Lecture 17c- Thermochemistry
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Bellwork‐ Hot water 1.Why is steam poten7ally more dangerous than hot water? 2. Which has a higher specific heat, water or sand? Explain using examples BUT STEAM CAN BURN LIKE FIRE WITH AN EXTRA 40.67 kJ/mol OF ENERGY. AND it is oEen hoIer than 100°C
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The last lecture for chapter 17 (honors and prep chemistry) covers thermochemical equations and section 17.4 for honors only.
Transcript of Lecture 17c- Thermochemistry
Bellwork‐Hotwater
1.Whyissteampoten7allymoredangerousthanhotwater?
2.Which has a higherspecific heat, water orsand?
Explain using examples
BUTSTEAMCANBURNLIKEFIREWITHANEXTRA40.67kJ/molOFENERGY.ANDitisoEenhoIerthan100°C
Let’sfillinthechartonyourhomework!
Bellwork‐Endoorexo2
Arethefollowingprocessesendothermicorexothermic?
1.Burningwood2.Boilingwater3.Diges7ngsugar4.Mixingtwoaqueoussolu7ons,thecombina7ongetscold.
5.ΔH=‐380kJ/mol6.Egg+heatover‐easy
Athermochemicalequa7ontellsustheamountofenergythatisexchangedduringtheprocess
C3H8(g)+O2(g)CO2(g)+H2O(g)ΔH=‐2220kJ
2NaHCO3(s)Na2CO3(s)+H2O(g)+CO2(g)ΔH=129kJ
C3H8(g)+O2(g)CO2(g)+H2O(g)ΔH=‐2220kJ
C3H8(g)+O2(g)CO2(g)+H2O(g)+2220kJ
2NaHCO3(s)Na2CO3(s)+H2O(g)+CO2(g)ΔH=129kJ
129kJ+2NaHCO3(s)Na2CO3(s)+H2O(g)+CO2(g)
TheycanbewriYenintwoways
C3H8(g)+O2(g)CO2(g)+H2O(g)ΔH=‐2220kJ
2220kJofenergyareproducedforeverymoleofC3H8(g)thatisburned.
5molesx‐2220kJ/mole=‐11,100kJ
Howmuchenergyisproducedby5moles?
2NaHCO3(s)Na2CO3(s)+H2O(g)+CO2(g)ΔH=129kJ
129kJisabsorbedbyevery2molesofNaHCO3(s)thatisdecomposed.
Whatistheenergychange(ΔH)forthedecomposi7onof25gofNaHCO3(s)?
25gx1mole84g
=.298moles 129kJ2molx =19.22kJ
Sec7on17.4‐HonorsonlyCalcula7ngHeatsofReac7on
Emeraldsarecomposedoftheelementschromium,aluminum,silicon,oxygen,andberyllium.
Whatifyouwantedtodeterminetheheatofreac7onwithoutactuallybreakingthegemsdowntotheircomponentelements?
Hess’slawallowsyoutodeterminetheheatofreacVonindirectly.
Hess’slawofheatsumma7onstatesthatifyouaddtwoormorethermochemicalequa7onstogiveafinalequa7on,thenyoucanalsoaddtheheatsofreac7ontogivethefinalheatofreac7on.
Manipulate the given equations toobtain the “equation of interest”.
change sign of∆H
switchingreactants &products
Flip reactions
Multiply ∆H bysame number
Multiply allcoefficientsby the samenumber
Multiplyreactions
Add ∆H valuesA →BPlus C→DGivesA+C →B+D
Add reactions
BUT YOU MUSTBYYOU CAN
StandardHeatsofForma7onForareac7onthatoccursatstandardcondi7ons,youcancalculatetheheatofreac7onbyusingstandardheatsofforma7on.
17.4
Thestandardheatofforma7on(∆Hf°)ofacompoundisthechangeinenthalpythataccompaniestheforma7onofonemoleofthecompoundfromitselementswithallsubstancesintheirstandardstatesat25°C.
17.4
17.4
TheStandardHeatofForma7onofWater
17.4
StandardHeatsofFormaVon17.4
