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tnoakes
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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
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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
- Frictionless
NO REAL PROCESS IS REVERSIBLE
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Closed system
only energy can be transferred, not mass
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Work
- Force acting through a distance
- May increase the temperature - no heat required necessarily
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Heat
- Energy transferred due to temperature difference
- Never stored
- Cannot be determined without analysing effect on system
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State functions
- 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
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Pathway functions
- dependent on how system changes from one state to another
- eg. W, Q
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Ideal gases definition
- molecules have zero volume
- no intermolecular forces
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When can real gases be modelled as ideal gases?
- at low pressure (eg. atmospheric)
- when compression is slow PV=nRT
- when process is reversible
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Which pressure do you use to calculate the work in an ideal gas expansion?
The resistive pressure, ie. the surroundings pressure
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Joule's experiment
- 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
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Equilibrium
- 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
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Irreversible process
- finite driving force
- energy dissipated: W to U, so not all work is transferred to surroundings
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Enthalpy
- State function: made up of state functions
- H=U+PV
- kJ/kg
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Heat capacity
- NOT ability to store heat - heat is not stored
- state function
- Cp (IG)= 29.099 J/mol K
- Cp=Cv+R for ideal gases
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Work produced in an irreversible process is
- smaller than the work produced in a reversible process
- W(irrev)=W(rev)*n
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Work consumed in an irreversible process is
- greater than the work consumed in a reversible process
- W(irrev)=W(rev)/n
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The final temperature of the system after an adiabatic process is always______in an irreversible process.
- larger
- adiabatic compression - extra W converted to U
- adiabatic expansion - some energy remains in gas as U
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Open energy balance
mass and energy transfer
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Causes of enthalpy changes (4)
- temperature: sensible heat
- phase changes
- reactions
- mixing: compositional changes, solution formation
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When does dH=CpdT apply?
- ideal gas processes
- incompressible fluids when dP is small
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Cocurrent flow
two streams pass through a heat exchanger in the same direction
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Sensible heat
Enthalpy change based solely on temperature different
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Define symbols in Gibbs phase rule, F= 2-Pi+N
- F=dof
- pi=number of phases
- N=number of chemical species
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phase
homogeneous region of matter
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vapour
gas below the critical temperature, Tc, which can be condensed into a liquid
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saturated steam
- water at the vapour pressure for a given temperature OR at the boiling point for that pressure
- mixture of liquid and vapour
- F=1
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superheated steam
- 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
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Why is there an enthalpy change during a phase change?
- H=U+PV
- U: bond breaking to change arrangement
- PV: volume change so work done
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What assumption should be made for mixed flow streams?
ideal gas mixture ie. no interactions between streams (equivalent to ideal gas assumption)
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Why does steam burn worse that boiling water?
latent heat>sensible heat (vertical gradient)
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What sign does the stoichiometric coefficient have for reactants?
negative
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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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