Our episode starts with a clarification of using the terms thermodynamically favored vs spontaneous.
Our episode starts with a clarification of using the terms thermodynamically favored vs spontaneous (1:19). To determine if a reaction is thermodynamically favored, we calculate Gibbs Free Energy using the Gibbs Free Energy of formation (2:45). Taking a closer look at Gibbs Free Energy we relate it to enthalpy and entropy and look at four different situations: exothermic and increase in entropy as well as endothermic and decrease in entropy don’t require calculations (3:37). But what if I have a reaction that is endothermic but increases entropy (5:46) or a reaction that is exothermic and decreases entropy (6:36)? We have to take the temperature into account!
Question: What is the standard state for a solution in terms of concentration?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Gibbs Free Energy
Lets Zoom Out:
Unit 9 - Applications of Thermodynamics
Topic 9.3 - Gibbs Free Energy and Thermodynamic Favorability
Big idea - Energy
Introduction:
In the past episodes we’ve talked about enthalpy and entropy. We’ve seen reactions happening “spontaneously”: exothermic reactions - like combustions reactions and reactions where the entropy increases, like in reactions that form gases. BUT: endothermic reactions, like the dissolution of sodium chloride in water and reactions that decrease entropy, like precipitation reactions, also happen without external input. Other reactions don’t happen, even though they might be exothermic or increase the entropy. So what determines if a reaction is thermodynamically favored? And what does that even mean?
Let’s zoom in:
Sometimes, reactions that are thermodynamically favored are described as “spontaneous”, like we just did in the introduction. The term thermodynamically favored, however, is preferred by the College Board. Why is that? Spontaneous sounds far more normal, why do chemists always have to sound so complicated? “Spontaneous” sounds like a reaction is happening all of a sudden, without reason. That is, however, not true! Chemical reactions are not reckless and unreasonable - everything they do has a reason and they can explain their “Why”! Okay, okay, that might be a bit too anthropomorphic, but: WE can explain why a reaction is thermodynamically favored or not by taking into account if a reaction needs the addition of constant external energy or not. Additionally, “spontaneous” sounds like it happens very quickly - but not all thermodynamically favored reactions happen quickly. For example, as pointed out in Episode 23, diamonds degrading to the more stable form graphite is thermodynamically favored - but it takes a VERY, VERY long time. So the College Board sticks with this term and doesn’t use the concept of “spontaneity”.
What are the factors that determine if a reaction is thermodynamically favored and how is it expressed and calculated? Enter: Gibbs Free Energy. Gibbs Free Energy tells us if a reaction is thermodynamically favored or not. If the change of Gibbs Free Energy under standard conditions - that means the reactants and products are in their standard state at 1 atm - is negative, then the reaction is thermodynamically favored. If Gibbs Free Energy is positive, the reaction is thermodynamically unfavored. Similar to enthalpy and entropy, we can calculate the change in Gibbs Free Energy for physical and chemical changes by using “products minus reactants” and, similar to enthalpy, we can again use the standard Gibbs Free Energy of formation taking into account the coefficients.
There are two factors to consider when determining if a reaction is thermodynamically favored or not: Enthalpy and Entropy. These two concepts are connected to Gibbs Free Energy by the equation: Change in Gibbs Free Energy equals the change in Enthalpy minus the temperature times the change in Entropy. All of the values here should be under standard conditions. So if enthalpy and entropy determine Gibbs Free Energy, what does that mean for our reaction? Let’s start with the two easier situations: 1) Our reaction is exothermic and increases entropy - that means we have a negative value minus a large positive value - your change in Gibbs Free Energy will definitely be negative and your reaction is thermodynamically favored. 2) We can do it vice-versa: A reaction that is endothermic and decreases the entropy - this means we have a positive value plus a positive value. Gibbs Free Energy will in any case be positive and the reaction is thermodynamically unfavorable. It will only happen with constant external energy input.
Let’s look at two other situations: endothermic with increase in entropy and exothermic with decrease in entropy.
The dissolutions of sodium chloride is an example for a change that is endothermic - the temperature decreases while you are doing it, but increases the entropy, since you are dissolving a solid. As you probably know from your kitchen, this reaction is thermodynamically favored - if not, please go to your kitchen now and add some salt to water! How can this be? When we look at the equation, we can see that we can get a negative Gibbs Free Energy if the temperature times entropy term is larger than the enthalpy. Therefore, these types of reactions happen at high temperatures - and the term “high” is somewhat relative, given the fact that your room temperature water is already sufficient.
The other situation is vice-versa: a reaction that is exothermic but decreases entropy. An example for this is freezing water. And, as you might realize, that only happens if the temperature is low! Let’s look at it from a math perspective: If you have a negative enthalpy, the term of temperature times negative entropy has to be smaller than the enthalpy because you are subtracting a negative value - and therefore adding it to a negative enthalpy making it more positive! Let me rephrase that with random numbers: negative 300 minus negative 500 is positive 200. That wouldn’t be thermodynamically favored. BUT: if I have negative 300 minus negative 100, that adds up to negative 200. And Gibbs Free Energy would still be negative! Therefore, these reactions happen at low temperatures.
Now that I am already talking about random numbers, one important note on UNITS - don’t forget, units are friends! Enthalpy is often reported as kJ/mol, whereas entropy is often reported as joules/mol! You need to make sure to have the same units - I usually suggest converting joules to kilojoules - otherwise you are off by a factor of 1000! And that makes a difference!
To recap:
The term “thermodynamically favored” is preferred over “spontaneous” to prevent misconceptions about the rate of the reaction. Thermodynamically favored reactions happen without constant external input and are measured as Gibbs Free Energy. A negative Gibbs Free Energy value indicates that a reaction is thermodynamically favored. Gibbs Free Energy can be calculated using products minus reactants or taking into account enthalpy and entropy. Reactions that are exothermic and increase entropy are always favored. Reactions that are endothermic and decrease entropy are never favored. Reactions that are either exothermic or increase entropy can be thermodynamically favored, depending on the temperature.
Coming up next on the APsolute RecAP Chemistry Edition: Galvanic and Electrolytic Cells
Today’s Question of the day is about the conditions for standard states.
Question: What is the standard state for a solution in terms of concentration?