Thermochemistry. Thermochemistry is concerned with the heat changes that occur during chemical...
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Transcript of Thermochemistry. Thermochemistry is concerned with the heat changes that occur during chemical...
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Thermochemistry
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ThermochemistryThermochemistry is concerned with the
heat changes that occur during chemical reactions.
Can deal with gaining or losing heat
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EnergyThe capacity for doing work or supplying
heat.- Energy is only detected because of its effects
Energy stored within the structural units of chemical substances is called Chemical Potential Energy (Gasoline)
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Heat (q)Energy that transfers from one object to
another because of a temperature difference between them
Heat is not detectable, only changes caused by heat
Heat always flows from a warmer object to a cooler object
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UniverseThe system and its surroundings make up
the universeSystem – part of the universe you focus onSurroundings – everything else in the
universe
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EndothermicA process that absorbs heat from the
surroundingsHeat flowing into a system from its
surroundings is defined as positive q
heat
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system
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ExothermicA process that releases heat to its
surroundingsHeat flowing out of the system to its
surroundings is defined as negative q
heat
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system
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Calorie vs. JouleCalorie – the quantity of heat needed to
raise the temperature of 1g of water 1oC1 Calorie = 1kilocalorie = 1000 calories
Joule – the quantity of heat needed to raise the temperature of 1g of water 0.2390oC1 J = 0.2390 cal 4.184J = 1 cal
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Heat CapacityThe amount of heat needed to increase the
temperature of an object exactly 1oCThe greater the mass of the object, the
greater its heat capacity.Which has more heat capacity, a drop of
water, or an entire pool?
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Specific HeatThe amount of heat it takes to raise the
temperature of 1g of the substance 1oCThe higher the specific heat, the longer it
takes to heat up.
q
m x ΔTC = =
heat
mass (g) x change in temp
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CalorimetryThe accurate and precise measurement of
heat change for chemical and physical processes.
The heat released by the system is equal to the heat absorbed by its surroundings
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Calorimeters
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Enthalpy (H)For systems at constant pressure, the heat
content is the same as a property called enthalpy of the system
If a reaction occurs at constant pressure, enthalpy can be used interchangeably with heat.
q = ΔH
ΔH = m x C x ΔT
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Thermochemical EquationAn equation that includes the heat of
change
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ
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Heat of ReactionThe heat change for the equation exactly
as written.
Heats of reaction at constant pressure (1atm @ 25o C) reported as ΔH
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Cont.Exothermic Endothermic
CaO(s) + H2O(l)
Ca(OH)2(s)
ΔH = -65.2kJ ΔH + 129kJ
Na2CO3(s) + H2O(g) + CO2(g)
2NaHCO3(s)
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Exothermic CaCl2(s) Ca2+(aq) + 2Cl-(aq)
ΔHsoln = -82.8kJ/mol
H2O(l)
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Endothermic
NH4NO3(s) NH4+(aq) + NO3-(aq)
ΔHsoln = 25.7 kJ/mol
H2O(l)
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Heat of CombustionThe heat of reaction for the complete
burning of one mole of a substance.
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + 890kJ
CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
OR
ΔH = - 890kJ
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Molar Heat of Fusion (ΔHfus)The heat absorbed by one mol of a
substance in melting from a solid to a liquid at a constant temperature.
H2O(s) H2O(l) ΔHfus = 6.01kJ/mol
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Molar Heat of Solidification(ΔHsolid)The heat lost when one mole of a liquid
solidifies at a constant temperature
H2O(l) H2O(s) ΔHsolid = -6.01kJ/mol
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Molar Heat of Vaporization (ΔHvap)The amount of heat necessary to vaporize
one mole of a given liquid
H2O(l) H2O(g) ΔHvap = 40.7kJ/mol
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Molar Heat of Condensation (ΔHcond) The amount of heat released when 1 mol of
vapor condenses
H2O(g) H2O(l) ΔHcond = -40.7kJ/mol
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Molar Heat of Solution (ΔHsoln) The heat change caused by dissolution of
one mole of substance
NaOH(s) Na+(aq) + OH-(aq)
ΔHsoln = -445.1 kJ/mol
H2O(l)
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Hess’s LawIndirect method to measure heat reaction
If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction
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Cont.Diamond to Graphite
a. C(s, graphite) + O2(g) CO2(g) ΔH= -393.5kJ
b. C(s, diamond) + O2(g) CO2(g) ΔH= -395.4kJ
Write equation a in reverse to cancel out unwanted information
c. CO2(g) C(s,graphite) + O2(g) ΔH= 393.5kJ
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Cont.Now, add equation b and c
b. C(s, diamond) + O2(g) CO2(g) ΔH= -395.4kJ
c. CO2(g) C(s,graphite) + O2(g) ΔH= 393.5kJ
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Cont.
b. C(s, diamond) + O2(g) CO2(g) ΔH= -395.4kJ
c. CO2(g) C(s,graphite) + O2(g) ΔH= 393.5kJ
• Now, add equation b and c
C (s, diamond) C (s, graphite) ΔH= -1.9kJ
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Standard Heat of Formation (ΔHf
o)The change in enthalpy that accompanies
the formation of one mole of a compound from its elements with all substances in their standard states at 25oC
ΔHo = ΔHfo (products) – ΔHf
o (reactants)