CHE 141 1st Edition Lecture 29Current LectureFree Energy and Efficiency - Rearrange delta G= delta H-T delta S in terms of delta H: delta H=delta G+T delta S- Delta H is the energy change from making/breaking bonds and is comprised of delta G and T delta S- Delta G is the energy available to do useful work- T delta S is the energy that disperses and is wasted - We use efficiency to evaluate the environmental impact of devices which use energy- Efficiency is simply the ratio of work done to energy consumed: work done/energy consumed x 100%Spontaneity and Temperature- Delta G= delta H- T delta So When delta H and delta S have opposite signs, the spontaneity of the reaction does not depend on To When delta H and delta S have the same sign, the spontaneity does depend on To The temperature at which the reaction changes from being spontaneous to being nonspontaneous is the temperature at which delta G changes signThese 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.o Therefore we just set delta G=0 and solve for T: T=delta H/delta So Make sure that temperature is in Ko You must express delta H and delta S in the same units, a good strategy is to always use jouleso Look at the signs of delta H and delta S to check whether reaction is spontaneous above or below TStandard Gibbs Free Energy of Formation delta G- The standard Gibbs free energy of formation is the change in free energy associated with a chemical reaction in which one mole of a substance is formed from its constituent elements in their most stable states under standard conditions- The SI units of the standard free energy of formation are joules per mole- The standard Gibbs free energy for a chemical reaction under standard conditions is delta GrxnProperties of Gibbs Free Energy- Like enthalpy and entropy, Gibbs free energy is a state function, delta G=Gfinal-Ginitial- If you know that the Gibbs free energy of a certain reaction is delta G then:- The free energy of the reverse reaction is –delta G- The reaction free energy for the reaction multiplied by n is n delta G- The reaction free energy for the reaction divided by n is delta G/n- We can calculate the standard free change in Gibbs free energy for a reaction delta Grxn in several ways depending on the info we haveo 1. Delta Grxn=delta Hrxn-T delta Srxno 2. Delta Grxn=sigmanGf(products)-sigmamGf(reactants)o 3. Delta Grxn=sigmadelta Gstep for a stepwise
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