Hi everyone. One important thing in chemistry is recognizing the type of chemical reaction that you see in front of you. The reason is because once you know what reaction you have, it becomes easier to predict the products you're going to get and how to improve the reaction. In general, there are two ways to classify chemical reactions.
One method splits reactions into three types. Precipitation reactions, acid-base reactions, and oxidation-reduction reactions. The second method groups the reactions into the five types listed here. We will discuss both of these methods in the next sections. Precipitation reactions are reactions where you produce a solid product where your reactants are both aqueous.
The picture here shows a typical appearance of this reaction. You can see that the two reactants, Ki and PbNO3, are both clear aqueous solutions. When Ki is added to lead nitrate, you immediately see a yellowish cloud forming.
Anytime you see a solution turning cloudy, that's a sign that a precipitate has formed. The precipitate, or solid, is preventing you from seeing through the solution because solid particles are densely packed together. Here's a video that illustrates the moment that Ki is added to PbNO3-2 and the formation of the precipitate. Not every addition of two aqueous reactants result in a precipitate formation. Sometimes you add two reactants and the two products you make are both aqueous.
In that case, we say that there is no reaction. An example will be the mixing of KI and NaCl. The products we get are KCl and NaI.
These are both soluble salts, so we will write no reaction on the product side as shown here. So how do we know whether two aqueous reactants will give us a precipitate or not? Well, we follow the steps that's outlined here in the notes. First, we start with the reactants, which are both soluble ionic compounds.
We can label these AB aqueous and CD aqueous. To create the products, what we do is we swap the cations A plus and C plus and partner them up with the opposing anions. What we have now is...
AD and CB. So we take this and partner up with that. We take this and partner up with that. This type of reaction is what we call a double displacement reaction because both cations are swapped.
You'll see in a later video that we also have a single displacement reaction. We then use the solubility rules to determine whether CB and AD are soluble or insoluble. If both CB and AD are soluble, we would write the reaction as the following.
AB plus CD gives no reaction, or sometimes you can also write it with a slash over the arrow. If one or both of these products are insoluble, we write their formulas and put an S next to the one that is insoluble and AQ for the one that is soluble. An example would be AB plus CD warming AD, solid, and CB, aqueous. Lastly, we will balance our equation. Okay, let me show you how to apply this idea in an example that's shown right here.
It says that we're going to write an equation for the precipitation reaction that occurs, if any, when solutions of sodium carbonate and copper chloride are mixed. We need to first write our reactants. Sodium carbonate is Na2CO3.
and this is going to be aqueous. And then copper to chloride is CuCl2. These are both aqueous species in this double displacement reaction.
And then the product is going to be a swap of the two cations. So I would take copper and now I would partner that up with carbonate. And then I would take sodium and I would partner that up with chloride. So my product is going to be sodium chloride and copper carbonate.
Now the next thing is I need to determine whether the formula I've written for the products are correct. To do that, I need to know my charges for my ions. In this case, Na is a plus one species.
Cl-is a negative one species, so that adds up to a neutral species, which means that that formula is already correct. Copper in this case is copper two, according to our question, so it's a plus two species. Carbonate is a polyatomic ion.
that has a charge of negative two. plus two and minus two, neutralize each other, giving us zero. So that formula is also correct. So now that we have both the correct formulas for the products, what we need to do is fill out what the states are for each of these products.
To do that, we go to our solubility rules. So here is our solubility rules shown on the left, and what we need to do is find out whether each of the following products, the ionic compounds or the salts, are soluble or insoluble. If you look at NaCl, That is a group 1A salt because it contains sodium.
And as a result, all group 1A is soluble. So we would write this as AQ. Copper carbonate, we would scan through the solubility rules to see if there's anything that has either copper or carbonate. And we see that there is a carbonate rule right here.
And it says all of them are insoluble with some exceptions being group 1A or ammonium. Copper is not one of those guys. So as a result.
this would be a solid. And that's the answer to that problem. We talk about double displacement reactions that generate a solid product, which we call a precipitate.
Here we will talk about double displacement reactions where the reactants are what we call acids and bases. Recall that acids are covalent compounds whose formulas start with H and is an aqueous state. Bases are soluble ionic compounds containing the hydroxide ion.
or OH-. So an example of an acid-base reaction is the reaction between hydrochloric acid and sodium hydroxide. To predict the products, you will use the same steps that we use for the precipitation reaction.
First, you will swap the cations, which is H+, and Na+. You will combine H+, with OH-, and Na+, with Cl-, forming NaCl, N, H+, and OH-. combine to form H2O or water. Then we put the state next to each of these compounds. Water is always a liquid in these reactions.
And for the salt, we use the solubility rules to determine what its state is. NaCl is a group one salt, so it should always be soluble or aq. Many acid-base reactions produce the products shown here, which is water and salt. which is just another way we call ionic compounds. A subset of acid-base reactions form unstable compounds that will decompose to form smaller molecules.
One of these decomposition products is usually a gas, so we call these acid-base reaction gas evolution reactions. There are only a few of these gases, so you will have to memorize them. They are listed here on this table.
For example, if your acid-base reaction forms H2CO3, aqueous or carbonic acid, this compound is unstable and it will break apart to form CO2 gas and H2O liquid. Let me work out an example for you. Let's take a look at the mixing of aqueous nitric acid and aqueous sodium carbonate.
So nitric acid is HNO3, sodium carbonate is Na2CO3. We would swap the cations. In this case, Na would get paired up with NO3 and H would get paired up with CO3.
So NaNO3 plus HCO3. Just like in the precipitation reaction, we will look at the charges to make sure that we have written the correct formula. Na has a charge of plus one.
NO3 is a polyatomic ion with a charge of minus one. So those two balance each other out, and that's the correct formula. Hydrogen has a charge of plus one. Carbonate is a polyatomic ion with a charge of minus two. So right now that formula doesn't balance each other.
In order to make it balance, we have to put a 2 next to the hydrogen to have a total charge of plus 2, which will balance out the negative 2 from the carbonate. So that's the correct formula. Once we do that, then we write the states for each of these.
NaNO3 is a group 1 salt because it has sodium in it, and so it's going to be soluble based on the solubility rules. H2CO3 is just an acid, so it's going to be soluble. Well...
As you can see, that H2CO3 is one of the products that's listed, which happens to be the one we have here. So that's going to decompose or break apart into two products. One of them is going to be H2O, which is always a liquid. And if you take out H2O from that formula, what's left is CO2, which is your gas.
So that's your final set of products. There are three products in that reaction. So here's an example of a gas evolution reaction.
In this case, this person is demonstrating that he has one reactant here and he's going to add the other reactant that's been kept at the top. the balloon and of course if it produces a gas the gas is going to fill up the balloon just like shown in these other examples here so he's adding the reactant as you can see there is bubbling occurring and look at what happens to that balloon right there the gases are filling it up the last type of reaction that is combined together with the precipitation acid-base reaction that's called oxidation reduction or we often abbreviate it as redox reactions these are reactions where electrons are being transferred from one reactant to another. So if you notice that a reaction is not double displacement, then those are automatically redox reactions. Double displacement reactions have this pattern of AB plus CD goes to AD plus CB. So as long as they don't look like this, then you would consider that a redox reaction.
So for example, if you look at this reaction here, Na plus Cl2. goes to NaCl. Now the reactants don't have this AB and CD pattern because the AB and CD are both ionic compounds.
So neither one of these guys is an ionic compound. This is just an element and that's also an element. So because of that we would call this a redox reaction. This question right here is asking you to recognize which of these is a redox reaction and which one is not. You look at reaction one, you have a reaction where you have an element plus another element forming.
a compound, that reaction doesn't have the pattern of AB plus CD goes to AD plus CB. You don't have two products in this case. So as a result, that must be a redox reaction.
If you look at the second reaction, you have HBr plus CaOH2 forming H2O and CaBr2. Now if you remember our discussion about acid-base reaction, this one is an acid and that's a base. And so that must be a double displacement reaction. But also it has that AB plus CD pattern.
This is your A, this is your B, this is your C, and that's your D. And you form A and D, which is the OH minus ion, I should make it clear. And then the CA here is your original cation here, combining with the other anion.
So in other words, that one is a double displacement reaction, so it is not a redox reaction. So there's certain examples of redox reaction that we will discuss in a lot more detail in the next video. For example, when you see a substance reacts with oxygen, that would be an example of redox reaction.
So any combustion reaction, a metal reacting with a nonmetal, which is basically these reactions right here, that's a redox reaction. And we will talk a lot more about other examples of redox reaction.