Now that we've done that one, let's take a look at example 2. In example 2, we have to do the same thing once again. So here, we have CN- but all of a sudden it becomes HCN. How did that happen? The CN- must have accepted an H+ because by accepting the H+ it's a base. Who's giving it that H+? It must have been the water. Water gives away an H+, making it an acid. When water gives away the H+, what happens to the water? The water becomes OH-. Here, we would say that this is the conjugate base. Whatever you are, your conjugate is the opposite. If this is a base, this is a conjugate acid. Then based on that, we'd say that these two are conjugates of one another and then these two also. Just realize, water acts as a base in the first example but as an acid in the second. Something that can act as both an acid and a base, we said, was called amphoteric. Water is the best example of an amphoteric species. Depending on what it's next to, it could act as either an acid or a base. So always be careful. And we know that this has to be the base because it's negative. We're still using the rules we've learned before.
And if we go back up to water, actually, how do we know HF is the acid and H2O is the base? Because remember, they're both going to have H connected to an electronegative element. F and O are both in the same period. Remember, we said when you're in the same period, what do we look at? We look at electronegativity. So HF is definitely a stronger acid than H2O. As a result, HF must be the acid, H2O must be the base. The rules we learned earlier play a huge role in what we're doing right now. Here, I gave us the product so it's easy to say who was the acid and who was the base. But on your exam, you may not get that luxury. Your professor might just give you HF plus water and ask you what are your products. So you still have to remember who would be the stronger acid. That person will be the acid. The other one would have to be the base.
Now that we've seen that, let's take a look at practice questions 1 and 2. For this one, we're asking, which of the following is an acid? I'll give you guys a huge help here. Remember, a Brønsted-Lowry acid has to have H+ in there. It has to have H. If it doesn't have H, it's out. Second, for something to give away an H, H needs to be connected to something that is electronegative. If H is connected to an electronegative element, it's going to make a polar bond. If that bond is polar, that means it's reactive. That means I can break it and take that H+ off. What bonds are nonpolar? Well, if H was connected to itself, this would be a nonpolar bond. So as a result, H2 does not represent an acid. Also, who else? We can also say that when H is connected to C, their electronegativities are not that far off from one another. So their electronegativities are very similar. So we would expect that bond not to be polar. So as a result, if you have H connected to carbon, we would expect that to not be an acidic bond. So, I gave you guys two huge helps to help you figure out which one of these could be a Brønsted-Lowry acid. Use the rules that we've learned earlier to identify something as either an acid or a base. That'll help guide you to what's a Brønsted-Lowry acid for this particular question.