thermochemistry exothermic reactions release heat to the surroundings. fe 2 o 3 + 2 al 2 fe + al 2...
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![Page 1: Thermochemistry Exothermic reactions release heat to the surroundings. Fe 2 O 3 + 2 Al 2 Fe + Al 2 O 3 + 851.5 kJ Potassium Permanganate Reaction Demo](https://reader036.vdocuments.mx/reader036/viewer/2022062421/56649c955503460f94951be3/html5/thumbnails/1.jpg)
ThermochemistryThermochemistry
Exothermic reactions release heat to the surroundings.
Fe2O3 + 2 Al 2 Fe + Al2O3 + 851.5 kJ
Potassium Permanganate Reaction Demo
Or ΔH = -851.5 kJ
Thermite Reaction Demo
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ThermochemistryThermochemistry
Endothermic reactions absorb heat from the surroundings.
Ba(OH)2 + 2 NH4NO3 + 102.2 kJ
Ba(NO3)2 + 2 NH3 + 10 H2O
Barium Hydroxide Reaction Demo
Or ΔH = +102.2 kJ
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EnthalpyEnthalpy
Energy that is gained or lost by substances during a reaction.
Symbolized by the letter H.
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EnthalpyEnthalpy
Example:Given equal amounts (mass) of
both, which produces more energy: Methane (Natural Gas!) or Octane (Gasoline!)
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EnthalpyEnthalpy
Combustion of Methane (Natural Gas!) CH4(g) + O2(g) CO2(g) + H2O(g)
ΔH = -890.4 kJ (Exothermic!)
22
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EnthalpyEnthalpy
Taken from reaction ratio!
Your furnace turns on and burns 32.0 g of methane, how much heat is produced?
32.0 g CH4
16.0 g CH4
1 mol CH4
1 mol CH4
-890.4 kJ= -1780 kJ
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
ΔH = -890.4 kJ
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EnthalpyEnthalpy
Combustion of Octane (Gasoline!) C8H18(l) + O2(g) CO2(g) + H2O(g)
ΔH = -5430 kJ (Exothermic!)
2 25 16 18
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EnthalpyEnthalpy
Taken from reaction ratio!
Your car turns on and burns 32.0 g of octane, how much heat is produced?
32.0 g C8H18
114.0 g C8H18
1 mol C8H18
2 mol C8H18
-5430 kJ
= -762 kJ
2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
ΔH = -5430 kJ
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EnthalpyEnthalpy
Methane produces more energy when burned…as long as mass is constant.
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Enthalpy of FormationEnthalpy of Formation
This is the amount of energy (enthalpy) that is involved in “creating” a compound.
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Enthalpy of FormationEnthalpy of Formation
The “ΔH” given in a reaction is calculated by using these values in this equation:
)(H - )(H H ReactantsProducts
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Enthalpy of FormationEnthalpy of Formation
Example:CH4 (g) + O2 (g) CO2 (g) + H2O (l)
What is ∆H for this reaction?1st Step - Balance the Equation
2 2
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CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l)
Enthalpy of FormationEnthalpy of Formation
2nd Step – Find Enthalpy Values Using Chart
-75 kJ/mol -393.5 kJ/mol
-286 kJ/mol
Any element that is “pure”, has an enthalpy of 0 kJ/mol
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Enthalpy of FormationEnthalpy of Formation
3rd – Plug into ΔH equation and solve:
ΔH = [ (-286) + (-393.5)] – [(-74.8) + 0]
Coefficient from balanced equation!
ΔH = (Hproducts) – (Hreactants)
2
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Enthalpy of FormationEnthalpy of Formation
Final Answer: -890.7 kJ/molNo significant digits needed since
all the numbers are found on tables!
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Hess’s LawHess’s LawA series of reactions can be added
together to find their overall enthalpy.
N2 + O2 2 NO
2 NO + O2 2 NO2+
N2 + 2 O2 2 NO2
ΔH = +181 kJ
ΔH = -131 kJ
ΔH = +68 kJ
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Hess’s LawHess’s Law
Example: C2H2 + H2 C2H4 ΔH = -174.4 kJ
C2H6 C2H4 + H2 ΔH = 137.0 kJ
What is the enthalpy for the net reaction:
C2H2 + 2 H2 C2H6
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C2H2 + H2 C2H4
C2H6 C2H4 + H2+
C2H2 + 2 H2 C2H6
ΔH = -174.4 kJ
ΔH = 137.0 kJ
ΔH = - 311.4kJ
If you cannot simply add to find the net reaction, “flip” a reaction to make it
work!
C2H4 + H2 C2H6 ΔH = -137.0 kJ