Free Energy (A)

Summary

Highlights

• Introduction to Gibbs Free Energy and its significance in chemistry. ⚛️
• Explanation of spontaneity in chemical processes using Gibbs Free Energy. 🔍
• Understanding the relationship between free energy and equilibrium constants. ⚖️
• Discussion on the impact of temperature on the spontaneity of reactions. 🌡️
• Step-by-step solving of example problems involving delta H and delta S calculations. 🧮

Key Takeaways

• Gibbs Free Energy is crucial in determining the spontaneity of processes. 🔑
• The change in Gibbs Free Energy (ΔG) can signal whether a reaction is spontaneous, non-spontaneous, or at equilibrium. 🔄
• Temperature plays a significant role in influencing reaction spontaneity. 🌡️
• Understanding the sign of delta G helps predict the behavior of chemical systems. 🔍
• Practical problems help apply theoretical concepts to real-world scenarios. 🌍

Overview

Chapters

• 00:00 - 00:30: Introduction to Free Energy and Gibbs Free Energy The chapter introduces the topic of free energy, specifically Gibbs Free Energy, as the concluding subject in a series of lectures on chemical thermodynamics. Delivered by chemistry lecturer Joey Laksa, it sets the stage for students to understand the concept that is pivotal for the completion of the chemical thermodynamics course.
• 00:30 - 02:30: Definitions and Calculations of Gibbs Free Energy This chapter focuses on Gibbs Free Energy, covering various key aspects such as definition, spontaneous reactions, and calculations. It explains how to calculate the change in free energy for reactions using both free energies and enthalpies of formation of reactants and products. Additionally, the chapter explores the effect of temperature on the spontaneity of reactions, thereby tying in concepts like standard free energy. Overall, it provides foundational knowledge for understanding energy changes in chemical processes.
• 02:30 - 03:30: Temperature Effects on Gibbs Free Energy The chapter discusses the temperature effects on Gibbs Free Energy, emphasizing the role of equilibria. It is divided into three parts, focusing on the application of the second law of thermodynamics. The main point is understanding whether a process is spontaneous, at equilibrium, or non-spontaneous by knowing the change in entropy of the universe.
• 03:30 - 06:00: Problem Solving: Concept of Spontaneity and Temperature This chapter discusses the concept of spontaneity in problem-solving, particularly focusing on the calculated value of delta S (entropy change) of the universe. It outlines that when delta S is greater than zero, the process is spontaneous; when it is zero, the process is at equilibrium; and when it is less than zero, the process is non-spontaneous in the forward direction but spontaneous in the reverse direction. The chapter also emphasizes the calculation of delta S of the universe to determine these states.
• 06:00 - 09:30: Problem Solving: Exothermic and Endothermic Reactions The chapter discusses the concept of spontaneity in chemical reactions, particularly focusing on exothermic and endothermic reactions. It introduces Gibbs free energy as a criterion for spontaneity, explaining it as a function combining the system's enthalpy and entropy to define the maximum net energy.
• 09:30 - 10:00: Conclusion The chapter discusses the concept of free energy in thermodynamics. It highlights the relationship between heat, work, and entropy at constant temperature and pressure. The free energy is defined as the maximum amount of energy that can be converted into work, and represents the thermal energy that is available for performance, excluding the entropy-related losses. The chapter concludes by explaining why this form of energy is termed as free energy, noting its theoretical proposal origins.

Free Energy (A) Transcription

• 00:00 - 00:30 hello everyone this is joey laksa your lecturer in chemistry for engineers we are now down to our last topic under chemical thermodynamics so let's get it started all right so this is our lesson which discuss about free energy which is as mentioned earlier the last topic under our discussion on chemical thermodynamics so after successful completion of this lesson
• 00:30 - 01:00 you are expected to be able to define gibbs free energy and describe its relation to spontaneity calculate free energy change for a process using free energies of formation for its reactants and products calculate free energy change for a process using enthalpies of formation and the entropies for its reactants and products explain how temperature affects the spontaneity of some processes and relate energy standard free energy
• 01:00 - 01:30 changes to equilibrium constants so this particular discussion is divided into three parts okay last meeting we learned from the application of the second law of thermodynamics that in order for us to know whether a process is spontaneous at equilibrium or non-spontaneous we should know the value of the change in entropy of the universe such that
• 01:30 - 02:00 if we have a calculated value of delta s of the universe that is greater than zero we have a spontaneous process if it is equal to zero we know that the process is at equilibrium and if it is less than zero we have a non-spontaneous process on the forward reaction but a spontaneous process on the reversed reaction and for us to be able to calculate for the delta s of the universe we should be able to solve for the changes in
• 02:00 - 02:30 entropy of both the system and the surroundings okay however it will be better if we have one criterion for spontaneity by examining the system only this is what gibbs free energy gives us gibbs free energy is a function that combines the system's enthalpy and entropy and it is defined as the maximum net energy
• 02:30 - 03:00 at constant temperature and pressure available for doing useful work from a process remember that we have heat that is convertible to work minus the entropy which is the available thermal energy that is no longer available for work the difference is the maximum amount of energy that can be converted to work this is the reason why we call this as the free energy it was proposed by
• 03:00 - 03:30 josiah gibbs now let us combine the second and the second law of thermodynamics and the definition of gibbs free energy for us to come up with a simpler criterion on spontaneity so let's start with wait let's start with our second law of thermodynamics which states that the
• 03:30 - 04:00 delta s of the universe is equivalent to the delta s of our system plus the delta s of our surroundings and we know for a fact that at constant pressure constant p we have the delta s
• 04:00 - 04:30 of our surroundings equivalent to negative delta h of the system over the temperature substituting this one to the statement mathematical statement of the second law you have we have u delta s universe is equivalent to delta s of the system minus
• 04:30 - 05:00 the delta h of the system over t multiplying both sides by negative t we have negative t delta s of our universe is equal to negative t delta s of system plus delta h of system so let us rearrange this one such that the delta s will move here okay so you have
• 05:00 - 05:30 negative t delta s of the universe is equal to the delta h of the system minus t delta s of the system however from the gibbs free energy definition you have g is equal to h minus t of s
• 05:30 - 06:00 okay such that delta g will give you delta h minus t delta s at constant temperature maybe we'll expand that so minus t delta s plus s delta t but we have their delta t is equal to zero because we are dealing with a constant temperature process
• 06:00 - 06:30 okay so rewriting this one we have delta g is equal to delta h minus t delta s okay so we seldom read this one as gatas so if you have a difficulty in recalling what delta g is just remember your milk
• 06:30 - 07:00 right so this can also be written as your delta g of your system is equal to your delta h of your cis drop into my penta system minus t delta s of system right this um if you try to see this is delta h system
• 07:00 - 07:30 minus t delta s system is the same as this one therefore your negative t delta s of the universe is equal to your delta g of your system okay so given this one let's try to check on the criteria if a system
• 07:30 - 08:00 when the system is spontaneous and when it is not so next slide for a spontaneous process spontaneous process we have delta s
• 08:00 - 08:30 of your universe is should be greater than zero but we know that our delta g of our system is equal to the negative t delta s of the universe so this part is positive right because we have greater than zero so a negative times a positive will give you negative number which is zero so for a
• 08:30 - 09:00 spontaneous process you have your delta g lesser than zero for none for non-spontaneous process no spontaneous process spontaneous process we have the delta s of the universe should be less than zero and we know that delta g
• 09:00 - 09:30 is equal to negative t delta s of the universe okay for spontaneous process this is negative because it should be less than zero so negative times negative will give you a positive value okay so for a non-spontaneous process your delta g is greater than zero and for process at equilibrium for process
• 09:30 - 10:00 at equilibrium okay we have the delta s of the universe is equal to zero we know that delta g is equal to negative t delta s of the universe okay and this one being zero a number multiplied by zero is always zero therefore your delta g is equal to
• 10:00 - 10:30 zero okay so summing it up there we have this table so please be careful and take note that the quality signs for the change in the entropy of the universe and that of the gibbs of free energy of a system are opposite in directions but gives the same meaning so for a spontaneous process you
• 10:30 - 11:00 have a positive value for the change of entropy of the universe but you have a negative value for the change in gibbs energy but for equilibrium they are the same to be equal to zero for non-spontaneous process on the forward reaction we have the change of the entropy in the universe having a negative sign but for delta g it should be positive sign
• 11:00 - 11:30 okay i hope this picture rings a bell it is about the dependency of the spontaneity of a process on the temperature well it is gibbs free energy equation here which explains that temperature there is one of the parameters that have great effect on whether the calculated delta g will have either negative or positive sign hence it will help in the
• 11:30 - 12:00 prediction whether a process is is spontaneous or not as can be seen in the picture you know for a fact that it is temperature which serves as the determining factor whether the process is spontaneous or not so at greater than zero degree celsius we know that um the ice melts and less than zero degree celsius we know that ice will form now let us
• 12:00 - 12:30 have a closer look on the effect of temperature on process spontaneity there so if we have a negative sign for a delta h of the system and a positive sign for uh the change in entropy of the system there will we will have a spontaneous process at all temperatures why so imagine this one negative a minus a positive sign here a positive
• 12:30 - 13:00 and a negative is a minus so uh the value will be of a greater negative or the magnitude will be of a negative sign but if you have a positive delta h and a negative delta s you will have a nonspontaneous process at any temperature imagine this one a positive minus minus so you have a positive here so you have a positive plus positive you have a positive delta g which is
• 13:00 - 13:30 a non-spontaneous process i hope you are with me so just a simple algebra here you may actually uh use some values if you want you want to try you'll say put a value of delta h say positive 100 and a delta s of say positive 500 or a negative 500 and have a temperature and try to play with those figures to check or to prove that this uh table is valid
• 13:30 - 14:00 or not okay but if you have both negative sign for delta h and delta s process can either be spontaneous or non-spontaneous depending on the temp temperature so spontaneous at low temperature and non-spontaneous at high temperature and then if you have both positive for the change of enthalpy and for the change of entropy of the system again you will have either spontaneous
• 14:00 - 14:30 or non-spontaneous process depending on the process temperature so let us solve this problem to further illustrate the concepts that were illustrated so we have the companion diagram shows how delta h and t delta s change with temperature for a hypothetical reaction okay what is the significance
• 14:30 - 15:00 of the point at 300 kelvin where uh delta h and t delta s are equal and in what temperature range is this reaction of spontaneous so for letter a we are asked about the significance of 300 kelvin so we have there the delta h delta g rather delta h we're given that delta h
• 15:00 - 15:30 is equal to t delta s at t is equal to 300 kelvin so we know from the gibbs free energy definition that delta g is equal to delta h minus t delta s okay so we can transpose t delta s there so you have t delta s plus t delta g is equal to
• 15:30 - 16:00 delta h the only way for us to have a t delta s equal to delta h is to make your delta g zero okay so this is zero therefore therefore delta g is zero which means that at 300 kelvin
• 16:00 - 16:30 the process is at equilibrium so that is the significance of the 300 kelvin it is the temperature when the process is at equilibrium so let's be at what uh temperature range is this reaction spontaneous
• 16:30 - 17:00 so when is t what is t when delta g is less than zero okay going back to our working equation delta g is equal to delta h minus t delta s okay so at 0
• 17:00 - 17:30 when a delta g is equal to 0 your delta this is equivalent to delta h minus 300 kelvin delta s so we have there 3 delta h is equivalent to 300 kelvin delta s let's call that equation one
• 17:30 - 18:00 okay and we have here we're looking for the uh delta g equivalent greater than or is it greater than or lesser than zero rather because we're looking for a temperature range when the reaction is continuous so delta g should be negative so delta g should be lesser than
• 18:00 - 18:30 zero okay we know that delta g is equivalent to delta h minus t delta s okay but we know also that delta h is equivalent to 300 k delta s from our equation one so that is 300 k delta s minus t delta s so let's transpose so that is t
• 18:30 - 19:00 delta s greater than 300 k delta s so let's cancel delta s from the equation and delta s because we can divide both sides of the equation by delta s therefore your t should be greater than 300 kelvin so that the process
• 19:00 - 19:30 reaction will or the process or reaction will become spontaneous okay problem number two for a certain chemical reaction the standard uh heat of reaction is negative 35.4 kilojoules and the cha the standard change in entropy is equivalent to negative 85.5 jolt per kelvin letter a
• 19:30 - 20:00 is the reaction exothermic or endothermic letter b does the reaction lead to an increase or decrease in the randomness or disorder of the system and letter c delta g or standard gives free energy for the reaction of 298 kelvin should be calculated and for letter d is the reaction spontaneous up to 98 kelvin under standard condition so for letter a
• 20:00 - 20:30 is a reaction exothermic so you have delta h there is equivalent to negative 35.4 kilojoules since we have a negative sign for delta h the reaction is spontaneous
• 20:30 - 21:00 thermal again because we have a negative sign for the delta h for the letter b letter b does the reaction lead to an increase or decrease in the randomness or disorder of the system we are talking about the change in entropy of the system which is negative 85.5 joel per kelvin so again you have a negative
• 21:00 - 21:30 sign which is an indication that the entropy um decrease in value so there is decrease in the disorder of the system of the sis stem okay and then for letter c
• 21:30 - 22:00 calculate delta g for the reaction at t is 298 so you have delta g of reaction the stage of the reaction is delta h of the reaction minus t delta s
• 22:00 - 22:30 of the reaction so it's a direct substitution type of problem now we have a delta h given delta s given and a temperature at 298 kelvin so you have delta g of reaction is equivalent to negative 35.4 kilojoules minus you have there 298 kelvin times
• 22:30 - 23:00 negative 85.5 joel kelvin but notice you have a kilojoule in the first term but joel in the second term therefore we need to change one of them so i opt to change the joule into kilojoule by converting it okay so let's check on the units kelvin cancels out joel cancels out you
• 23:00 - 23:30 have kilojoules and kilojoules there so we can subtract them so you have the delta g of your reaction is equal to let's calculate 298 times negative 85.5 divided by one thousand memory negative thirty five point four minus the answer we have negative
• 23:30 - 24:00 nine point kilo joules okay for letter d is the reaction spontaneous at 298 kelvin under standard conditions yes the reaction is spontaneous because you have a negative
• 24:00 - 24:30 value for delta g positive delta s of universe are spontaneous but for delta g it is negative so i will cut the discussion here because this is around how many minutes you gonna make the 30-minute cha so
• 24:30 - 25:00 and i will cut the discussion here and we'll be back for the second part see you later