Chemistry and Energy
description
Transcript of Chemistry and Energy
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How are they related?
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Energy Encountered Daily
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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
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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
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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
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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 ___________.
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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
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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
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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?
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How to solve
1. Use your stoichiometry2. Treat heat as a reactant or
productSnO2(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
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If an Object feels hot, it means it is undergoing a change with a H that is:
a. positiveb. negativec. whether the object feels hot
or not is unrelated to its Hd. I don’t know
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If the object feels hot, it means it is undergoing:
a. an exothermic reactionb. an endothermic reactionc. whether it feels hot or not is
unrelated to whether it is undergoing an exothermic or an endothermic change
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How does ice melt?
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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
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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
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Molar Heat of Vaporization Heat needed to vaporize 1 mole
of a liquid Hvap H2O(l) H2O(g) Hvap = 40.7
kJ/mol
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Molar Heat of Condensation Heat released when 1 mole of
vapor condenses Hcond
H2O(g) H2O(s) Hcond = -40.7 kJ/mol
Hvap = -Hcond
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Phase Change Diagram for Water
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Phase Change Diagram
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The House that Heats Itself http://www.sciencefriday.com/videos/
watch/10007
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Calorimetry
Method used to determine the heat involved in a physical or chemical change.
Relies on the law of conservation of energy
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Calorimeter
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Simple Calorimeter
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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)