5.1 The First Law of Thermodynamics, Enthalpy, and Phase Changes
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| ENERGY | ||
|---|---|---|
| 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 |
Enthalpy (H)
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?
