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Spontaneous Process 10,141 views

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Spontaneous processes are reactions which proceed without requiring an input of energy because the products are at a lower, more stable energy state than the reactants. Spontaneous processes often require activation energy, but do not require a prolonged input of energy.

Alright so we're going to talk about spontaneous processes and when you hear the word spontaneous a lot of things like come to mind. One thing might be something that happens instantly, when you think of spontaneity or something being spontaneous it happens instantly. That's not the case in Chemistry, there's spontaneous reactions like when you think uranium decaying. Uranium takes a long time to decay, we don't have to do anything to make it decay, it just decays on its own which makes it spontaneous but it decays over millions and millions and millions of years or there's a reaction of like diet coke and mentles you drop some mentles in diet coke it explodes instantly. They're both spontaneous we didn't do anything to make that reaction occur they both just occur on their own. That's kind of what spontaneous means in Chemistry. It's a physical or chemical change that occurs with no outside intervention at ordinary condition. And ordinary conditions are things like 1 atmospheric pressure and 25 degrees Celsius everyday in life that would be your ordinary condition.

Some energy might be supplied initially and that's okay like for example when you're talking about lighting your Bunsen burner for example if you have a methane gas running you need, it's the only way it's going to combust or create that flame in your Bunsen burner is if you create a spark, that small spark is just initial like to make the reaction get started but once it's started we do nothing that can make that reaction continue that we call the spontaneous process. Let's look over here at some spontaneous reactions or not so spontaneous reactions and we'll discuss them. So first we have methane CH4 reacts with oxygen what's coming out of your Bunsen burner we know that when it reacts it'll produce carbon dioxide and water totally okay it actually the net difference between the reactants and products is going to be 89 kilo joules it's going to release 89 kilo joules of energy. So let's see if, this we call, we know it's spontaneous we've done it before in lab but if the reverse reaction is spontaneous if we take carbon dioxide and water and combined together are we going to be able to get oxygen and methane gas? Well no it takes a lot of energy, it takes 89 kilo joules but not only that, we have to continuously push the reaction along. So if we continue to force the reaction to occur put in, in some sort of like have catalysts or have other like things that will actually make the reaction go without it happening on its own we're going to call that non-spontaneous and I know for a fact this is not spontaneous so we're going to say no this that's not.

Let's talk about rust we know if you leave your bike outside in the rain or if things are outside expose the elements the iron will react with the oxygen in the air and it will form rust or iron III oxide. We know it releases 1,625 kilo joules of energy so this reaction will happen without us doing anything. We actually sometimes don't want it to happen and it happens all the time. So we definitely think this reaction is spontaneous, it will happen without us doing a thing to it. So yeah this is a spontaneous reaction however, we can't take rust and then go back and get iron and oxygen and get our nice iron back. And it takes a lot of energy to do that we're going to push a lot of weight [IB] a lot of things to happen to make this reaction occur it requires 1,625 kilo joules of energy. It's not going to happen unfortunately once your bike is rusted, it's rusted so I can't do anything to get that back unless I do some major work on it and in the lab or something. So if you notice corelation here exothermic reactions or reactions that release energy are our spontaneous ones but our endothermic reactions are the ones that are not spontaneous. So for a really long time we kind of have assumed that all exothermic reactions are spontaneous and all endothermic reactions or processes even are non-spontaneous but is that true? So let's go down here, what about this process, we have solid water aka ice and liquid water, water.

And so we know at ordinary conditions that solid ice will melt. If you put an ice cube on this table it's going to melt. So without me doing anything I can just leave it there and it'll melt. So, but this actually requires 6.0 in kilo joules of energy so we just disproved our whole theory there so spontaneity does not just depend on enthalpy or delta H it must be depending on something else. So let's go back and also look over here, so spontaneity depends on a combination of enthalpy which we just discussed or delta H or energy and entropy. Entropy is a measurement of disorder there's actually a whole video on entropy alone. So if you want to learn about entropy you might want to check out that video. The symbol for entropy is s delta s and so a combination of this so we'd like things to be disorderly, we like things to have chaos and we also like to be less than energy. So a combination of these 2 guys Gibbs came up with this combination to tell us if something's spontaneous or not and he said Gibbs for energy the amount of energy that is actually released in a system is a combination of delta H subtracted from the temperature that the reaction is occurring at times the entropy or the disorder that's actually happening. So we like things being disorderly and we also like things left [IB] and a combination of those 2 things are going to make something spontaneous or not.