CHEM 1210 Lecture Notes - Lecture 18: Hotchkiss H35, Heat Capacity

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Lecture 18 October 1st
Constant pressure reaction
Either “h” or qp
1. This is a state function (path independent)
2. An extensive property (depends on amount)
■ CH4+2O2–>CO2+2H2O
1 mole of methane
H= -890 KJ (this means an exothermic reaction)Δ
Because it is extensive: using half of each reactant means that half of the
product is produced
** positive H meansΔ
endothermic
Negative H meansΔ
exothermic
3. H changes sign when the reaction is reversedΔ
■ CO2+2H2O—>CH4+2O2
H= 890 KJΔ
Now endothermic
4. Phases of matter:
2H O (l) → 2H O(g) 2 2
H= 88 KJ (endothermic)Δ
Example:
○ CH4+2O2–>CO2+2H2O
H= 890 KJΔ
If 24.0 grams of methane were reacted?
Every mole of methane produced 890 KJ of heat
1. Convert 24.0 grams of methane to mol
2. Multiply by 890 KJ of heat
Have KJ of heat from 24.0 grams of CH4
1340 KJ of heat produced
Heat comes from forming bonds
Thermochemical reactions
Constant pressure → think enthalpy
R→ P+heat exothermic H= -Δ
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Document Summary

Because it is extensive: using half of each reactant means that half of the. H= -890 kj (this means an exothermic reaction) product is produced. H changes sign when the reaction is reversed. Every mole of methane produced 890 kj of heat. Have kj of heat from 24. 0 grams of ch 4. It always takes energy to break bonds. Always endothermic if bonds are being broken. Going from gas to liquid, there is even more energy released, h goes down. In a reaction bonds need to be broken first, then a reaction can happen. No matter the whole reaction, as long as first and final steps are the same, we know h . Hess had a large impact on arrhenius and gibbs. His law basically stated: you can produce the same amount of heat by doing things differently. 2a+ 5b a b h= -90 kj. What is enthalpy change for 2ab +b a b.

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