- Oxidation–reduction (redox) reactions
- Worked example: Using oxidation numbers to identify oxidation and reduction
- Balancing redox equations
- Dissolution and precipitation
- Precipitation reactions
- Double replacement reactions
- Single replacement reactions
- Molecular, complete ionic, and net ionic equations
- Molecular, complete ionic, and net ionic equations
- 2015 AP Chemistry free response 3a
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.
We can see that
is replaced by in compound to make a new compound and elemental . Another thing you might notice is that starts out as an ion in solution but appears in its elemental form on the product side. The reactant 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 .
Let's try to make sense of that definition with an example reaction.
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.
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
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
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
can be replaced by copper to form . In the process, elemental silver, , 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
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
and in the generic pattern above, or copper and silver in our example reaction. If element is more reactive than element , then will replace in a compound. If element is less reactive than element , 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:
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
would replace in a single replacement reaction, but 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
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 would get replaced by 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
might be replacing in the compound . is a metal that usually forms cations with a charge of 3+. We can verify this because is neutral and phosphate has a 3- charge, so the aluminum cation must have a 3+ charge. Since is also a metal that forms cations, we might expect to replace the metal 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
and the new ionic compound .
That gives the following reaction:
We are not quite done though since our reaction is not currently balanced. We can fix this by multiplying
on the reactant side by two, by three, and 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:
- 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.
- 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.
Want to join the conversation?
- In problem 1, why is the correct answer NaCl(aq)+Br2?
what happened to the second Cl? did it just disappear? and where did we get the second Br?(60 votes)
- In Problem 2, why Zinc as opposed to Silver?(12 votes)
- Silver is below copper in the activity series, so it will not displace copper.
Zinc is above copper, so it will displace copper.(28 votes)
- What is the difference between a single replacement and a double replacement reaction?(6 votes)
- 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
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.(27 votes)
- Are the products of single replacement reactions related to the electronegativities of the elements in the reactants?(6 votes)
- I was also wondering the same question, and I googled out this aritle. http://www.madsci.org/posts/archives/1999-09/936330555.Ch.r.html(8 votes)
- How do we know Mg has +3 charge and PO4 has - 3 charge? Explain it to me.(7 votes)
- 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(2 votes)
In this formula, how do we know that copper has a +2 charge ?(5 votes)
- 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 ?
mg3( po4)2 + 2al = ??(5 votes)
- reaction would not occur. in example, compound with magnesium is more stable than compound with aluminum so phosphate tends to keep bonding with magnesium(3 votes)
- 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?(3 votes)
- The reaction CuSO4 + AgNO3 would not be a single replacement reaction since the reactants are both compounds, i.e. there are no pure elements. Also the reaction of CuSO4 + AgNO3 would cause a double displacement reaction where you will not be able to obtain copper on its own as a precipitate.
The correct answer is Zn, zinc, since it is more reactive than copper and is a pure element. Check a reactivity table or list to find which elements will want to react with compounds in a single displacement reaction.(3 votes)
- Where is sodium in the reactivity series? Id it before Ca or after it? Because in secondary school level China, we learnt the reactivity series as K Ca Na Mg, etc. while in Singapore secondary school teachers say it's K Na Ca Mg, etc. Thank you!!(3 votes)
- why is sodium less reactive than potassium?
sodium comes before potassium,then why is it so?(1 vote)
- Why is it Cu 2+, but Ag +? They are in the same column and nowhere does it say that(3 votes)
- Transition metals have variable valencies. So, transition metals in the same column might have different valencies.(2 votes)