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Because all the compounds are water-soluble and are strong electrolytes, they have been written in the ionic form. They completely dissolve in water. If we eliminate spectator ions from the equation, nothing remains. Hence, there is no reaction: Equation 7: K + ( aq ) + Cl ( aq ) + Na + ( aq ) + NO 3 ( aq ) no reaction size 12{K rSup { size 8{+{}} } \( "aq" \) +"Cl" rSup { size 8{ - {}} } \( "aq" \) +"Na" rSup { size 8{+{}} } \( "aq" \) +"NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) rightarrow "no reaction"} {}

Metathesis reactions occur when a precipitate, a gas, a weak electrolyte, or a nonelectrolyte is formed. The following equations are further illustrations of such processes.

Formation of a gas

Molecular equation: Equation 8: 2HCl ( aq ) + Na 2 S ( aq ) 2NaCl ( aq ) + H 2 S ( g ) size 12{"2HCl" \( "aq" \) +"Na" rSub { size 8{2} } S \( "aq" \) rightarrow "2NaCl" \( "aq" \) +H rSub { size 8{2} } S \( g \) } {}

Complete ionic equation: 2H + ( aq ) + 2Cl ( aq ) + 2Na + ( aq ) + S 2 ( aq ) 2Na + ( aq ) + 2Cl ( aq ) + H 2 S ( g ) size 12{"2H" rSup { size 8{+{}} } \( "aq" \) +"2Cl" rSup { size 8{ - {}} } \( "aq" \) +"2Na" rSup { size 8{+{}} } \( "aq" \) +S rSup { size 8{2 - {}} } \( "aq" \) rightarrow "2Na" rSup { size 8{+{}} } \( "aq" \) +"2Cl" rSup { size 8{ - {}} } \( "aq" \) +H rSub { size 8{2} } S \( g \) } {}

Net ionic equation: 2H + ( aq ) + S 2 ( aq ) H 2 S ( g ) size 12{"2H" rSup { size 8{+{}} } \( "aq" \) +S rSup { size 8{2 - {}} } \( "aq" \) rightarrow H rSub { size 8{2} } S \( g \) } {}

or

2H + + S 2 H 2 S ( g ) size 12{"2H" rSup { size 8{+{}} } +S rSup { size 8{2 - {}} } rightarrow H rSub { size 8{2} } S \( g \) } {}

Formation of a weak electrolyte

Molecular equation:

HNO 3 ( aq ) + NaOH ( aq ) H 2 O ( l ) + NaNO 3 ( aq ) size 12{"HNO" rSub { size 8{3} } \( "aq" \) +"NaOH" \( "aq" \) rightarrow H rSub { size 8{2} } O \( l \) +"NaNO" rSub { size 8{3} } \( "aq" \) } {}

Complete ionic equation:

H + ( aq ) + NO 3 ( aq ) + Na + ( aq ) + OH ( aq ) H 2 O ( l ) + Na + ( aq ) NO 3 ( aq ) size 12{H rSup { size 8{+{}} } \( "aq" \) +"NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) +"Na" rSup { size 8{+{}} } \( "aq" \) +"OH" rSup { size 8{ - {}} } \( "aq" \) rightarrow H rSub { size 8{2} } O \( l \) +"Na" rSup { size 8{+{}} } \( "aq" \) " NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) } {}

Net ionic equation:

H + ( aq ) + OH ( aq ) H 2 O ( l ) size 12{H rSup { size 8{+{}} } \( "aq" \) +"OH" rSup { size 8{ - {}} } \( "aq" \) rightarrow H rSub { size 8{2} } O \( l \) } {}

In order to decide if a reaction occurs, we need to be able to determine whether or not a precipitate, a gas, a nonelectrolyte, or a weak electrolyte will be formed. The following brief discussion is intended to aid you in this regard. Table 1 summarizes solubility rules and should be consulted while performing this experiment.

The common gases are CO 2 size 12{"CO" rSub { size 8{2} } } {} , SO 2 size 12{"SO" rSub { size 8{2} } } {} , H 2 S size 12{H rSub { size 8{2} } S} {} , and NH 3 size 12{"NH" rSub { size 8{3} } } {} . Carbon dioxide and sulfur dioxide may be regarded as resulting form the decomposition of their corresponding weak acids, which are initially formed when carbonate and sulfite salts are treated with acid:

H 2 CO 3 ( aq ) H 2 O ( l ) + CO 2 ( g ) size 12{H rSub { size 8{2} } "CO" rSub { size 8{3} } \( "aq" \) rightarrow H rSub { size 8{2} } O \( l \) +"CO" rSub { size 8{2} } \( g \) } {}

and

H 2 SO 3 ( aq ) H 2 O ( l ) + SO 2 ( g ) size 12{H rSub { size 8{2} } "SO" rSub { size 8{3} } \( "aq" \) rightarrow H rSub { size 8{2} } O \( l \) +"SO" rSub { size 8{2} } \( g \) } {}

Ammonium salts form NH 3 size 12{"NH" rSub { size 8{3} } } {} when they are treated with strong bases:

NH 4 + ( aq ) + OH NH 3 ( g ) + H 2 O ( l ) size 12{"NH" rSub { size 8{4} rSup { size 8{+{}} } } \( "aq" \) +"OH" rSup { size 8{ - {}} } rightarrow "NH" rSub { size 8{3} } \( g \) +H rSub { size 8{2} } O \( l \) } {}

Table 1 solubility rules

Water-soluble salts  
Na + ,K + ,NH 4 + size 12{"Na" rSup { size 8{+{}} } ",K" rSup { size 8{+{}} } ",NH" rSub { size 8{4} rSup { size 8{+{}} } } } {} All sodium, potassium, and ammonium salts are soluble.
NO 3 ,CIO 3 ,C 2 H 3 O 2 size 12{"NO" rSub { size 8{3} rSup { size 8{-{}} } } ",CIO" rSub { size 8{3} rSup { size 8{-{}} } } ",C" rSub { size 8{2} } H rSub { size 8{3} } O rSup { size 8{2 - {}} } } {} All nitrates, chlorates, and acetate are soluble.
Cl size 12{"Cl" rSup { size 8{ - {}} } } {} All chlorides are soluble except AgCl, Hg 2 Cl 2 size 12{"Hg" rSub { size 8{2} } "Cl" rSub { size 8{2} } } {} , and PbCl 2 size 12{"PbCl" rSub { size 8{2} } * } {} .
Br size 12{"Br" rSup { size 8{ - {}} } } {} All bromides are soluble except AgBr, Hg 2 Br 2 size 12{"Hg" rSub { size 8{2} } "Br" rSub { size 8{2} } } {} , PbBr 2 size 12{"PbBr" rSub { size 8{2} } * } {} , and HgBr 2 size 12{"HgBr" rSub { size 8{2} } * } {} .
I size 12{I rSup { size 8{ - {}} } } {} All iodides are soluble except AgI, Hg 2 I 2 size 12{"Hg" rSub { size 8{2} } I rSub { size 8{2} } } {} , PbI 2 size 12{"PbI" rSub { size 8{2} } } {} , and HgI 2 . 0 size 12{"HgI" rSub { size 8{2} } "." 0} {}
SO 4 2 size 12{"SO" rSub { size 8{4} rSup { size 8{2 - {}} } } } {} All sulfates are soluble except CaSO 4 size 12{"CaSO" rSub { size 8{4} } * } {} , SrSO 4 size 12{"SrSO" rSub { size 8{4} } } {} , BaSO 4 size 12{"BaSO" rSub { size 8{4} } } {} , Hg 2 SO 4 size 12{"Hg" rSub { size 8{2} } "SO" rSub { size 8{4} } } {} , PbSO 4 size 12{"PbSO" rSub { size 8{4} } } {} , and Ag 2 SO 4 size 12{"Ag" rSub { size 8{2} } "SO" rSub { size 8{4} } } {} .

 

Water-insoluble salts  
CO 3 2 size 12{"CO" rSub { size 8{3} rSup { size 8{2 - {}} } } } {} , SO 3 2 size 12{"SO" rSub { size 8{3} rSup { size 8{2 - {}} } } } {} , PO 4 3 size 12{"PO" rSub { size 8{4} rSup { size 8{3 - {}} } } } {} All carbonates, sulfites, phosphates, and chromates are insoluble except those of CrO 4 2 size 12{"CrO" rSub { size 8{4} rSup { size 8{2 - {}} } } } {} alkali metals and NH 4 + size 12{"NH" rSub { size 8{4} rSup { size 8{+{}} } } } {} .
OH size 12{"OH" rSup { size 8{ - {}} } } {} All hydroxides are insoluble except those of alkali metals and Ca ( OH ) 2 size 12{"Ca" \( "OH" \) rSub { size 8{2} } * } {} , Sr ( OH ) 2 size 12{"Sr" \( "OH" \) rSub { size 8{2} } * } {} , and Ba ( OH ) 2 size 12{"Ba" \( "OH" \) rSub { size 8{2} } } {} .
S 2 size 12{S rSup { size 8{2 - {}} } } {} All sulfides are insoluble except those of the alkali metals, alkaline earths, and NH 4 + size 12{"NH" rSub { size 8{4} rSup { size 8{+{}} } } } {} .

*Slightly soluble.

Table 2 strong electrolytes

Salts All common soluble salts
Acids HClO 4 size 12{"HClO" rSub { size 8{4} } } {} , HCl, HBr, HI, HNO 3 size 12{"HNO" rSub { size 8{3} } } {} , and H 2 SO 4 size 12{H rSub { size 8{2} } "SO" rSub { size 8{4} } } {} are strong electrolytes; all others are weak.
Bases Alkali metal hydroxides, Ca ( OH ) 2 size 12{"Ca" \( "OH" \) rSub { size 8{2} } } {} , Sr ( OH ) 2 size 12{"Sr" \( "OH" \) rSub { size 8{2} } } {} , and Ba ( OH ) 2 size 12{"Ba" \( "OH" \) rSub { size 8{2} } } {} are strong electrolytes; all others are weak.

Which are the weak electrolytes? The easiest way of answering this question is to identify all of the strong electrolytes, and if the substance does not fall in that category then it is a weak electrolyte. Note, water is a nonelectrolyte. Strong electrolytes are summarized in Table.2.

In the first part of this experiment, you will study some metathesis reactions. In some instances it will be very evident that a reaction has occurred, whereas in others it will not be so apparent. In the doubtful case, use the guidelines above to decide whether or not a reaction has taken place. You will be given the names of the compounds to use but not their formulas. This is being done deliberately to give practice in writing formulas from names.

Questions & Answers

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Source:  OpenStax, Gen chem lab. OpenStax CNX. Oct 12, 2009 Download for free at http://cnx.org/content/col10452/1.51
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