Figure 2-86. A solution of MgCl2 and Na2SO4 is just a cocktail of ions produced on dissociation of the two salts.
It would be incorrect to state that MgCl2
do not react. They do, but the products of their reaction, MgSO4
and NaCl, also react nonstop to give back the starting salts. In other words, there is exchange between MgCl2
to give MgSO4
and NaCl, but, being fully reversible, this reaction does not go to completion
. 2.6.3. Reactions That Go to Completion Revisited.
Some reactions go to completion and some do not. For instance, neutralization reactions go to completion and so does the formation of insoluble BaSO4
from soluble sources of Ba2+
. In contrast, the reaction between MgCl2
does not go to completion. As in real life we need to make chemical and materials, it is critical to know what reactions go to completion and what reactions do not. A chemical reaction proceeds to completion if:
- Water (or another weak electrolyte) is produced in the reaction. Acid-base neutralization is a typical example of such a chemical transformation.
- A precipitate is formed in the reaction. The formation of insoluble BaSO4
(Figures 2-84 and 2-85) is a good example. Another typical example is the reaction of a soluble silver salt with a soluble chloride salt to give insoluble AgCl, for example:AgNO3 + KCl = AgCl↓ + KNO3
Complete ionic equation: Ag+ + NO3- + K+ + Cl- = AgCl↓ + K+ + NO3-
Net ionic equation: Ag+ + Cl- = AgCl↓
- A gas is formed, which bubbles off the reaction mixture. For example, mixing sodium sulfide (Na2
S) with hydrochloric acid (HCl) produces H2
S, a gas that escapes from the reaction solution (video
): Na2S + 2 HCl = 2 NaCl + H2S↑
Complete ionic equation: 2 Na+ + S2- + 2 H+ + 2 Cl- = 2 Na+ + 2 Cl- + H2S↑
Net ionic equation: S2- + 2 H+ = H2S↑ 2.6.4. Exercises.
1. Write molecular, complete ionic, and net ionic equations for the following neutralization reactions. Assume complete neutralization, i.e. involvement of all of the OH groups of the metal hydroxides and all of the protons on the acid molecules. For the neutralization of H3
with NaOH write two sets of complete and net ionic equations: (A) assuming that H3
is a weak electrolyte and (B) assuming that only the first proton of H3
(a) KOH + HNO3
(d) NaOH + H3
2. Write regular, complete ionic, and net ionic equations for the following exchange reactions.
+ KOH (Cu(OH)2
(d) NaF and CaCl2
3. To a solution of 100 g of BaCl2
was added Na2
in excess. The experiment was then repeated using 100 g of BaBr2
in place of the BaCl2
. Which of the two reactions produced a larger quantity of BaSO4
? Solve this problem mentally, without performing calculations. Answer
4. Heating solid NaCl with concentrated H2
in the presence of a small amount of water produces HCl gas and NaHSO4
. Write regular, complete ionic, and net ionic equations for this reaction. Could HCl gas be made using dilute rather than concentrated sulfuric acid? Answer
5. Of the chemical reactions presented below, identify those proceeding to completion and write both complete and net ionic equations for them. Mark precipitates and gases in the products with arrows.
+ 2 HCl = 2 NaCl + CO2
+ NaCl = KCl + NaNO3
+ 2 Na3
= 2 AlPO4
+ 3 Na2
+ 2 KCl = MgCl2
+ 2 KBr
= 2 AgCl + BaSO4
(h) FeS + 2 HCl = FeCl2