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)

system and surroundings
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?

phase changes