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Double Replacement Reactions
 

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).


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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