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Video transcript
Our bodies are constantly active. Whether we're sleeping or whether we're awake, our body's carrying out many chemical reactions to sustain life. Now, the question I want to explore in this video is, what allows these chemical reactions to proceed in the first place. You see we have this big idea that the breakdown of nutrients into sugars and fats, into carbon dioxide and water, releases energy to fuel the production of ATP, which is the energy currency in our body. Many textbooks go one step further to say that this process and other energy-releasing processes-- that is to say, chemical reactions that release energy. Textbooks say that these types of reactions have something called a negative delta G value, or a negative Gibbs-free energy. In this video, we're going to talk about what the change in Gibbs free energy, or delta G as it's most commonly known is, and what the sign of this numerical value tells us about the reaction. Now, in order to understand delta G, we need to be talking about a specific chemical reaction, because delta G is quantity that's defined for a given reaction or a sum of reactions. So for the purposes of simplicity, let's say that we have some hypothetical reaction where A is turning into a product B. Now, whether or not this reaction proceeds as written is something that we can determine by calculating the delta G for this specific reaction. So just to phrase this again, the delta G, or change in Gibbs-free energy, reaction tells us very simply whether or not a reaction will occur. Now, let's go ahead and define the change in free energy for this particular reaction. Now as is implied by this delta sign, we're measuring a change. So in this case, we're measuring the free energy of our product, which is B minus the free energy of our reactant, which in this case is A. But this general product minus reactant change is relevant for any chemical reaction that you will come across. Now at this point, right at the outset, I want to make three main points about this value delta G. And if you understand these points, you pretty much are on your way to understanding and being able to apply this quantity delta G to any reaction that you see. Now, the first point I want to make has to do with units. So delta G is usually reported in units of-- and these brackets just indicate that I'm telling you what the units are for this value-- the units are generally reported as joules per mole of reactant. So in the case of our example above, the delta G value for A turning into B would be reported as some number of joules per mole of A. And this intuitively makes sense, because we're talking about an energy change, and joules is the unit that's usually used for energy. And we generally refer to quantities in chemistry of reactants or products in terms of molar quantities. Now, the second point I want to make is that the change in Gibbs-free energy is only concerned with the products and the reactants of a reaction not the pathway of the reaction itself. It's what chemists call a "state function." And this is a really important property of delta G that we take advantage of, especially in biochemistry, because it allows us to add the delta G value from multiple reactions that are taking place in an overall metabolic pathway. So to return to our example above, we had A turning into a product B. But what if the product B turned into another product C? If we wanted to calculate the overall Gibbs-free energy for A going to C, we could instead calculate the individual delta G for each step of the reaction that is A going to the product B, and B going to the product C. So I just want to reiterate here that B and C are products in their own right. They're not transition states. But what we're seeing here is that in some cases we may not be able to measure the change in Gibbs-free energy going from A to C directly. So instead, we can add together the individual change in Gibbs-free energy for each step, because remember Gibbs-free energy is a state function. And if we do that, we ultimately get the change in Gibbs-free energy for the overall reaction of A going to C. Now one fun way that I kind of remember the state function like quality of delta G, as well as some other variables in chemistry, is that my chemistry professor used to tell us that life is not a state function. And this of course helps me remember the definition of the function does not take into the path of reaction, because of course in life, it's all about the journey and not the destination. But in chemistry, sometimes it's the opposite. Now, the third point that I want to make is that delta G unlike temperature, for example, which can be readily measured in a lab for a particular situation, delta G is something that can be calculated but not measured. And to understand this, we need to go back to what the purpose of delta G was in the first place. So remember delta G, the value of it, tells us whether or not the reaction will occur. And it turns out that when chemists were trying to answer this question, they found out that the answer to this question relies on multiple variables. There's not just one thing that determines whether or not a reaction will occur. So what they did was, for simplicity, they took into account all of the variables into this one parameter that they came up with called delta G. And the way they did this was by creating an equation. So they said, the change in Gibbs-free energy is equal to the change in enthalpy, or heat content, of a particular reaction minus the temperature of the reaction times the change in entropy, or broadly speaking randomness, between products and reactants in a particular reaction. Therefore, as I mentioned before, we can go ahead and calculate one single value that takes into account all of the variables that affect the extent and degree to which a reaction will occur. And it turns out that we can actually measure the change in enthalpy, the temperature, and the change in entropy for a reaction, so that works out quite well. Now, at this point, you probably have a question of OK, I see that I have an equation to calculate delta G for a reaction, but what does this value that kind of pops out of this equation tell me about a reaction? So let's go ahead and go back to our hypothetical reaction of A going to B. Let's draw a diagram that will help us understand this reaction better. So I'm going to go ahead and draw a y-axis and an x-axis. On the y-axis will be the quantity free energy in units of joules, let's say. And on the x-axis will be the quantity of a reaction coordinate. And this is kind of an abstract parameter that simply is a way for us to kind of monitor the progress of a reaction over time. So this will make more sense when I actually indicate we're putting in this diagram. So let's say that our reactants A have a much higher free energy than the products of our reaction, which is B in this case. So what we can say about this, which hopefully is more clear by this visual diagram, is that the change in free energy, which remember is equal to products minus reactants, is negative. Or we say it's less than 0. On the other hand, let's say that we started off with reactant A that had a much lower free energy than the product B. Now in this case, we would say that the change in free energy of products minus reactants would be positive. Now, the key takeaway here is that for any chemical reaction that has a negative delta G value, we say that the reaction proceeds spontaneously. That is, it proceeds without an input of energy. So I'm just going to write spontaneous there. On the other hand, when a delta G value is positive, that is when the conversion of reactants to products requires a gain of energy, we say that it's a non-spontaneous reaction and cannot proceed unless there is an input of energy. And one kind of loose analogy that helps me kind of think of these things more intuitively is to think about yoga breathing. So imagine that you're taking a deep, deep breath in, and all of this breath that you have inside of your body makes you feel kind of unstable and wanting to burst. So I kind of think of that as starting off at a high free energy state. So let's say we're starting off with A. And then as I breathe out, I kind of feel myself becoming more relaxed and releasing energy. And that brings me to B, which has a lower free energy. And that of course, breathing out, is a spontaneous process.