Pre-AICE Chemistry
Replacement Reactions
Intro: These reactions are somewhat easier than the decomposition
reactions in the last section. There are two formats to remember and
only a few hints/strategies to keep in mind.
1. “Metal replaces metal”. This format will have the generic
equation
A + BX  AX + B
where the A and the B represent metals and the X is simply a nonmetal or a polyatomic.
 The element A will be represented by its atomic symbol. No
ion charge because it is simply an element.
 The BX is a compound and its formula must be correctly
written with “SOCCR”.
 Important: on the product side, the AX is also a compound.
Therefore, you will also have to write its formula using
“SOCCR”.
 Finally, on the product side – the element B will be
represented by its atomic symbol with NO CHARGE. It is no
longer an ion.
Example: “A coil of magnesium ribbon is placed into a beaker
containing aqueous copper(II) nitrate.”
This is a Replacement Reaction – we know this because the reactants
are an element and a compound.
The word equation for this will be:
magnesium + copper(II) nitrate  magnesium nitrate + copper
Writing the formulas and balancing the equation will give:




The magnesium is represented as simply “Mg”
The copper(II) nitrate solution is represented as “Cu(NO3)2“
The magnesium nitrate will be represented as “Mg(NO3)2”
The copper will simply be written as “Cu”
Putting all of this together will give the equation:
Mg + Cu(NO3)2  Mg(NO3)2 + Cu
Now for the other pattern:
2. “Non-metal replaces non-metal” This format will have the generic
equation
X + AY  AX + Y
where the X and Y are non-metals and the A is a metal.
Now for the notes on this format:
 Remember that the X as a reactant is an element. It does not have
a charge. Remember too that it may be a diatomic.
 Just like the previous format, the compound AY must have a
chemical formula written with SOCCR.
 Same thing for the product AX.
 Finally, the product Y is an element – be sure to represent it
correctly.
An example: “Gaseous chlorine is bubbled through a solution of sodium
iodide”
The word equation for this will be:
”Chlorine + sodium iodide  sodium chloride + iodine”
(note that this time, the element is a non-metal, so you have to
switch out the non-metals.)
Converting this to an equation will give:




The chlorine is represented as Cl2 (diatomic)
The sodium iodide is written as NaI
The sodium chloride is written as NaCl
The iodine (now an element) is written as I2 (diatomic)
Putting all of this together gives the unbalanced equation:
Cl2 + NaI  NaCl + I2
Adding the coefficients gives:
Cl2 + 2 NaI  2 NaCl + I2
A final example: “A piece of sodium metal is added to a beaker of
distilled water.”
This is a replacement reaction. It has the reactants “element +
compound” and so it fits into the format. Since sodium is a metal, this
will be a “metal replaces metal” system.
The important trick to remember in trying to get this reaction written
correctly is that you have to think of the water as “H(OH)” instead of the
more common H2O. The reason for this is that in chemical reactions,
water behaves as if it is has two parts, an H and an OH.
So, word equation for this reaction would be:
Sodium + “hydrogen hydroxide”  sodium hydroxide + hydrogen




Represent the sodium as Na (it is an element)
Represent the water as H(OH) (discussed above)
Represent the sodium hydroxide as NaOH. (remember SOCCR)
Represent the hydrogen as H2 (remember that it is diatomic)
So the unbalanced equation will be:
Na + H(OH)  NaOH + H2
Balancing gives:
2 Na + 2 H(OH)  2 NaOH + H2