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Single replacement reactions

Definition of single replacement (or single displacement) reactions. Predicting and determining the products using the reactivity series. 

What is a single replacement reaction?

A single replacement reaction, sometimes called a single displacement reaction, is a reaction in which one element is substituted for another element in a compound. The starting materials are always pure elements, such as a pure zinc metal or hydrogen gas, plus an aqueous compound. When a replacement reaction occurs, a new aqueous compound and a different pure element will be generated as products. The general pattern of a single replacement reaction is shown below.
             Pure elements!              
We can see that A is replaced by C in compound AB to make a new compound CB and elemental A. Another thing you might notice is that A starts out as an ion in solution but appears in its elemental form on the product side. The reactant C does the opposite: it starts out in its elemental form on the reactant side, but it ends up as an ion in aqueous solution as part of the compound CB(aq).
Let's try to make sense of that definition with an example reaction.
AgNO3(aq)+Cu(s)  ?
Clear, colorless
You probably noticed that the products of the above reaction haven't been specified yet. In fact, it is possible the reaction won't happen at all! We will figure out in the next section whether we would predict that this reaction would occur and what products it might form. Meanwhile, we can use our keen observational skills to start thinking about what is going on.
Color change plus the formation of precipitate? Looks like a chemical reaction! Image credit: Toby Hudson from Wikimedia Commons, CC BY-SA 3.0 AU
What does this reaction look like in real life?
We start with a clear, colorless solution of silver(I) nitrate, then we drop in some shiny copper wire. The solution turns aquamarine blue, and the copper wire starts to look grey and fuzzy. Cool!
Let's now try to explain this phenomenon using chemistry.

Determining the products of single replacement reactions

If we are trying to figure out whether a single displacement reaction will occur, there are two main questions we need to answer
1. What are the two elements that might swap places in our proposed reaction?
In general, elements that form anions can replace the anion in a compound, and elements that form cations can replace the cation in a compound. The following guidelines can be used to determine what kind of ions a given element might form.
  • Metals will usually form cations. This includes groups 1 and 2, some of group 13 and 14 elements, and the transition metals.
  • The common non-metals in single replacement reactions are the group 17 elements, which generally form anions with a 1- charge.
  • Hydrogen usually forms the cation H+ in a single replacement reaction.
In our reaction with copper metal and aqueous silver(I) nitrate, the copper metal will likely react to form copper cations because it is a transition metal. The copper cations can replace the silver cations in the compound AgNO3(aq) to form a new compound.
2. What is the new compound that will form as a product?
Once we know what element might be replaced in our ionic compound, we can predict the products that might be formed. In this example, the silver atoms in AgNO3(aq) can be replaced by copper to form Cu(NO3)2(aq). In the process, elemental silver, Ag(s), would also form as a product. We can write out the full—and balanced!—reaction as follows:
Does this match our observations? It turns out that aqueous solutions of Cu(NO3)2 are blue-green, which explains the solution's color change. The grey fuzz growing on the copper would be from silver metal precipitating out on the surface of the wire.
Can you think of other measurements we might make to check our conclusions?

Predicting if a single replacement reaction will occur

Once we know which elements might get swapped in our single displacement reaction, we can predict whether the reaction will occur based on knowledge of the relative reactivities of the two elements—elements C and A in the generic pattern above, or copper and silver in our example reaction. If element C is more reactive than element A, then C will replace A in a compound. If element C is less reactive than element A, then there will be no reaction.
The reactivity series—also called the activity series—ranks elements in order of their reactivity for certain types of reactions, including single replacement reactions. The more reactive elements will replace the less reactive elements in the reactivity series, but not the other way around. There are separate rankings for elements that form cations and elements that form anions.
For elements that tend to gain electrons to form anions, the order of reactivity from most reactive to least reactive goes as follows:
Most reactive     F2>Cl2>Br2>I2     Least reactive
For these elements, you can also look at their arrangement on the periodic table—group 17—to remember the order of reactivity. The higher the element's position in the column, the more reactive it will be. Based on this activity series, we would predict that Br2 would replace I2 in a single replacement reaction, but Br2 would not react with a compound containing fluoride ions.
For the cation-forming elements, the reactivity series is longer, and the trends are not as straightforward. You can see an example of the reactivity series for cations below.
Reactivity is pretty complicated! After all, there are lots of different kinds of reactions, so what kind of reactivity are we really ranking here? Some properties that are taken into account in the reactivity series include reactivity with water and acids, as well as how readily an element loses electrons to form cations. As a result of the different ways reactivity can be defined, however, you might see some elements ranked in a different order depending on your teacher or textbook. For this article, we will use the reactivity series above as our reference for solving our examples.
The process of using the reactivity series is the same for both cations and anions:
More reactive elements will replace less reactive elements in a compound.
Let's think back to our experiment combining AgNO3(aq) and copper wire. In the cation reactivity series, we see that copper is ranked higher than silver, so we would expect copper to be more reactive than silver in a single replacement reaction. Therefore, we would predict that Ag+ would get replaced by Cu2+ in a compound, which matches our results. Hooray!

Example: Predicting the products of a single replacement reaction

Let’s consider the following reaction:
The first question we might ask is what element Mg might be replacing in the compound AlPO4. Al is a metal that usually forms cations with a charge of 3+. We can verify this because AlPO4 is neutral and phosphate has a 3- charge, so the aluminum cation must have a 3+ charge. Since Mg is also a metal that forms cations, we might expect Mg to replace the metal Al in our compound. If we check the cation reactivity series, we see that magnesium is more reactive than aluminum, so we predict the single replacement reaction will occur.
What products do we expect from this single replacement reaction? We expect to form elemental Al(s) and the new ionic compound Mg3(PO4)2.
That gives the following reaction:
AlPO4(aq)+Mg(s)Al(s)+Mg3(PO4)2(aq)                   Warning: Not balanced!
We are not quite done though since our reaction is not currently balanced. We can fix this by multiplying AlPO4 on the reactant side by two, Mg(s) by three, and Al(s) on the product side by two. This gives us our final balanced equation:


Single replacement reactions have the overall form shown below
where one element is substituted for another element in a compound to generate a new element and a new compound. Other things to remember for single replacement reactions include:
  1. Elements that are likely to form cations—usually metals or hydrogen gas—will replace the cation in a compound, and elements that are likely to form anions—usually group 17 halogens—will replace anions in a compound.
  2. A higher ranked element in the activity series is more reactive in a single replacement reaction. We predict a single replacement reaction will occur when a less reactive element can be replaced by a more reactive element in a compound.

Try it!

Problem 1

What are the predicted products for the following single replacement reaction?
Choose 1 answer:

Problem 2

If we want to precipitate copper metal from an aqueous solution of CuSO4, which of the following reactants should we add to our solution?
Choose 1 answer:

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