Chemical reactions can be classified into five types based on how the reactants interact to form products. So far in this lesson, you have learned the characteristics of three types of chemical reactions: synthesis, decomposition, and combustion. The last two types of reactions you will learn about involve the replacement of an element in a compound. Study the information on the tabs to learn about each one.
In a double replacement reaction two compounds exchange cations (positive ions) creating two new compounds. Remember, in ionic compounds the cations are the first ion and the anions (negative ions) are the second.
Watch the video to see some examples of double replacement reactions and what the equations for these types of reactions look like.
Double replacement reaction shows yellow and pink dot together plus green and blue dot right arrow to green and pink dot together and yellow and blue dot together.
You may want to use the study guide to follow along. If so, click below to download the study guide.
We've looked at reactions where compounds come together or break apart, but now we are going to look at a set of reactions where the interaction between the reactants is a little more complex. In double replacement reactions, two compounds exchange their cations, which - you'll recall - are the positive ions.
In these types of reactions, there will be exactly 2 reactants, which will both be compounds, and 2 products, which will also be compounds. At the end of the reaction, all of the elements will of course be the same, but the cations from the two compounds will have swapped places, like switching partners in a dance.
You can see an example of a double replacement reaction here. In this chemical reaction, silver nitrate is reacting with sodium chloride - table salt - to form silver chloride and sodium nitrate. The cations, silver and sodium, switched places, and two new compounds were formed.
To better understand this, let's look at a few examples.
This chemical reaction is occurring between barium chloride and potassium carbonate. In the first compound, barium is the cation, and in the second compound potassium is the cation. In a double replacement reaction, the cations, barium and potassium, will switch places. This gives us barium carbonate and potassium chloride. Notice that - because the cations and anions are present in different ratios in the two compounds, we have to use a coefficient of 2 next to the newly formed potassium chloride.
Another example is iron 2 sulfide reacting with hydrogen chloride (or hydrochloric acid). Here, the cations are iron 2 and hydrogen, so those two will swap places. You'll need to know the charges of the ions to make sure that you have the ratios correct. The result of that swap is hydrogen sulfide and iron 2 chloride.
Now let's work one through together. This example reads, “Write the balanced chemical equation for the reaction between sodium hydroxide (NaOH) and iron 3 nitrate (Fe(NO3)3).”
Well, let's write down that reaction: NaOH plus Fe(NO3)3. Now, let's keep track of the charges of each of the ions involved. Sodium is 1 plus, hydroxide is 1 minus, iron 3 is of course 3 plus, and nitrate is 1 minus. Since sodium is 1 plus and nitrate is 1 minus, they combine to form NaNO3 - sodium nitrate. Since iron is 3 plus, and hydroxide is 1 minus, we need three hydroxides for each iron, creating Fe(OH)3 - iron 3 hydroxide.
Now let's see if we're balanced. Well, we only have one hydroxide on the left side, and three on the right, so we can fix that with a coefficient of three over here. Now we have three nitrate ions and three sodiums on the left, but only one of each on the right, so we fix that with a coefficient of three over here.
So, in this double replacement reaction, our reactants were sodium hydroxide and iron III nitrate, our products were sodium nitrate and iron III hydroxide, and this equation is balanced.
Let's do one more of these. This example reads, “Write the balanced chemical equation for the reaction between barium nitrate (Ba(NO3)2) and phosphoric acid (H3PO4).”
So in this reaction we have Ba(NO3)2 reacting with H3PO4. The charge on a barium ion is 2 plus, nitrate is 1 minus, hydrogen is 1 plus, and phosphate is 3 minus. When we swap the cations, barium and phosphate are now paired, and if we criss-cross those charges, we see that the come together in a 3:2 ratio. Hydrogen and nitrate both have a charge of 1, so they just combine to form HNO3.
Right off the bat, we can see that there is only one barium on the left, and three on the right, so we need to use a coefficient on the left to balance the barium. But now we have 6 nitrates on the left, so we need to put a coefficient of 6 on the HNO3, which is nitric acid. Lastly, we need to balance the hydrogen and phosphate by putting a coefficient of 2 over here on the left side. And now it's balanced.
So our reactants were barium nitrate and phosphoric acid. The barium and hydrogen, our two cations in this reaction, swapped places, and the result was barium phosphate and nitric acid. And we have checked that this equation is indeed balanced.
Question
If sodium phosphate reacted with silver nitrate in a double replacement reaction, what would be the names of the two products?
silver phosphate and sodium nitrate
In a single replacement reaction one element takes the place of another element in a compound. Single replacement reactions occur between a pure element and a compound, and the products of these types of reactions are a different pure element and a different compound.
Watch the video to see some examples of single replacement reactions and what the equations for these types of reactions look like.
Single replacement reaction shows yellow dot plus green dot and purple dot together right arrow green dot plus yellow dot and purple dot together.
You may want to use the study guide to follow along. If so, click below to download the study guide.
The last type of chemical reaction that we're going to look at are single replacement reactions. As you might have guessed from the name, they have a lot in common with double replacement reactions. But while double replacement reactions occur between two compounds, single replacement reactions occur between an element and a compound, where the element swaps in for one of the elements in the compound.
When these types of reactions take place, the elements that switch places are similar to each other. The most important similarity they have is that their ions are either both positive or both negative. That is to say, cations swap with cations, and anions swap with anions.
In this video, you're seeing a single replacement reaction take place. Solid zinc is reacting with hydrogen chloride. Zinc is an anion, so it switches out with the hydrogen - which is also an anion - in the hydrogen chloride. When the reaction goes to completion, the result is zinc chloride and pure hydrogen gas.
Now that we've seen what this reaction looks like at the visible scale, let's try to quantify this reaction to understand it better.
So, in this reaction, hydrochloric acid was poured over solid zinc. Zinc is Zn. Now, let's go ahead and keep track of the phase of each substance in this reaction. In this case, the zinc is a solid, so we mark that with an “s.” That solid zinc reacts with hydrochloric acid, which you'll recall is an aqueous solution of hydrogen chloride.
Now, in order to know how this reaction will proceed, we need to know the charges of the ions of all these elements involved. Zinc ion has a charge of 2 plus. Hydrogen is 1 plus, and chloride is 1 minus. Since zinc is a cation, it's going to switch out with the cation of the compound, which is hydrogen. We can criss-cross the charges on zinc and chlorine, and we see that the new compound created will have 1 zinc atom and two chlorine atoms.
So one of our products will be zinc chloride, ZnCl2. You can look at a chart to see whether certain substances dissolve in water or precipitate out. Zinc chloride dissolves in water, so it is present as an aqueous solution. Hydrogen got booted from the compound, and remember that hydrogen is a HONCLBRIF element, so it is the diatomic H2, which is a gas.
Looking at this chemical equation, there is only one hydrogen and one chlorine on the left, but there are two of each on the right, so we can fix that by putting a coefficient of 2 next to the hydrogen chloride.
So our reactants were zinc and hydrogen chloride, our products were zinc chloride and hydrogen, and we have checked, and this is balanced.
In the next example, bromine is poured onto an aqueous solution of sodium iodide. Well, to start off, we know that bromine is a HONCLBRIF element, so our first reactant will be Br2. In this reaction, our elemental bromine is a liquid. That reacts with an aqueous solution of sodium iodide, NaI.
Now let's look at the charges for each of these parts. Bromide carries a 1 minus charge, sodium ion is 1 plus, and iodide is 1 minus, same as bromide. As they are both anions, and indeed both in the same group on the periodic table, the bromine and the iodine are going to switch places with each other, so the bromine is now going to bond with the sodium. When we criss-cross the charges, we see that sodium and bromine will combine in a one-to-one ratio, so our products are sodium bromide, which dissolves in water, so it's an aqueous solution, and elemental iodine, which can be a few different phases at room temperature, but in this reaction, it precipitates out as a solid.
There are two bromines on the left, so we have to put a 2 next to the sodium bromide. But now there are 2 sodium atoms on the right, so we have a put another 2 next to the sodium iodide on the left. And now everything looks good.
Our reactants were elemental bromine and sodium iodide, our products were sodium bromide and elemental iodine. We've checked, and this equation is indeed balanced.
This reaction is a classic example of a single replacement reaction. We have an elemental substance, bromine, reacting with a compound, sodium iodide. The element swaps places with its fellow anion or cation, and the reaction produces a new elemental substance and a new compound.
Question
A pure element \((X)\) forms positive ions \((X^{1+})\). In a single replacement reaction with sodium chloride, would element \(X\) replace the sodium ion \((Na^{1+})\) or the chloride ion \((Cl^{1-})\)? Why?
Element \(X\) would replace the sodium ion. Since element \(X\) forms a cation, it will replace the cation in the compound, which is sodium.
Question
Explain the difference between double replacement reactions and single replacement reactions in terms of the reactant and products of each.
The reactants of a double replacement reaction are two compounds, and the products are two different compounds. The reactants of a single replacement reaction are an element and a compound, and the products are a different element and a different compound.
