Chapter 18 Spontaneity
How do you determine the ΔSsurroundings?
-ΔHsystem / T
What is Gibbs free energy of element in its standard state?
0, tho this rule does NOT apply to std entropy, which all have positive values.
What is ΔSuniverse in terms of Gibb's free energy?
ΔSuniverse = -ΔG/T
Looking at ΔG= ΔH - TΔS, under what enthalpy and standard entropy conditions is a process spontaneous, non spontaneous, and when does temp play a role?
1. First, remember that a negative ΔG is spontaneous and a positive ΔG is non-spontaneous (opposite to ΔS of universe). 2. When the enthalpy is positive (endothermic) and the entropy is negative, the process will always be positive (+ΔG) which is non spontaneous under ALL temps. 3. When the enthalpy is negative and the entropy is positive, then ΔG will always be negative, which is spontaneous under ALL temps. 4. When the enthalpy is positive and the entropy is positive, then you want high temps to make ΔG negative. Here, temp matters because the two signs are the same. 5. When the enthalpy is negative and the entropy is also negative, you want a low temp to keep the positive TΔS low, so the negative ΔH can shine. Again, temperature matters here because the signs are the same.
Temperature determines magnitude of entropy in the surroundings. The greater the temperature, the _______ it effects ΔSuniverse. The smaller the temperature, the _______ it effects ΔSuniverse.
1. Less (greater denominator equals smaller number) 2. More (smaller denominator equals bigger number)
What are the five cases in which entropy will be positive
1. When a solid melts to a liquid 2. From solid to gas or liquid to gas 3. When a solid dissolves into an aqueous solution 4. An increase in temperature within a phase (closer to a state of matter with higher energy) 5. An increase in the number of gas particles
Entropy
A measure of the degree of disorder or randomness in the system; you can also think of it as how spread out something (energy) is across the system Measure of molecular randomness, a measure of how dispersed a system's energy is High entropy, positive Delta S is moving from low disorder and spreaded outness to maximum spreaded outness and disorder. Low entropy, negative Delta S, is moving from High disorder and spreaded outness, to low disorder and spreaded outness.
Second law of thermodynamics
ALL spontaneous processes result in an increase in the entropy of the universe.
What does the universe want in terms of entropy?
An increase! Naturally, just like exothermic processes are more energetically favorable, processes that result in an increase in entropy are also energetically favorable. So more spreading out, more randomness, etc.
ΔS of universe will be positive under what conditions?
If both ΔSsystem and ΔSsurroundings are positive If positive ΔSsystem is greater than a negative ΔS surroundings If negative ΔSsystem is less than positive ΔS of surroundings
How is entropy a probability concept?
Entropy, as a measure of the dispersal of a system's energy, increases when there are more energetically equivalent ways to arrange the components of that system. W (the possible arrangements of particles) = X^n where X is the number of locations and n is the number of molecules. The greater the X, the more Ws possible. Instead of all occupying one side or the other, there is a greater probability of occupying BOTH sides. For example, if you have 2 locations and 2 particles, probability of both on either side is .25, while probability of one on each side is .5. This is all to wat tat the most probable state is the one with the highest spreading out, the tendency towards disorder and "pread outness" is a statistical probability.
What are the units for entropy of a reaction and how do you determine the entropy of a reaction when given a specific amount of moles?
Entropy= J/mol, so multiply by moles to get entropy of reaction for a certain amount of moles.
When the system of the universe is ____, an entropy _____, it is spontaneous. When the ΔG is ______ the reaction is spontaneous.
For ΔSuniverse, spontaneous reactions occur when the value is POSITIVE, an entropy INCREASE. For ΔG, spontaneous reactions occur when the values is negative. The relationship between large and small temps remains the same between the two. If enthalpy change is positive and entropy change is negative, the process is non-spontaneous If enthalpy change is negative and entropy change is positive, the process is spontaneous (for these two, just think about the equations for both processes) If enthalpy and entropy are both positive, you want a high temp to make sure the process is spontaneous If enthalpy and entropy are both negative, you want a low temp to make sure the process is spontaneous. (correlation between positive values and high temp, negative values and low temp).
Why does the gas phase have the highest entropy and why does increasing temperature increase entropy? How about increasing the number of gas particles?
Gasses have the highest entropy because the distance between particles and the overall KE of the particles is the greatest in the gas phase (Increasing kinetic energy causes particles to move faster, which disperses their energy more throughout the system and increases their entropy) For gas particles, Any increase in the number of particles provides more ways for the systems energy to be distributed.
A process is spontaneous (negative ΔG) when the enthalpy is positive and the entropy is positive at _____ temps
High
A process is spontaneous (+ΔSuniverse) when the ΔSsystem is positive and the ΔH of system is positive at what temps?
High temps
How does enthalpy of a system, then, play a role in sponteneity? Think both mathematically and critically. This was a detail included in lecture so pay attention!
It determines the sign of ΔSsurrounding! If a system is endothermic, the ΔSsurrounding will be negative. You can think of the system taking in energy so the surrounding loses energy and decreases randomness. If a system is exothermic, the ΔSsurrounding will be positive. Energy is released into the surroundings, thereby increasing randomness.
A process is spontaneous (negative ΔG) when the enthalpy is negative and the entropy is negative at _____ temps
Low
A process is spontaneous (+ΔSuniverse) when the ΔSsystem is negative and the ΔH of system is negative at what temps?
Low temps
What happens when you are trying to calculate ΔG for processes at temperatures other than 298K?
Multiply the TΔS by the temp! Simple as that.
What is the most basic difference between sponteneous and non-spontaneous
Spontaneous processes occur without any help from outside forces, while non-spontaneous processes cannot occur at a given condition without help. Another key detail is that spontaneous processes HAPPEN under indicated conditions, while non-spontaneous processes do not. Non-spontaneous processes do NOT occur under indicated conditions no matter how long you wait.
Units that standard ΔG, ΔS, and ΔH will come in (on table)
Std ΔG is listed in KJ Std ΔS is listed in J (the only one with J and always positive) Std ΔH is listed in KJ
The way you consider enthalpy and entropy of their surroundings, as well as their relationship to temperature and the entropy of the universe is all dependent on what
The -sign; you really need to be careful and think critically; remember that dividing by a small number equals large product while dividing by a big number = small product.
First law of thermodynamics
The energy of the universe is constant; the system and the surroundings make up the total energy of the universe. The energy of the system plus the energy of the surrounding is equal to ZERO. The two are equal and opposite.
Third law of thermodynamics
The entropy of a pure crystalline substance is 0, and based off this, we can determine the entropy values of various substances, called the standard molar entropies.
What determines whether or not a reaction is spontaneous under given conditions?
The entropy of the UNIVERSE, which is the sum of the entropy of the system and the entropy of the surroundings
Thinking about entropy as a probability value?
The more arrangements that are possible (spreaded outness) for particles across a certain amount of space, the more likely they are to occupy those spaces and increase their randomness. Tendency to move towards disorder is a statistical probability.
What is the purpose of Gibb's Free Energy
To predict sponteneity at temps other than 298 (which we were unable to do before because the entropy of the system in all problems of Delta Ssystem calculation were at 298). It also allows us to determine at what temp a process will become spontaneous.
While we CAN determine ΔG by taking product - reactant equation for Gibbs Free energy, why is it better to use enthalpy of reaction (ΔH) and entropy of reaction (ΔS) for calculating Gibbs Free energy of solutions?
Using ΔG = ΔH - TΔS better reflects how enthalpy AND entropy AND tempreture effects Gibbs free energy. Remember that boiling points and melting points occur at equilibrium, when ΔG is equal to 0. Using that principle, we can determine at what temp a process will naturally occur spontaneously.
So when is the only time that temperature has a bearing in determining spontaneity (ΔSuniverse)?
When the signs for ΔS of system and surrounding are different So if ΔSsystem is positive and ΔSsurrounding is positive (which would require a negative ΔHsystem), ΔS of universe is obviously going to be positive. The same principle applies to ΔSuniverse when both the system and surrounding have a negative ΔS, in which case the ΔSuniverse is negative. When ΔSsystem is positive and ΔSsurrounding is negative, we want high temperatures to keep -surroundings small (larger denominator=smaller number) and the ΔSuniverse positive. When ΔSsystem is negative, and surrounding is positive, we want low temp to keep the ΔSsurrounding large (small denominator=large number) and ΔSuniverse positive).
Review of enthalpy concepts; how does enthalpy relate to spontaneity?
When the system absorbs energy, the reaction/system is endothermic When the system releases energy, the reaction/system is exothermic Though enthalpy plays a role in distinguishing between spontaneous and non-spontaneous processes, it is not the key factor or distinguisher between them. So for example, at room temp, ice melts, and this is an endothermic as well as spontaneous process. At temperatures below 0, the processes would still be endothermic, however, it would not be spontaneous.
What are the rules for sponteneity when talking about Gibb's Free energy; recall that they are opposite of delta Suniverse rules.
When ΔG > 0 (positive), the system is non-spontaneous When ΔG < 0 (negative), the system is spontaneous When ΔG = 0, the system is at equilibrium
Core importance of Gibbs
While ΔSuniv looks at what happens in terms of entropy of the system AND the surrounding, The Gibbs free energy allows chemists to predict spontaneity using data for the system under consideration, without having to calculate ΔSsurr or ΔSuniv. Remember that ΔSuniv wants to be positive (increase the entropy of the universe) while ΔG wants to be negative (energetically favorable, works the same way as enthalpy).
What is another way you can calculate ΔG?
ΔG * = (Sum of Moles x G* in Products) - (Sum of Moles x G* in Reactants)
When we talk about spontaneous vaporization or melting, what is the value of ΔG?
ΔG = 0 (phase changes happen at equilibrium). Remember that we have specific rules about above that temperature and below that temp that are further elaborated upon in the study guide. Basically tho, If both Enthalpies and entropy are negative, terms below equilibrium are spontaneous. If both entropy and Enthalpy are positive, temps above equilibrium are spontaneous.
What is the equation for Gibb's Free Energy?
ΔG = ΔH - TΔS You can use standard enthalpies of ΔH and ΔS for this.
Unlike ΔSuniverse, in which temperature matters when.... ΔG is only dependent on tempreture when...
ΔS of universe WANTS to be positive, add entropy to the universe. Because it is equal to ΔSsystem -ΔHsystem/T, the temp only plays a role when the signs of the system and the surrounding are the same (or when entropy and enthalpy are the same). ΔG WANTS to be negative; because it is equal to ΔH - TΔS, the temp only matters when enthalpy and entropy are the same in sign.
What is the equation for determining the standard enthalpy of a chemical reaction? ***It is important to distinguish this from standard entropy of the universe and the the std entropy of the surroundings because...
ΔSrxn (system) = ΣMoles (S products) - Σ moles (S reactants) Remember that std entropy is measured in J not Kj because everything else is in KJ so it will require a metric conversion when you start adding things together. BE VERY CAREFUL WITH YOUR SIGNS; RECALL THAT ITS FINAL MINUS INITIAL, SO PRODUCT MINUS REACTANTS (dont forget to multiply by molar coefficients).