Writing Chemical Reactions is based on our understanding of the Solubility Rules.
When writing a chemical reaction our reactants break up into ions and then reassemble to create products. This type of reaction is referred to as a double displacement or metathesis.
A balanced molecular equation is an equation showing the neutral forms of compounds.
All that happens is that soluble ionic compounds separate and become ions in the chemical equationl. This represents the total or full ionic equation.
Concept: Writing Molecular Reactions6m
Hey, guys! Welcome back! In this new video, we finally get to see how exactly do we come up with the products in a balanced equation. Now up to this point, we've been given chemical reactions in which they gave us the reactants and the products. Now it's up to us to figure out how to write those products just from the information they give us for the reactants.
Let's take a look at this first example. It says predict whether a reaction occurs, and write the balanced molecular equation.
Now first thing we're going to say is a reaction only occurs if we create a gas, a liquid, or a solid. So we have to produce one of these types of products. If we don't create a gas, a liquid or a solid then no reaction occurs. If all our products are aqueous that also means we don't form one of the three phases, so then no reaction occurs. So remember, you have to form a gas, a liquid or a solid.
Now let's take a look at this first equation here. Here we have lithium hydroxide (aqueous), aqueous just means it's surrounded by water, plus magnesium sulfate (aqueous). To do this it's going to be important that you guys remember your solubility rules.
We're going to say that lithium is a Group 1A ion. Group 1A ions regardless of who they're with, they're always soluble. So lithium hydroxide will be a soluble compound, meaning that when it's surrounded by water it's going to be broken up into its ionic forms. Lithium is in Group 1A so it exists as Li+1. Remember polyatomic ions that we learned, they're annoying to memorize but they are essential for this.
Hydroxide is a polyatomic ion and it's OH-1.
For magnesium sulfate, remember the rules for sulfates, sulfates are soluble here. They're only insoluble when they're with certain types of elements. So here this is going to break up into Mg2+ and then sulfate here is going to be SO42-. Remember, it's essential you guys remember your polyatomic ions.
Now that we've broken up everyone into their ions, it's time for us to switch partners. So these guys are going to act like swingers and switch partners with each other. So remember in chemistry opposites attract. This positive is going to be attracted to this new negative sulfate. The one from the lithium is going to come over here. The two is going to come over here.
So that's going to create our first product. And remember when we create the product the positive ion is always listed first. So Li first, the two from the sulfate comes down here and then we have SO4 plus, now the positive Mg would be attracted to the negative hydroxide ion, the two from here moves here, the one from here moves here. Remember Mg is the positive ion so it has to be written first. Mg, that two that moved over to the OH goes for the entire structure. So you can't just put OH2 because that would just say that the two is going to the hydrogen. But the two is being shared by both the hydrogen and the oxygen, so you have to put brackets and then a little two outside.
Now we have to remember our solubility rules. Lithium is a Group 1A ion. Group 1A ions regardless of who they're with, they're always aqueous. So this is going to be aqueous.
Next, we have hydroxide ion, solubility rules for hydroxide ion says hydroxides are solids, they're insoluble unless they're connected to a Group 1A element. Magnesium is not that so we don't satisfy that. If hydroxide is connected to NH4+ it would be aqueous, still haven't reached there. Or if hydroxide is connected to calcium, barium, or strontium then it would be aqueous. Magnesium doesn't fit any one of those three categories, therefore, this is going to be insoluble compound, it's going to be what we call a precipitate or solid.
We're going to say that a reaction does occur because we create a solid, that's what you have to see here. We created a solid as one of our products and as long as we create one product that is at least a gas, a liquid or a solid, we say a reaction occurs.
Now we have to balance it. On the left side, we have one lithium and on the right side, we have two, so we're going to throw a two here. On the left side, we have one sulfate, one SO4, and on the right, we have one sulfate, so we're good there. On the left, we have one magnesium and we have one magnesium. And then on the left, we have -- this two gets distributed to the OH also so now we have two OH's just like we have two OH's here.
So our equation is now balanced. This is the approach you have to take in order to figure out what your products are. First, you break up the reactants into their ions then they switch partners. The positive ion will be attracted to a new negative ion and vice versa.
Concept: Balanced Molecular Equation5m
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 +1. CO3 is a polyatomic ion. It's carbonate. Carbonate is CO3 2-. 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 +1. Then bromine breaks off into Br -1. Remember, it's -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 negative ion. Here we're going to write NaBr. Then we're going to say this two 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, so it's going to be H and we're going to say there's a little 2 there, 2, CO3.
Based on the rules, we're going to say carbonates are soluble when they're with group 1A ions. Hydrogen happens to a group 1A ion, so this will be soluble. This will be aqueous. 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. Here's the thing, anytime we from a carbonic acid as a product, it's going to break down even further. It's going to decompose naturally. 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 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. 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 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. This is also gas evolution.
And it can also happen for one more structure, NH4OH. This is ammonium hydroxide. Again, if you ever form one of these as products, you have to remember that they're going to break down further. So this is going to give me H2O liquid. You see that you're making H2O on each of them. Everything else left behind comes together to form my gas, NH3 gas. All three of these are gas evolutions.
So 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, so we're going to put a two here. That gives me two Br's, 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.
In a balanced molecular equation reactant compounds exist as ions that recombine to form products.
Concept: Total Ionic Equation2m
From there we have to write the total ionic. For the total ionic, we're going to say all compounds that are aqueous, aq, break up into ions. If it's not aqueous, it does not break up, so the water that's a liquid and the CO2 that's a gas, those will not break up. All the other compounds in our equation are aqueous, so they will break up.
I'm actually going to take myself out of the shot guys so we can have more room to write everything out.
We already know how the reactants are going to break down because we saw that earlier, so now we're going to say when they break down we're going to have 2Na+ aqueous, you have to make sure it's balanced, plus CO3 2- aqueous, remember this breaks up because it's aqueous, plus, there's a 2 in front of that HBr, so 2 H+ aqueous plus 2Br- aqueous gives me H2O liquid and it doesn't break up because it's a liquid. Only aqueous things break up. CO2 gas plus 2Na+ aqueous plus 2Br- aqueous. That represents our total ionic. I broke down everything that is aqueous.
In a total ionic equation only the compounds that are aqueous can break up into ions.
Concept: Net Ionic Equation2m
Finally, we have to do the net ionic equation. The net ionic equation is the total ionic equation, but we have to remove the spectator ions. What's a spectator ion? A spectator ion is an ion that looks exactly alike on both sides of the arrows. What we're going to say here is, Na+ is a spectator ion because it looks exactly the same on both sides of the equation. As a reactant, it looks exactly the same as a product. But that's not our only spectator ion. Our other spectator ion would be Br-. It looks exactly the same on both sides of the arrow.
We're going to say in this equation our spectator ions are Na+ and Br-. The net ionic equation does not include the spectator ions. We totally get rid of them. At the end what we're going to have left are CO3 2- aqueous plus 2 H+ aqueous gives me H2O liquid plus CO2 gas. This is the ways and attempts that we take when we have to write out the molecular equation and based on our solubility rules, this helps us determine the total ionic and the net ionic.
In a net ionic equation the spectator ions are removed and what remains is the net ionic equation.
Remember when asked to write a complete and balanced molecular equation that a reaction only occurs if a solid, liquid or gas is formed as a product.
Problem: Predict whether a reaction occurs, and write the balanced total and net ionic equations.4m
If your chemical equation produces only aqueous products then the answer is NO REACTION.
Problem: Predict whether a reaction occurs, and write the balanced total and net ionic equations.3m
Consider the metathesis reaction between FeF3 and (NH4)2S in water. Write the balanced molecular, total ionic and net ionic equations. Also, identify any spectator ions and include physical states for each equation.
Total Ionic Equation:
Net Ionic Equation:
Which are spectator ions in the reaction of aqueous solutions of ammonium perchlorate + sodium bromide
A. ammonium and bromide
B. sodium and perchlorate
C. ammonium and perchlorate
D. sodium and bromide
E. all are spectator ions
Consider the double displacement reaction of calcium nitrate with potassium sulfide. Which response identifies all of the products formed and no others?
1. CaSN and PO2
2. Ca and S and K and H2 O
3. CaSO2 and K3 N
4. None of the other answers is correct
5. CaSO3 and KNO2
6. CaSO3 and KNO3
7. CaSO4 and PNO3
8. CaS and KNO3
9. CaN3 and K2O and SO2