So it looks at, it only cares about really gases and also aqueous. Aqueous just means that the solvent is water, so a compound is dissolved in water. It would be aqueous. Now, knowing this, let's take a look at the following example. It says, Write the equilibrium expression for the following reaction. Here we have N2O5 (aqueous) gives us 4NO2 (aqueous) plus O2 (aqueous). It's important to look at the phases because remember if it's a solid or a liquid, we ignore it. Now we're going to say here k= products over reactants. So we're going to have our products as NO2 and remember, you have to use the coefficients in these calculations. The number in front of NO2 is a 4, so that 4 will become the power. So it's going to be NO2 to the 4 times O2. O2 just has a coefficient of 1 in front of it, which we don't have to show. Divided by N2O5. Again, the coefficient in front of N2O5 is a 2, so that becomes the power. So we would say that this represents our equilibrium expression or our equilibrium equation. Same thing. Equilibrium expression, equilibrium equation. Now that we've seen this one, let's look at B.
For B, we're going to say here, we're going to ignore this compound because it's a solid and we're going to ignore this compound because it's a solid. Here we're going to say k= products over reactants like before. But here's the thing, we're going to say we don't have any products available. But you have to put something on top. We're going to say it's equal to 1. Solids and liquids are ignored and in place of them, we're going to put 1. We're going to say this because things such as pressure don't really affect solids and liquids like they do gases and aqueous compounds. That's why they're equal to 1 because their number is being held constant. So we're going to replace solids and liquids with the number 1. And on the bottom we'd have O2. Now, a different way your professor could give you this is, we know there's a one here, so this could also be O2-1. So be aware of both types of answers. Both are correct. Both are saying the same thing. Inverse 1 just means 1 over whatever it is.
For the next one, again we ignore solids. The only thing that we don't ignore is this gas. Here, k= products over reactants, so just equal Xe3 because of the 3 over 1. But we don't need to put the one because anything over 1 is the same exact thing. So that would be our equilibrium C. We ignore the solids and the liquids.
Now that you've seen this, I want you guys to attempt to answer a question that follows this one, the practice one. Here, I want you to tell me who's greater in amount? Is it products or is it reactants? From that, you have to remember what do we say about K. When it's greater than 1, who's favored? When it's less than 1, who's favored? When it's equal to 1 who's favored. Remembering that will be a great way for you to approach this problem. Good luck, guys.