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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.
start color #e84d39, start text, A, end text, end color #e84d39, start text, B, end text, left parenthesis, a, q, right parenthesis, plus, start color #1fab54, start text, C, end text, end color #1fab54, right arrow, start color #df0030, start text, A, end text, end color #df0030, plus, start color #1fab54, start text, C, end text, end color #1fab54, start text, B, end text, left parenthesis, a, q, right parenthesis
space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, \downarrow, space, space, space, space, space, space, space, space, \downarrow, space, space, space, space, space, space, space, space, space
start color #11accd, space, space, space, space, space, space, space, space, space, space, space, space, space, start text, P, u, r, e, space, e, l, e, m, e, n, t, s, !, end text, space, space, space, space, space, space, space, space, space, space, space, space, space, space, end color #11accd
We can see that start color #e84d39, start text, A, end text, end color #e84d39 is replaced by start color #1fab54, start text, C, end text, end color #1fab54 in compound start color #e84d39, start text, A, end text, end color #e84d39, start text, B, end text to make a new compound start color #1fab54, start text, C, end text, end color #1fab54, start text, B, end text and elemental start color #e84d39, start text, A, end text, end color #e84d39. Another thing you might notice is that start color #e84d39, start text, A, end text, end color #e84d39 starts out as an ion in solution but appears in its elemental form on the product side. The reactant start color #1fab54, start text, C, end text, end color #1fab54 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 start color #1fab54, start text, C, end text, end color #1fab54, start text, B, end text, left parenthesis, a, q, right parenthesis.
Let's try to make sense of that definition with an example reaction.
start color #e84d39, start text, A, g, end text, end color #e84d39, start text, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis, plus, start color #1fab54, start text, C, u, end text, end color #1fab54, left parenthesis, s, right parenthesis, right arrow, space, space, start text, question mark, end text
space, space, space, space, space, space, space, space, space, space, \downarrow
start color #ca337c, start text, C, l, e, a, r, comma, space, c, o, l, o, r, l, e, s, s, end text, end color #ca337c
space, space, space, space, space, start color #ca337c, start text, s, o, l, u, t, i, o, n, end text, end color #ca337c
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 start text, H, end text, start superscript, plus, end superscript 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 start text, A, g, end text, start text, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis 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 start color #e84d39, start text, A, g, end text, end color #e84d39, start text, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis can be replaced by copper to form start color #1fab54, start text, C, u, end text, end color #1fab54, left parenthesis, start text, N, O, end text, start subscript, 3, end subscript, right parenthesis, start subscript, 2, end subscript, left parenthesis, a, q, right parenthesis. In the process, elemental silver, start color #e84d39, start text, A, g, end text, end color #e84d39, left parenthesis, s, right parenthesis, would also form as a product. We can write out the full—and balanced!—reaction as follows:
2, start color #e84d39, start text, A, g, end text, end color #e84d39, start text, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis, plus, start color #1fab54, start text, C, u, end text, end color #1fab54, left parenthesis, s, right parenthesis, right arrow, start color #1fab54, start text, C, u, end text, end color #1fab54, left parenthesis, start text, N, O, end text, start subscript, 3, end subscript, right parenthesis, start subscript, 2, end subscript, left parenthesis, a, q, right parenthesis, plus, 2, start color #e84d39, start text, A, g, end text, end color #e84d39, left parenthesis, s, right parenthesis
Does this match our observations? It turns out that aqueous solutions of start text, C, u, end text, left parenthesis, start text, N, O, end text, start subscript, 3, end subscript, right parenthesis, start subscript, 2, end subscript 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 start color #1fab54, start text, C, end text, end color #1fab54 and start color #e84d39, start text, A, end text, end color #e84d39 in the generic pattern above, or copper and silver in our example reaction. If element start text, C, end text is more reactive than element start text, A, end text, then start text, C, end text will replace start text, A, end text in a compound. If element start text, C, end text is less reactive than element start text, A, end text, 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:
start color #1fab54, start text, M, o, s, t, space, r, e, a, c, t, i, v, e, end text, end color #1fab54, space, space, space, space, space, start text, F, end text, start subscript, 2, end subscript, is greater than, start text, C, l, end text, start subscript, 2, end subscript, is greater than, start text, B, r, end text, start subscript, 2, end subscript, is greater than, start text, I, end text, start subscript, 2, end subscript, space, space, space, space, space, start color #e84d39, start text, L, e, a, s, t, space, r, e, a, c, t, i, v, e, end text, end color #e84d39
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 start text, B, r, end text, start subscript, 2, end subscript would replace start text, I, end text, start subscript, 2, end subscript in a single replacement reaction, but start text, B, r, end text, start subscript, 2, end subscript 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 start text, A, g, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis 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 start text, A, g, end text, start superscript, plus, end superscript would get replaced by start text, C, u, end text, start superscript, 2, plus, end superscript in a compound, which matches our results. Hooray!


## Example: Predicting the products of a single replacement reaction

Let’s consider the following reaction:
start text, A, l, P, O, end text, start subscript, 4, end subscript, left parenthesis, a, q, right parenthesis, plus, start text, M, g, end text, left parenthesis, s, right parenthesis, right arrow
The first question we might ask is what element start text, M, g, end text might be replacing in the compound start text, A, l, P, O, end text, start subscript, 4, end subscript. start text, A, l, end text is a metal that usually forms cations with a charge of 3+. We can verify this because start text, A, l, P, O, end text, start subscript, 4, end subscript is neutral and phosphate has a 3- charge, so the aluminum cation must have a 3+ charge. Since start text, M, g, end text is also a metal that forms cations, we might expect start text, M, g, end text to replace the metal start text, A, l, end text 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 start text, A, l, end text, left parenthesis, s, right parenthesis and the new ionic compound start text, M, g, end text, start subscript, 3, end subscript, start text, left parenthesis, P, O, end text, start subscript, 4, end subscript, right parenthesis, start subscript, 2, end subscript.
That gives the following reaction:
start text, A, l, P, O, end text, start subscript, 4, end subscript, left parenthesis, a, q, right parenthesis, plus, start text, M, g, end text, left parenthesis, s, right parenthesis, right arrow, start text, A, l, end text, left parenthesis, s, right parenthesis, plus, start text, M, g, end text, start subscript, 3, end subscript, left parenthesis, start text, P, O, end text, start subscript, 4, end subscript, right parenthesis, start subscript, 2, end subscript, left parenthesis, a, q, right parenthesis, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, space, start color #aa87ff, start text, W, a, r, n, i, n, g, colon, space, N, o, t, space, b, a, l, a, n, c, e, d, !, end text, end color #aa87ff
We are not quite done though since our reaction is not currently balanced. We can fix this by multiplying start text, A, l, P, O, end text, start subscript, 4, end subscript on the reactant side by two, start text, M, g, end text, left parenthesis, s, right parenthesis by three, and start text, A, l, end text, left parenthesis, s, right parenthesis on the product side by two. This gives us our final balanced equation:
2, start text, A, l, P, O, end text, start subscript, 4, end subscript, left parenthesis, a, q, right parenthesis, plus, 3, start text, M, g, end text, left parenthesis, s, right parenthesis, right arrow, 2, start text, A, l, end text, left parenthesis, s, right parenthesis, plus, start text, M, g, end text, start subscript, 3, end subscript, start text, left parenthesis, P, O, end text, start subscript, 4, end subscript, right parenthesis, start subscript, 2, end subscript, left parenthesis, a, q, right parenthesis

## Summary

Single replacement reactions have the overall form shown below
start color #e84d39, start text, A, end text, end color #e84d39, start text, B, end text, left parenthesis, a, q, right parenthesis, plus, start color #1fab54, start text, C, end text, end color #1fab54, right arrow, start color #df0030, start text, A, end text, end color #df0030, plus, start color #1fab54, start text, C, end text, end color #1fab54, start text, B, end text, left parenthesis, a, q, right parenthesis
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?
start text, N, a, B, r, end text, left parenthesis, a, q, right parenthesis, plus, start text, C, l, end text, start subscript, 2, end subscript, right arrow

#### Problem 2

If we want to precipitate copper metal from an aqueous solution of start text, C, u, S, O, end text, start subscript, 4, end subscript, which of the following reactants should we add to our solution?

## Want to join the conversation?

• In problem 1, why is the correct answer NaCl(aq)+Br​2​​?

what happened to the second Cl? did it just disappear? and where did we get the second Br?
• Bromine forms a diatomic molecule when it is by itself. Hence the Br2 and not 2Br.
• In Problem 2, why Zinc as opposed to Silver?
• Silver is below copper in the activity series, so it will not displace copper.
Zinc is above copper, so it will displace copper.
• What is the difference between a single replacement and a double replacement reaction?
• A single-replacement reaction is a chemical reaction in which one element replaces another in a compound.
The general equation is
A + BC → AC + B
Examples are
Zn + 2HCl → ZnCl₂ + H₂, where Zn replaces H in HCl, and
F₂ + 2NaCl → 2NaF + Cl₂, where F replaces Cl in NaCl.

A double-replacement reaction is a reaction in which the metals in two ionic compounds exchange partners.
The general equation is
AB + CD →AD + CB
An example is
CuCl₂ + 2AgNO₃ → Cu(NO₃)₂ + 2AgCl, where Cu and Ag exchange partners.
• Are the products of single replacement reactions related to the electronegativities of the elements in the reactants?
• How do we know Mg has +3 charge and PO4 has - 3 charge? Explain it to me.
• mg is +2
the electronic configuration is 2,8,2. To achieve octet state it gives 2 electrons . Now it has 2 more protons so the overall charge is +2.
p needs 5 more electrons. each oxygen gives 2 electrons , so 4 will give 8
5-8 is 3 . it has 3 more electrons so the overall charge is -3
• what will happen if the element reacting with a compound is less reactive then the element present in the compound . will the reaction not occur ?
eg
mg3( po4)2 + 2al = ??
• reaction would not occur. in example, compound with magnesium is more stable than compound with aluminum so phosphate tends to keep bonding with magnesium
• 2AgNO3(aq)+Cu(s)→Cu(NO3)2(aq)+2Ag(s)
In this formula, how do we know that copper has a +2 charge ?
• You can't really know it other than memorising. Cu2+ is generally more stable than Cu+.
• So I have to ignore Zn + 2 H2O = Zn(OH)2 + H2 in the aqueous solution here? I don't understand why I'm supposed to be ignoring this reaction in question 2.

If I ignored the fact I am asked to find a single replacement reaction (which in fact isn't specified but implied from context), then my reasoning tells me that the Ag in AgNO3 will give up its place to form CuNO3 because we can see in the reactivity that Cu is more reactive than Ag. Ag can form AgSO4 and then balance the equation. Why is this incorrect?