DOC PREVIEW
ISU CHE 141 - Chemical Thermodynamics 2
Type Lecture Note
Pages 5

This preview shows page 1-2 out of 5 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 5 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 5 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 5 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

CHE 141 1st Edition Lecture 28Outline- Calculating delta S from Reversible Heat Flow- Predicting the Spontaneity of a Reaction - Gibbs Free EnergyCalculating delta S from Reversible Heat Flow- If heat is flowing out of the system (exothermic)o Delta Hsys<0o qsys<0o qsurr>0o delta S >0- If heat is flowing into the system (endothermic)o Delta Hsys>0o qsys>0o qsurr<0o deltaSsurr<0Predicting the Spontaneity of a Reaction These 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 can determine spontaneity from the sign of deltaSuniv which is calculated using the 3rd law of thermodynamics: deltaSuniv=deltaSsys+deltaSsurr- We need to know deltaSsys and delta Ssurr but its difficult to measure/calculate delta Ssurr accurately- It is possible to just use delta Ssys and deltaHsys to predict the spontaneity of a reaction: deltaSsurr=qsurr/T- At constant T andP qsurr=deltaHsurr: delta Ssurr=deltaHsurr/T- The amount of the heat surrounding absorb equals the amount of heat the reaction gives off- Qsurr=-qsys- deltaHsurr=-deltaHsys- deltaSsurr=-deltaHsys/T- substitute deltaSsurr=-deltaHsys/T into delta Suniv=delta Ssys+delta Ssurr and get TdeltaSuniv=TdetaSsys-deltaHsys - Now we can identify the change in entropy of the univers delta Suniv exclusively in terms of the properties of the system delta Ssys and delta HsysGibbs Free Energy- We don’t need to know about the surroundings to calculate spontaneity- J. Willard Gibbs defined a state function called free energy later assigned the symbol G inhis honor - The change in free energy is defined as deltaG=-TdeltaSuniv then get delta G=delta H-TdeltaS- Gibbs free energy (G) is a thermodynamic state function that is a measure of energy available to perform useful work. The SI units for Gibbs free energy are joules- Gibbs free energy change (deltaG) is the change in free energy associated with a chemicalprocess and provides the criterion for spontaneous changes- For spontaneous processes Suniv >0, deltaG=-TdeltaSuniv- Thus we can use delta G as a criterion for spontaneity just likeSuniv- If delta G>0 the reaction is spontaneous in the forward reaction- If delta G=0 the reaction is at equilibrium- If delta G>0 the reaction in the forward direction is nonspontaneous - The main driving forces which determines the sign of delta G and thus whether a reactionwill ocurr spontaneously are enthalpy and entropy- We can predict the sign of delta G by considering the signs of delta H and delta So Delta H negative, delta S positiveo Delta H positive, delta S negativeo Delta H negative, delta S negativeo Delta H positive, delta S positive- When delta H negative, delta S positive the reaction is exothermic (deltaH<0)o Change in entropy for reaction is positive (deltaS>0)o Delta G=deltaH-TdeltaSo deltaG=(negative)-T(positive)o delta G is negative at all temperatures (delta G<0)o reaction is spontaneous at all temperatures- When delta H positive, delta S negativeo Reaction is endothermic (delta H<0)o Change in entopy for reaction is neative (delta S<0)o Delta G=(negative)-T(negative)o Delta G is positive at all temperatures (delta G>0)o Reaction is nonspontaneous in the direction written at all temperatures- Delta H negative, delta S negativeo Reaction is exothermic (deltaH<0)o Change in entropy for reaction is negative (delta S<0)o Delta G=negative-T(negative)o Delta G is negative at low temperatures (T delta S<delta H) and positive at high temperatures (TdeltaS>deltaH)o Reaction is spontaneous at low T (T< deltaH/deltaS) and nonspontaneous at high T (T>deltaH/deltaS)- Delta H positive, delta S positiveo Reaction is endothermic (delta H>0)o Change in entropy for a reaction is positive (delta S>0)o Delta G=(positive)-T(positive)o Delta G is positive at low temperatures and negative at high temperatures o Reaction is nonspontaneous at low T and spontaneous at high T- Crystallization of a pure compound is spontaneous only below 49 degrees celcius. Predictthe signs of the reaction enthalpy delta H and reaction entropy- Reaction is only spontaneous at low temperatures therefore delta G is negative at low temperature - Delta H and delta S must both be negative- This makes sense as we would expect delta S to be negative for


View Full Document

ISU CHE 141 - Chemical Thermodynamics 2

Type: Lecture Note
Pages: 5
Documents in this Course
Load more
Download Chemical Thermodynamics 2
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Chemical Thermodynamics 2 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Chemical Thermodynamics 2 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?