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Molecular, complete ionic, and net ionic equations

In the molecular equation for a reaction, all of the reactants and products are represented as neutral molecules (even soluble ionic compounds and strong acids). In the complete ionic equation, soluble ionic compounds and strong acids are rewritten as dissociated ions. In the net ionic equation, any ions that do not participate in the reaction (called spectator ions) are excluded. As a result, the net ionic equation shows only the species that are actually involved in the chemical reaction. Created by Sal Khan.

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Video transcript

- [Instructor] What we have here is a molecular equation describing the reaction of some sodium chloride dissolved in water plus some silver nitrate, also dissolved in the water. They're going to react to form sodium nitrate, still dissolved in water, plus solid silver chloride and if you were to look at each of these compounds in their crystalline or solid form before they're dissolved in water, they each look like this. But once you get dissolved in water, and that's what this aqueous form tells us, it tells us that each of these compounds are going to get dissolved in water, they're no longer going to be in that crystalline form, crystalline form. Instead, you're going to have the individual ions disassociating. So for example, in the case of sodium chloride, the sodium is going to disassociate in the water. Sodium is a positive ion, or cation, and so it's going to be attracted to the partially negative oxygen end. Remember, water is a polar molecule. That's what makes it such a good solvent. Now, the chloride anions, similarly, are going to dissolve in water 'cause they're going to be attracted to the partially positive hydrogen ends of the water molecules and the same thing is gonna be true of the silver nitrate. Silver ... The silver ion, once it's disassociated, is going to be positive and the nitrate is a negative. It is an anion. Now, in order to appreciate this and write an equation that better conveys the disassociation of the ions, we could instead write the equation like this. This makes it a little bit clearer that look, the sodium and the chloride aren't going to be necessarily together anymore. The sodium is going to dissolve in the water, like we have here. The chloride is gonna dissolve in the water. The silver ions are going to dissolve in the water and so are the nitrate ions. So this makes it a little bit clearer and similarly on this end with the sodium nitrate stays dissolved so we can write it like this with the individual ions disassociated. But the silver chloride is in solid form. You can think of it as precipitating out of the solution. This is not ... This does not have a high solubility, so it's not going to get dissolved in the water and so we still have it in solid form. Now you might say, well which of these is better? Well it just depends what you are trying to go for. This form up here, which we see more typically, this is just a standard molecular equation. Molecular ... Molecular equation. It's in balanced form. We always wanna have our equations balanced. This right over here is known as a complete ionic equation. The complete's there because we've put in all of the ions and we're going to compare it to a net ionic equation in a second. Complete ionic equation, sometimes just known as an ionic equation. Now why is it called that? Well, 'cause we're showing the individual ions as they're disassociated in water. Now, what would a net ionic equation be? Well let's think about that a little bit. Let me free up some space. So one thing that you notice, on both sides of this complete ionic equation, you have the same ions that are disassociated in water. So for example, on the left-hand side you have the sodium that is dissolved in water and you also have on the right-hand side sodium dissolved in the water. It's not, if you think about the silver chloride being the thing that's being produced, this thing is in ionic form and dissolved form on both sides of this reaction and so you can view it as a spectator, and that's actually what it's called. It's called a spectator ion. Spectator ion. If you wanna think of it in human terms, it's kind of out there and watching the reaction happen. Its value in this reaction is, well it was part of the sodium chloride and its providing ... So the sodium chloride is providing the chloride that eventually forms the silver chloride, but the sodium is just kind of watching. Similarly, you have the nitrate. The nitrate is dissolved on the left and the nitrate is dissolved on the right. So the nitrate is also a spectator ion. So if you wanna go from a complete ionic equation to a net ionic equation, which really deals with the things that aren't spectators, well you just get rid of the spectator ions. You get rid of that. You get rid of that. You get rid of that. You get rid of that, and then you see what is left over. Well what we have leftover is we have some dissolved chloride, and we write aqueous to show that it is dissolved, plus some dissolved silver, plus some dissolved silver. Once again, to show that it's dissolved we write aqueous and if you put those two together, you are going to get some solid silver ... Solid silver chloride. And what's useful about this form, one it's more compact and it's very clear what is actually reacting, what is being used to build, and you can say hey, however you get your chloride into the solution, however you get your silver into the solution, these are the things that are going to react to form the solid. Instead of using sodium chloride, maybe you use potassium chloride and the potassium in that case would be a spectator ion. But either way your net ionic equation would be what we have here.