# Thermodynamics 1st law

 The flashcards below were created by user tnoakes on FreezingBlue Flashcards. 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 conditionsNo path hysteresisNever more than differentially removed from equilibrium so transverses a series of equilibrium statesDriving force must be differential and have no resistanceFrictionless NO REAL PROCESS IS REVERSIBLE Closed system only energy can be transferred, not mass Work Force acting through a distanceMay increase the temperature - no heat required necessarily Heat Energy transferred due to temperature differenceNever storedCannot be determined without analysing effect on system State functions defined by a system's current set of conditions (P,T,V), independent of the pathway used to achieve themeg. U, H, S, P, V, T Pathway functions dependent on how system changes from one state to anothereg. W, Q Ideal gases definition molecules have zero volumeno intermolecular forces When can real gases be modelled as ideal gases? at low pressure (eg. atmospheric)when compression is slow PV=nRTwhen process is reversible Which pressure do you use to calculate the work in an ideal gas expansion? The resistive pressure, ie. the surroundings pressure Joule's experiment showed the U=f(T) for an ideal gasgas expansion in insulated water bath, temperature change measured and found to be negligiblethis is because IGs have no intermolecular forces so no energy can be stored Equilibrium system unchanging on macroscopic scale due to no net driving forcesystems where resistance=driving force~=0 are not at true equilibrium but can be modelled with thermodynamics if process is slowthermodynamics: systems at equilibrium or transitioning between equilibrium states Irreversible process finite driving forceenergy dissipated: W to U, so not all work is transferred to surroundings Enthalpy State function: made up of state functionsH=U+PVkJ/kg Heat capacity NOT ability to store heat - heat is not storedstate functionCp (IG)= 29.099 J/mol KCp=Cv+R for ideal gases Work produced in an irreversible process is smaller than the work produced in a reversible processW(irrev)=W(rev)*n Work consumed in an irreversible process is greater than the work consumed in a reversible processW(irrev)=W(rev)/n The final temperature of the system after an adiabatic process is always______in an irreversible process. largeradiabatic compression - extra W converted to Uadiabatic expansion - some energy remains in gas as U Open energy balance mass and energy transfer Causes of enthalpy changes (4) temperature: sensible heatphase changesreactionsmixing: compositional changes, solution formation When does dH=CpdT apply? ideal gas processesincompressible fluids when dP is small Cocurrent flow two streams pass through a heat exchanger in the same direction Sensible heat Enthalpy change based solely on temperature different Define symbols in Gibbs phase rule, F= 2-Pi+N F=dofpi=number of phasesN=number of chemical species phase homogeneous region of matter vapour gas below the critical temperature, Tc, which can be condensed into a liquid saturated steam water at the vapour pressure for a given temperature OR at the boiling point for that pressuremixture of liquid and vapourF=1 superheated steam dry steam above the vapour pressure at a given T OR above the boiling point at that pressureF=2 (define both T and P)useful for turbines - no water droplet damage Why is there an enthalpy change during a phase change? H=U+PVU: bond breaking to change arrangementPV: 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? negative How are adiabatic reactors different? System is insulated so enthalpy change of rxn results in a larger sensible heat changeadiabatic exit temperature is higher for an exothermic rxn Authortnoakes ID243187 Card SetThermodynamics 1st law DescriptionThermodynamics 1st law, ench291 Updated2013-10-29T08:34:28Z Show Answers