Now that we know how to write the molecular equation, which is what we just did, it's important that we learn how to, from the molecular equation, give us the total and the net ionic equations. We're going to take the same approach we learned earlier. Here we have Na2CO3 plus HBr.
What we're going to say here is we're going to break this up into its ions. We can do this because they're both aqueous. Na is going to break off and Na is in group 1A, so it's plus 1. CO3 is a polyatomic ion.
It's carbonate. Carbonate is CO3 2 minus. That's where this little 2 came from.
It came from the carbonate. HBr breaks up into H+, because remember hydrogen is in group 1A, so it's plus 1. And then bromine breaks off into Br-1. Remember it's minus 1 because it's in group 7A. Next what we're going to do is we're going to switch partners.
This positive ion is attracted to this new negative ion. So here we're going to write NaBr. Then we're going to say this 2 from this carbonate is going to move down here. This one over here is going to move over here.
Remember, since the hydrogen is the positive ion, it's listed first. It's going to be H and we're going to say there's a little 2 there, 2Cl3. Now, based on the rules, we're going to say carbonates are soluble when they're with group 1A ions. Hydrogen happens to be a group 1A ion, so this will be soluble. This will be aqueous.
And then for NaBr, group 1A ions such as sodium are soluble with everyone. So this is also aqueous. But here there's a little thing that happens.
This is what we need to realize with this special type of question. Here we created H2CO3. H2CO3 is carbonic acid.
And here's the thing, anytime we form carbonic acid as a product, it's going to break down even further. It's going to decompose naturally. So we're going to say H2CO3 breaks down to give me water as a liquid. Plus, CO2 as a gas.
I know this is kind of like, whoa, what happened? What just happened here? This is called a gas evolution reaction.
Gas evolution means that you form gas as a product. CO2 happens to be one of the gases. So remember, the answer you have there, H2CO3 aqueous, would not be the correct answer. You have to realize that any time you form carbonic acid, it breaks down.
one, to give me water liquid plus CO2 gas. Everything matches up on both sides. We have two hydrogens on both sides, one carbon on both sides, three oxygens on one side. What you also need to realize, it doesn't only happen for carbonic acid. It can also happen for H2SO3.
This is sulfurous acid. It can also happen for this one. This will also break down into H2O liquid plus SO2 gas.
So this is also gas evolution. And it can also happen for one more structure, NH4OH. So this is ammonium hydroxide.
So again, if you ever form one of these as products, you have to remember that they're going to break down further. This is going to give me H2O liquid. You see that you're making H2O in each of them. Everything else left behind comes together to form my gas, NH3 gas.
All three of these are gas evolutions. That H2CO3 that I have there, I'm actually going to erase it now because it decomposes and what I really get is H2O liquid plus CO2 gas. We're going to say a reaction does occur because we created a liquid and a gas. What we have to do next, we have to just make sure everything balances out.
We have one hydrogen on the left side, but we have two on the right side. We're going to put a two here. That gives me two Brs, but on the right side we only have one, so I'm going to put a two here. We also have two sodiums now on both sides so it's totally balanced.
This would represent my molecular balanced equation. Remember, this is a gas evolution type of question.