In this video, we're going to focus on precipitation reactions and also write in that ionic equations. So let's say if you have the reaction between silver nitrate and calcium chloride. Now the first thing you need to do is you need to be able to determine the products of this reaction. So precipitation reactions are double replacement reactions.
The first and the last ions will pair up together and the two in the middle will pair up as well. Silver has a plus one charge. And you can tell based on the formula because nitrate has a minus 1 charge, and because there's only 1 nitrate, silver has to have a plus 1 charge to neutralize the negative 1 charge of the nitrate ion.
Chloride has a negative 1 charge. All of the halogens like fluoride, chloride, bromide, iodide, they all contain negative 1 charges as an ion. Because the magnitude of the silver and chloride ions, because the magnitude of the charges are the same, you could simply write them in a 1 to 1 ratio. Or, if you prefer, you can use the crisscross method, which is going to be Ag1Cl1.
So you really don't need to write the one, so we're just going to write it as AgCl. Now, when calcium pairs up with nitrate, let's see the formula that's going to be produced as a result. Typically, the positively charged ion is written before the negatively charged ion. So using the crisscross method to write the formula, it's going to be Ca1NO3 2. Now whenever you have multiple polyatomic ions, you need to enclose it inside a parenthesis.
So the other product is CaNO3 2. Now, the next thing we need to do is balance the equation. So, feel free to pause the video and balance it, and then unpause it to see if you have the right answer. Notice that we have two nitrates on the right side. So, we've got to put a 2 in front of AgNO3. And we have two chloride ions on the right, I mean on the left.
So, we need to put a 2 in front of AgCl. Notice that the reaction is now balanced. Our next step is to write the phases of every substance. Silver nitrate, is it in the aqueous phase or in the solid phase? Nitrates are always soluble.
So anytime you see NO3, it's going to be in the aqueous phase. Now what about calcium chloride and silver chloride? Chlorides are generally soluble with the exception of silver, mercury, and lead. So silver chloride is insoluble, but calcium chloride is soluble.
Now that we have our complete molecular equation, the next thing we need to do is write the total ionic equation. So every substance that is in the aqueous phase, we need to separate it into ions. Anything else, like the silver chloride, which is in the solid phase, we're going to leave it the way it is. So silver nitrate contains the Ag plus ion. and the NO3 ion.
And it has two of each, so you need to distribute the two. Calcium chloride has the Ca2 plus ion. All of these ions are in the aqueous phase, by the way. Now on the right side, we're going to leave the AgCl the way it is, and this is going to remain in the solid phase. And for calcium nitrate, it's going to be Ca2+, and 2 NO3 ions.
What are the spectator ions in the total ionic equation? The spectator ions are those ions that look exactly the same on both sides. So this includes the nitrate ions and the calcium ions.
Now what remains is the net ionic equation. So notice that we have a coefficient of 2 for what remains. If we divide everything by 2, the net ionic equation is going to be Ag plus plus Cl minus produces AgCl.
So this is the net ionic equation. And of course the ions are in the aqueous phase. So that's how you can write the net ionic equation from a precipitation reaction. So let's go ahead and try another example.
So let's say if we have a solution of lead nitrate and sodium bromide. Typically, the solutions on the left side, these substances are dissolved in water, so it's usually in the aqueous phase. Nitrates are always soluble, and alkali metals like lithium, sodium, potassium, they're always soluble.
So make sure you know your solubility rules. So go ahead, pause the video, predict the products of this reaction, balance the equation, write the total and net ionic equation after that. So let's begin by writing the products of the reaction. So Pb is going to pair up with Br.
Now what is the charge on the lead ion? Notice that it has two nitrates. Each nitrate ion has a negative 1 charge, which means that Pb has to have a plus 2 charge to neutralize it.
Now bromide is a halide, so we know it has a negative 1 charge. So using the crisscross method, it's going to be Pb1Br2. So now let's pair up the other two ions, sodium and nitrate.
Sodium is an alkali metal in group 1, and alkali metals usually form plus 1 charges. Nitrate we know has a minus 1 charge. So because the charges are the same, these two ions will combine in a one-to-one ratio. So we don't need to put the one for NO3.
So we could just write NaNO3 any time the charges are the same. You could just write them together. So now our next step is to balance the equation. So notice that we have two nitrates on the left side, so we've got to put a 2 in front of NaNO3.
We have two bromine atoms on the right, so we need to put a 2 in front of NaBr. And so now the reaction is balanced. So now, we need to determine the phases of the ions on the right side.
Nitrates are always soluble, so this is in the aqueous phase. And lead bromide, the halides like chloride, bromide, iodide, they're generally soluble except with silver, lead, and mercury. So this is an exception.
So lead to bromide is insoluble. So now at this point, we need to separate everything in the aqueous phase into ions. So we have the Pb2 plus ion.
2 nitrate ions 2 Na ions don't forget to distribute the 2 and 2 bromide ions Now on the right side, we're going to leave PbBr2 the way it is, and we're going to separate this compound into two ions. So we can see that the speck to the ions are the sodium ions and the nitrate ions. So now what remains is the net ionic equation, which is the reaction between the lead ion and the bromide ion. And this is going to produce lead bromide.
So that is it for this video. Thanks for watching and have a great day.