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Single Replacement Reactions
 

In a single replacement(or displacement) reaction one element replaces another element in a compound. They follow one of the following two patterns:
A + BC --> BA + C (single anion replaces anion in compound) or
A + BC --> AC + B (single cation replaces cation in compound)
An important thing to remember with single displacement reactions is elements that form cations can only replace cations and elements that form anions can only replace anions. See the two examples below:
Example #1:
          Cl2(g) + 2 KBr(aq) --> 2 KCl(aq) + Br2(l)
There are a few things we must be aware of in the above equation. First, chlorine is an element that forms an anion, so it can only replace an element that forms an anion, in this case bromine. Second, chlorine and bromine are both diatomic molecules and must be written with a subscript of 2 when they are alone. Do not be confused by the letters in parenthesis after each molecule’s formula. They tell you the state that each molecule is in.
Example #2:
          2 Al(s) + Fe2O3(aq) --> Al2O3(aq) + 2 Fe(s)
In the equation above, the element that is all by itself is aluminum. Aluminum forms a cation, so it can only replace iron (Fe), which also is a cation.

Not all single replacement reactions can occur. Before you can write the products for any single displacement equation, you MUST check the activity series to see if the displacement is possible. There is a cation and an anion activity series. I listed both to the right.

Elements higher on the chart can replace elements below it, but elements lower on the chart cannot replace elements higher on the chart. For example, as we saw above in our two examples, chlorine, Cl, can replace bromine, Br, and aluminum, Al, can replace iron, Fe. Look at your charts. Note that on the Anion Activity Series, chlorine is higher than bromine. Note on the Cation Activity Series chart that aluminum is higher than iron. If an element cannot replace the other element, the reaction is labeled N.R. for no reaction. See below for an example. If I tried to reverse the reaction in example #2, lets see what would happen.
      Fe(s) + Al2O3(aq) --> N.R. Since iron is lower on the activity series chart than aluminum, it cannot replace it. When this occurs we label it N.R. for no reaction.

One more important issue: Whenever you see the formula for water, H2O, you should treat it as a compound formed between H+ (cation) and OH- (anion). In fact, it would be helpful to you to change the formula to HOH whenever you see H2O. Think of it as hydrogen hydroxide.


Activity Series
cations
anions
Li
F
Rb
Cl
K
Br
Cs
I
Ba
Sr
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H
Cu
Hg
Ag
Pt
Au
Now let’s go step by step.
Predict the products when solid sodium reacts with water.
Description of Action
Action
1. Write the formulas for the reactants you are given.Na(s) + HOH(l) -->
Remember, water is now HOH! Unless it is stated otherwise, water is always a liquid (l).
2. Once you recognize that this is a single replacement problem, determine whether the non-bonded element is a cation or anion. Then determine which element it will replace. Remember cations can only replace cations and anions can only replace anions. Sodium is our non-bonded element and it is a cation. Therefore, it will try to replace Hydrogen, the cation in the compound.
3. Check your activity series chart to see if this replacement is possible. If the reaction cannot occur, label it N.R. If it can occur, go to step 4. Sodium is higher up on the activity series chart than hydrogen so this replacement can occur.
4. Switch the appropriate elements and write the elements that will form a compound with their charges as superscripts. If the element that is being replaced is diatomic, be sure to write it with a two for its subscript. Na(s) + HOH(l) --> Na1+ (OH)1-(aq) + H2(g)
5. Cross charges between the ions in the compound and remove any + signs, - signs and ones. Na(s) + HOH(l) --> NaOH(aq) + H2(g)
6. Balance the equation. 2Na(s) + 2HOH(l) --> 2NaOH(aq) + H2(g)

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