In a double replacement(or displacement)
reaction you begin with two compounds as your reactants. The products of
the reaction are two compounds but the anions have switched. The general
pattern of a double replacement reaction is:
AB + CD --> AD + CB (remember cations
go first in an ionic compound)
There is no need to check the activity
series when solving double replacement reactions. Instead you need to
check the solubility table to ensure that your reactants are soluble.
If either of your reactants are insoluble, the reaction cannot occur and
you should label your product as N.R. meaning no reaction. If one of your
products is insoluble it is referred to as a precipitate. See below
for a couple examples.
PbCl2(aq) + Li2SO4(aq)
--> PbSO4(aq) + 2LiCl(aq)
One more note, if a multivalent (an element with more than one charge) element
(like lead above) is used, you must use the same charge once you switch
the cations. In other words, if we use lead(II) as a reactant, we must also
use lead(II) as a product. You cannot switch to lead(IV).
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Now
let’s go step by step.
Predict the product when aqueous lead(II) nitrate reacts with aqueous
sodium chromate.
Description
of Action |
Action |
| 1. Write the formulas for the reactants you are given. |
Pb(NO3)2 +
Na2CrO4 --> |
| 2. On the products side, write each anion,
and cross its charge with the charge of the
other cation. Leave all subscripts
behind unless it is part of the polyatomic ion. Remember if you
are using a multivalent cation, you must use the same charge on
each side. |
Pb(NO3)2 +
Na2CrO4 --> PbCrO4
+ NaNO3 |
| 3.
Balance the equation. |
Pb(NO3)2 +
Na2CrO4 --> PbCrO4
+ 2NaNO3 |
|
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