Week 3 Enthalpy Flashcards
What is enthalpy , H,h of combustion
Internal energy + (pressure * volume) of a thermodynamic system ie. energy required to create system and ‘make room for it’
Can think of it as the ‘total energy content’ of a system
Enthalpy of combustion is the ‘total energy available from a reaction for heating’
Difference between intensive property and extensive property
Intensive is a property that does NOT depend on the amount of material present… e.g t,T… enthalpy per mole,h ….. entropy per mole, s… density, r (usually lowercase letters)
Extensive DOES depend on the amount… e.g total enthalpy, H… total entropy, S… mass, M
What are the equations for total enthalpy
H= hm where h is the enthalpy per unit mole and m is quantity of moles present
What do you assume when applying the first law of thermodynamics
- No work is done (other than that associated with volume change of substances)
- Kinetic energy negligible
What is the heat of reaction in terms of enthalpy of combustion?
Heat of reaction = - enthalpy of combustion
Same thing with minus sign
What is a calorific value ( or heating value )
Standard heat of reaction at constant pressure for a reaction in which a fuel is burned completely.
Higher calorific value (HCV) or (HHT) is where water in products Is in the liquid phase
Lower calorific value (LCV) or (LHV) is where water in products is in the vapour phase
What is combustion efficiency and the equations
A measure of how effectively the heat content of a fuel is transferred into useable heat
ηz = actual mole fraction z in product / mole fraction of z in product for complete combustion
Where z is a mixture/chemical compound e.g. CO2
For adiabatic combustion :
ηc = (actual temp rise in combustion) / ( adiabatic temp rise for complete combustion )
What is heating efficiency equation
η subscript h = useful heat transfer/ max possible heat transfer
