Use your stoichiometry
2. Treat heat as a reactant or product
SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ
Sn(s) + 4HNO3(aq)
59.5 g Sn 1 mol Sn 192 kJ
1
g Sn 1 mol Sn
If an Object feels hot, it means it is
undergoing a change with a H that
is:
a. positive
b. negative
c. whether the object feels hot or not is
unrelated to its H
d. I don’t know
If the object feels hot, it means it is
undergoing:
a. an exothermic reaction
b. an endothermic reaction
c. whether it feels hot or not is unrelated
to whether it is undergoing an
exothermic or an endothermic change
How does ice melt?
HEAT DURING A CHANGE OF
STATE
Molar Heat of Fusion
Heat absorbed by one mole of a substance
during melting
Constant temperature
Hfus
H2O(s) H2O(l)
H = 6.01 kJ/mol
Molar Heat of Solidification
Heat lost when 1 mole of a liquid solidifies
Temperature is constant
Hsolid
Hfus = -Hsolid
H2O(l) H2O(s) H = -6.01 kJ/mol
Molar Heat of Vaporization
Heat needed to vaporize 1 mole of a liquid
Hvap
H2O(l) H2O(g)
Hvap = 40.7 kJ/mol
Molar Heat of Condensation
Heat released when 1 mole of vapor
condenses
Hcond
H2O(g) H2O(s)
Hvap = -Hcond
Hcond = -40.7 kJ/mol
Phase Change Diagram for Water
Phase Change Diagram
The House that Heats Itself
http://www.sciencefriday.com/videos/watch/
10007
CALORIMETRY
Calorimetry
Method used to determine the heat involved
in a physical or chemical change.
Relies on the law of conservation of energy
Calorimeter
Simple Calorimeter
Calorimetry Math
Heat gained by the water = q
Heat lost by the system = -q
mCT = q
T = Tf –Ti , m = mass, C = specific heat
q gained by water = q lost by system
q water = - q system
mCT = -mCT
(mass H2O)(spec. heat H2O)(T H2O) = - (mass
sys)(spec. heat sys)(T sys)