CHE 141 1st Edition Lecture 31Temperature Dependence of the Equilibrium Constant- lnK=-deltaHstatndardrxn/R (1/T)+delta S standard rxn/R- thus the plot of lnK against 1/T yields a straight line with gradient=-deltaH standard rxn/R and y-intercept=delta S standard rxn/R- thus we can calculate delta H standard rxn and delta S standard rxn from a plot of ln K against 1/T- gradient=-deltaHstandardrxn/R- deltaHstandardrxn<0 exothermic- gradient of lnK against 1/T is positive for an exothermic process- delta H standard reaction>0 endothermic- gradient of ln K against 1/T is negative for an endothermic process- lnK=-delta H standard rxn/R (1/T)+delta Sstandardrxn/R- assuming delta S standard reaction is independent of T, K will vary with temperature as ln(K2/K1)=-deltaH standard rxn/R(1/T2-1/T1)- where K1 and K2 are equilibrium constant at T1 and T2 (van’t Hoff equation)Coupled Reactions- coupled reaction: uses a highly spontaneous reaction as a source of energy to power a nonspontaneous reaction, such that the overall coupled reaction is spontaneousThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- we know that delta G standard rxn for a stepwise reaction is equal to the sum of the delta G standard rxn for each of the steps- we calculate overall delta G standard for a coupled reaction in the same way- So long as the magnitude of delta G standard rxn for the highly spontaneous reaction is greater than delta G standard rxn for the nonspontaneous reaction, their sum will result in a negative delta G standard rxn (spontaneous) overall- Free energy allows us to determine whether a reaction is spontaneous- Work must be done to force a reaction to occur in the direction we wish- S(s)+O2(g)SO2(g) delta G standard rxn=-300.1kJo The free energy change for the overall reaction is the sum of the free energy changes of the two constituent reactionso Adding this to Cu2(s)2Cu(s)+S(s) delta G standard rxn=86.2kJo Even though the extraction of copper metal from Cu2S(s) is nonspontaneous the second reaction is so highly spontaneous that it provides enough free energy to drive the nonspontaneous reactiono The overall reaction has a negative delta G standard reaction thus occurs spontaneouslyo Example of coupled reaction- The coupling of two or more reactions such that a non-spontaneous chemical process can occur is very important in living systems- Many of the reactions that are essential for life do not occur spontaneously and are oupled with spontaneous reaction- The laws of thermodynamics govern the chemical reactions that are constantly occurring within living systems- Living systems are far from efficient and require a continuous input of energy thus organisms extract energy from food in order to live- In living systems, spontaneous reactions delta G<0 typically involve breaking food down- This energy is used to do work and carry out essential biological functions which are usually nonspontaneous reactions delta G>0- The ultimate source of all food is sunlight- Green plants store energy from sunlight in their tissues as molecules such as glucose- Photosynthesis is a nonspontaneous process requiring energy from
View Full Document
Unlocking...