thermochemistry. thermochemistry is concerned with the heat changes that occur during chemical...
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Thermochemistry
ThermochemistryThermochemistry is concerned with the
heat changes that occur during chemical reactions.
Can deal with gaining or losing heat
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)
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
UniverseThe system and its surroundings make up
the universeSystem – part of the universe you focus onSurroundings – everything else in the
universe
EndothermicA process that absorbs heat from the
surroundingsHeat flowing into a system from its
surroundings is defined as positive q
heat
system
ExothermicA process that releases heat to its
surroundingsHeat flowing out of the system to its
surroundings is defined as negative q
heat
system
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
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?
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
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
Calorimeters
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
Thermochemical EquationAn equation that includes the heat of
change
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ
Heat of ReactionThe heat change for the equation exactly
as written.
Heats of reaction at constant pressure (1atm @ 25o C) reported as ΔH
Cont.Exothermic Endothermic
CaO(s) + H2O(l)
Ca(OH)2(s)
ΔH = -65.2kJ ΔH + 129kJ
Na2CO3(s) + H2O(g) + CO2(g)
2NaHCO3(s)
Exothermic CaCl2(s) Ca2+(aq) + 2Cl-(aq)
ΔHsoln = -82.8kJ/mol
H2O(l)
Endothermic
NH4NO3(s) NH4+(aq) + NO3-(aq)
ΔHsoln = 25.7 kJ/mol
H2O(l)
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
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
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
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
Molar Heat of Condensation (ΔHcond) The amount of heat released when 1 mol of
vapor condenses
H2O(g) H2O(l) ΔHcond = -40.7kJ/mol
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)
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
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
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
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
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)