5.1 The First Law of Thermodynamics, Enthalpy, and Phase Changes
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|Energy – the ability to do work (SI unit is Joules)||4.18J = 1 cal||1 Cal = 1kcal = 1000cal|
1st Law of Thermodynamics
Energy cannot be created or destroyed (conservation of energy).
ΔE = q + w q = heat w = work PV work (w = -PΔV)
|Signs for q and w|
|q > 0 heat is transferred into the system|
|q < 0 heat is transferred out of the system|
|w > 0 work is done on the system|
|w < 0 work is done by the system|
ΔH = qp
Enthalpy is a state function (depends only on the initial and final state of the system (independent of path).
Endothermic (ΔH > 0) vs. Exothermic reactions (ΔH < 0)
2H2(g) + O2(g) ➔ 2H2O(l) ΔH°rxn = -572kJ
What is the enthalpy change when 3 moles of O2 are consumed in the reaction above?
What is the enthalpy change when 9g H2O(l) are produced in the reaction above?