Thermodynamics 1st law
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What is the 1st law?
- Energy is conserved: if one form of energy disappears, another for must reappear simultaneously.
- Change in E(sys)+E(surr)=0
What is a reversible process?
- Can be reversed with an infinitesimal change in external conditions
- No path hysteresis
- Never more than differentially removed from equilibrium so transverses a series of equilibrium states
- Driving force must be differential and have no resistance
NO REAL PROCESS IS REVERSIBLE
only energy can be transferred, not mass
- Force acting through a distance
- May increase the temperature - no heat required necessarily
- Energy transferred due to temperature difference
- Never stored
- Cannot be determined without analysing effect on system
- defined by a system's current set of conditions (P,T,V), independent of the pathway used to achieve them
- eg. U, H, S, P, V, T
- dependent on how system changes from one state to another
- eg. W, Q
Ideal gases definition
- molecules have zero volume
- no intermolecular forces
When can real gases be modelled as ideal gases?
- at low pressure (eg. atmospheric)
- when compression is slow PV=nRT
- when process is reversible
Which pressure do you use to calculate the work in an ideal gas expansion?
The resistive pressure, ie. the surroundings pressure
- showed the U=f(T) for an ideal gas
- gas expansion in insulated water bath, temperature change measured and found to be negligible
- this is because IGs have no intermolecular forces so no energy can be stored
- system unchanging on macroscopic scale due to no net driving force
- systems where resistance=driving force~=0 are not at true equilibrium but can be modelled with thermodynamics if process is slow
- thermodynamics: systems at equilibrium or transitioning between equilibrium states
- finite driving force
- energy dissipated: W to U, so not all work is transferred to surroundings
- State function: made up of state functions
- NOT ability to store heat - heat is not stored
- state function
- Cp (IG)= 29.099 J/mol K
- Cp=Cv+R for ideal gases
Work produced in an irreversible process is
- smaller than the work produced in a reversible process
Work consumed in an irreversible process is
- greater than the work consumed in a reversible process
The final temperature of the system after an adiabatic process is always______in an irreversible process.
- adiabatic compression - extra W converted to U
- adiabatic expansion - some energy remains in gas as U
Open energy balance
mass and energy transfer
Causes of enthalpy changes (4)
- temperature: sensible heat
- phase changes
- mixing: compositional changes, solution formation
When does dH=CpdT apply?
- ideal gas processes
- incompressible fluids when dP is small
two streams pass through a heat exchanger in the same direction
Enthalpy change based solely on temperature different
Define symbols in Gibbs phase rule, F= 2-Pi+N
- pi=number of phases
- N=number of chemical species
homogeneous region of matter
gas below the critical temperature, Tc, which can be condensed into a liquid
- water at the vapour pressure for a given temperature OR at the boiling point for that pressure
- mixture of liquid and vapour
- dry steam above the vapour pressure at a given T OR above the boiling point at that pressure
- F=2 (define both T and P)
- useful for turbines - no water droplet damage
Why is there an enthalpy change during a phase change?
- U: bond breaking to change arrangement
- PV: volume change so work done
What assumption should be made for mixed flow streams?
ideal gas mixture ie. no interactions between streams (equivalent to ideal gas assumption)
Why does steam burn worse that boiling water?
latent heat>sensible heat (vertical gradient)
What sign does the stoichiometric coefficient have for reactants?
How are adiabatic reactors different?
- System is insulated so enthalpy change of rxn results in a larger sensible heat change
- adiabatic exit temperature is higher for an exothermic rxn
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