0.8 Metathesis: to exchange or not?  (Page 2/4)

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: $K^{+}(\text{aq})+{\text{Cl}}^{-}(\text{aq})+{\text{Na}}^{+}(\text{aq})+{\text{NO}}_{3^{-}}(\text{aq})\to \text{no reaction}$

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: $\text{2HCl}(\text{aq})+{\text{Na}}_{2}S(\text{aq})\to \text{2NaCl}(\text{aq})+H_{2}S(g)$

Complete ionic equation: ${\text{2H}}^{+}(\text{aq})+{\text{2Cl}}^{-}(\text{aq})+{\text{2Na}}^{+}(\text{aq})+S^{2-}(\text{aq})\to {\text{2Na}}^{+}(\text{aq})+{\text{2Cl}}^{-}(\text{aq})+H_{2}S(g)$

Net ionic equation: ${\text{2H}}^{+}(\text{aq})+S^{2-}(\text{aq})\to H_{2}S(g)$

or

${\text{2H}}^{+}+S^{2-}\to H_{2}S(g)$

Formation of a weak electrolyte

Molecular equation:

${\text{HNO}}_3(\text{aq})+\text{NaOH}(\text{aq})\to H_{2}O(l)+{\text{NaNO}}_3(\text{aq})$

Complete ionic equation:

$H^{+}(\text{aq})+{\text{NO}}_{3^{-}}(\text{aq})+{\text{Na}}^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})\to H_{2}O(l)+{\text{Na}}^{+}(\text{aq}){\text{NO}}_{3^{-}}(\text{aq})$

Net ionic equation:

$H^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})\to H_{2}O(l)$

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 ${\text{CO}}_2$ , ${\text{SO}}_2$ , $H_{2}S$ , and ${\text{NH}}_3$ . 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:

$H_2{\text{CO}}_3(\text{aq})\to H_{2}O(l)+{\text{CO}}_2(g)$

and

$H_2{\text{SO}}_3(\text{aq})\to H_{2}O(l)+{\text{SO}}_2(g)$

Ammonium salts form ${\text{NH}}_3$ when they are treated with strong bases:

${\text{NH}}_{4^{+}}(\text{aq})+{\text{OH}}^{-}\to {\text{NH}}_3(g)+H_{2}O(l)$

Table 1 solubility rules

*Slightly soluble.

Table 2 strong electrolytes

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.