Spontaneous vs Nonspontaneous Reactions - Video Tutorials & Practice Problems
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Understanding Spontaneity
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Now, spontaneity, it determines whether a reaction or process under certain conditions occurs or not. Now, when we say spontaneous versus non spontaneous, what exactly do we mean? Well, when we say a spontaneous reaction, we mean that it is a natural process which does not require a constant outside energy source. Now, a spontaneous reaction, we're going to say that it favors uh formation of products at equilibrium. So your reactants go towards uh making products. When we say non spontaneous reaction, a non spontaneous reaction is an unnatural process which requires a constant outside energy source in order for it to occur. We're gonna say here, it does not favor formation of products at equilibrium and rather stay its react in form. Now, here, spontaneity does not determine the speed of a reaction or process. When we take a look at a reaction, there's two things you need to keep in mind. We're talking about thermodynamics which deals with spontaneity and non spontaneous um reactions. So that determines if the reaction can go or not. On the other side, we look at K connects, which looks at the speed of a reaction. OK? It doesn't matter if a reaction takes a million years to happen or if it takes a second to happen, if it can happen naturally, it's a spontaneous reaction speed does not matter. So if we take a look here, we have two types of reactions here. We're gonna say we have diamond, carbon, diamond going towards basically graphite, carbon graphite. We see that on the top arrow, it's spontaneous. This is a natural process over time, diamonds can transform into graphite. So that's uh just uh I guess a cheaper form of carbon if you want to say. But the good thing is this takes millions of years. So you can buy diamonds and you don't have to worry about them converting into graphite over time. OK? Like it will take millions of years for that to happen. So here we say the rate in the forward reaction is going to be slow. It happens very slowly. And we're gonna say that if you're spontaneous in one direction, then you're non spontaneous in the opposite direction. So going from graphite to diamond is a non spontaneous process. It does not occur naturally. We'd have to basically input a lot of energy, a lot of heat, a lot of pressure in order to transform graphite into diamonds because the process is not a natural process. That's what makes diamonds also so precious. Now, here we have another type of reaction here, we have uh CO2 plus water giving us glucose plus oxygen. Here we'd say that this is an example of photosynthesis, right? So we have sunlight here, sunlight is the outside energy source that's needed without it. This cannot occur. OK. So we'd say here this reaction happens pretty fast. If we have the outside energy source present, it's just gonna push it forward, not going in the opposite direction where glucose plus oxygen gives us co2 and water. So here in that direction, we are a spontaneous reaction. So it's easy for glucose and oxygen to basically produce CO2 plus water. So this is cellular respiration on our part. OK. So we exude or, or or breathe out CO2 into the environment. OK. So that is a natural process, right? So again, remember a spontaneous process is natural. It can occur without the need of outside energy. A non spontaneous one is not natural and it needs a constant flow of energy for to even have a possibility of forming. Without that energy source, it does not naturally occur and there will be no products being formed, right? So keep this in mind also, spontaneous reactions can take a second to happen or less where they can take millions of years. The fact that it can happen makes it spontaneous.
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Spontaneous vs Nonspontaneous Reactions Example
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Here, it says to label each process as spontaneous or non spontaneous. Remember, a spontaneous process occurs naturally without the need of outside energy. A non spontaneous process is an unnatural process that requires a constant flow of energy for it to occur. So all you have to do is imagine if we're describing this process in the real world, can you imagine it happening? If you can, then it's a natural process and it's spontaneous. So here a as a sugar cube dissolved in room temperature water, we can picture ourselves taking a sugar cube, placing it in water, we know what would happen. The sugar cube would dissolve in the water since it can happen in the natural world, it's spontaneous. Next, mixing of three gasses in a container, gasses move randomly when put in a container. So if we took three different gasses and put it in a container, we can imagine them mixing together randomly, they don't just stay separate from one another, they just move around freely. So eventually they wouldn't mix. So this would be spontaneous as well because no outside energy is needed. Heat flows from cold object to warm object. So think about this. What's the natural flow of heat, heat traditionally moves from a hotter object to a colder object here, this is saying the exact opposite. So this would be a non spontaneous reaction because it's an unnatural process. It's not something we would typically see in the natural world, ice cube melts at negative one °C. Well, at zero °C, this is the temperature in which water starts to freeze. So if we go even lower than that, then we would expect the water to continue to freeze. So we shouldn't expect it to start melting. The temperature is too low. So here, this would be a non spontaneous process as well. A ball rolls down a hill. Now, here we're gonna add a little bit more to this. So we have a ball and let's just say that a gust of wind or something basically pushes it over the cliff. What would be the natural progression of the ball? The ball wouldn't just hover in the air. It would naturally go down the hill because it's naturally going down the hill. It would be a spontaneous process. Here, it's aided by gravity, which is not an outside source. It's a natural force. Finally, we have plant respiration where we have oxygen, glucose, produce co2 water and we have some a TP involved. Now we said that when it comes to uh photosynthesis, we need an outside energy source. So it's a non spontaneous process. The opposite of photosynthesis would be cellular respiration or plant respiration. This is a natural process. So it would be a spontaneous process. And that's how we describe each one of these processes as being.
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