Enthalpy - Lompoc Unified School District

Enthalpy - Lompoc Unified School District

Enthalpy Internal Energy Equation E = Q + W = Q + PVE = Q + W = Q + PVE = Q + W = Q + PVV If the reaction is carried out at a constant volume (V = 0) , then E = QE = Q + W = Q + PVV = 0) , then E = Q + W = Q + PVE = Q

If volume is constant, any heat added or removed changes the internal energy Constant PVressure If pressure is constant, Q = E = Q + W = Q + PVE + PVE = Q + W = Q + PVV Heat needed to bring about any change is the

sum of internal energy plus PV-V work Constant pressure reactions are common in chemistry Enthalpy

Sum of internal energy (V = 0) , then E = QE) and work (V = 0) , then E = QPVV) It is a state function E = Q + W = Q + PVH = Q = E = Q + W = Q + PVE + PVE = Q + W = Q + PVV Flow of heat is equal to change in enthalpy Enthalpy is called the heat of reaction Since in many reactions the change in volume is

small, E = Q + W = Q + PVH is very often the same as E = Q + W = Q + PVE Enthalpies in Reaction E = Q + W = Q + PVH = H(V = 0) , then E = Qproducts) H(V = 0) , then E = Qreactants) Exothermic Reactions

E = Q + W = Q + PVH is negative 2H2(V = 0) , then E = Qg) + O2(V = 0) , then E = Qg) 2H2O(V = 0) , then E = Qg) + 484 kJ 2H2(V = 0) , then E = Qg) + O2(V = 0) , then E = Qg) 2H2O(V = 0) , then E = Qg)

E = Q + W = Q + PVH = -484 kJ Endothermic Reaction E = Q + W = Q + PVH is positive 68 kJ + N2(V = 0) , then E = Qg) + 2O2(V = 0) , then E = Qg) 2NO2(V = 0) , then E = Qg)

N2(V = 0) , then E = Qg) + 2O2(V = 0) , then E = Qg) 2NO2(V = 0) , then E = Qg) E = Q + W = Q + PVH = + 68 kJ Enthalpy is Extensive It depends on the amount For the reaction below CH4(V = 0) , then E = Qg) + O2(V = 0) , then E = Qg) CO2(V = 0) , then E = Qg) + H2O(V = 0) , then E = Qg) E = Q + W = Q + PVH = 802kJ how much heat is produced when 4.50

g of methane gas is burned? Reverse Reactions If a reaction is reversed its E = Q + W = Q + PVH is numerically the same, but opposite in sign

CO2(V = 0) , then E = Qg) + H2O(V = 0) , then E = Qg) CH4(V = 0) , then E = Qg) + O2(V = 0) , then E = Qg) E = Q + W = Q + PVH = +802kJ Different State Have Different E = Q + W = Q + PVH Enthalpy change is different for different

states of matter of reactants and products 2H2(V = 0) , then E = Qg) + O2(V = 0) , then E = Qg) 2H2O(V = 0) , then E = Qg) E = Q + W = Q + PVH = -484 kJ 2H2(V = 0) , then E = Qg) + O2(V = 0) , then E = Qg) 2H2O(V = 0) , then E = Ql) E = Q + W = Q + PVH = -572kJ PVotential Energy Diagrams Used to represent the general change in

energy over the course of a reaction Activation Energy Minimum amount of energy required to initiate a chemical reaction

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