420
Reactions Prediction and Synthesis
6 H 3 PO 4 is used in preparing HBr and HI in order to avoid the strong
oxidizing action of hot concentrated H 2 SO 4 , which will also yield such
products as the halogens, SO 2 , and H 2 S
7 In order to get H 2 CO 3 and H 2 SO 3 , the gases CO 2 and SO 2 must be
passed into H 2 O
Many organic acids are insoluble in water, and so can be prepared from their
soluble salts by simply adding a dilute solution of a strong inorganic acid to their
aqueous solutions Hot concentrated H 2 SO 4 and H 3 PO 4 should never be used
with organic acids All you would probably get is a mess of carbon Two
inorganic acids, H 3 BO 3 and H 4 SiO 4 precipitate on acidification of solutions of
their soluble salts
X Is a Hydroxide
As noted by the solubility rules, most hydroxides are only slightly soluble and
would be prepared as in method 1 for a salt NaOH, KOH, and NH 3 are
commonly available, but the choice of base must take into account the possibility that soluble OH~ or NH 3 complexes may form (see Table 25-1)
NH 3 is easily prepared, as a gas, by warming any NHJ salt with an excess of
NaOH, using a generator of the type shown in Figure 27-2a
X Is a Miscellaneous Compound
1 Metal o\idt s With the exception of the alkali metals, prepare by vigorously heating the metal hydroxide to eliminate water Do not try to burn the
metal in air
2 bitlfitli s of alkahiu caith nntals Al and Ci Prepare by heating the pulverized metals with an excess of sulfur and with protection from the air They
cannot be precipitated by H 2 S from solution, they hydrolyze when they come in
contact with water
3 Acid.<,a/ts suchasNaHCO 3 KH 2 PO 4 , or K 2 HPO 4 Calculate the pH of the
solution you wish to make (see p 362) Choose an indicator (Table 23-2) whose
K t is close to the calculated [H
+ ] in this solution (see p 366) Titrate the acid to
the color change, then stop and evaporate off the water An alternative method
for NaHCO 3 NaHSO 3 and NaHS is simply to saturate an NaOH solution with
the appropriate gas (CO 2 , SO 2 , or H 2 S), the resulting solution will contain the
desired salt
4 Metasalts andp\iosalts Many solid acid salts serve as starting materials
for the preparation of were/salts and /r>>osalts because they lose water when
heated vigorously For example dihydrogen salts will yield wctasalts, such as
sodium nu /aarsenate
NaH 2 AsO 4 -^ NaAsO, + H 2 Ot
Reactions Prediction and Synthesis
6 H 3 PO 4 is used in preparing HBr and HI in order to avoid the strong
oxidizing action of hot concentrated H 2 SO 4 , which will also yield such
products as the halogens, SO 2 , and H 2 S
7 In order to get H 2 CO 3 and H 2 SO 3 , the gases CO 2 and SO 2 must be
passed into H 2 O
Many organic acids are insoluble in water, and so can be prepared from their
soluble salts by simply adding a dilute solution of a strong inorganic acid to their
aqueous solutions Hot concentrated H 2 SO 4 and H 3 PO 4 should never be used
with organic acids All you would probably get is a mess of carbon Two
inorganic acids, H 3 BO 3 and H 4 SiO 4 precipitate on acidification of solutions of
their soluble salts
X Is a Hydroxide
As noted by the solubility rules, most hydroxides are only slightly soluble and
would be prepared as in method 1 for a salt NaOH, KOH, and NH 3 are
commonly available, but the choice of base must take into account the possibility that soluble OH~ or NH 3 complexes may form (see Table 25-1)
NH 3 is easily prepared, as a gas, by warming any NHJ salt with an excess of
NaOH, using a generator of the type shown in Figure 27-2a
X Is a Miscellaneous Compound
1 Metal o\idt s With the exception of the alkali metals, prepare by vigorously heating the metal hydroxide to eliminate water Do not try to burn the
metal in air
2 bitlfitli s of alkahiu caith nntals Al and Ci Prepare by heating the pulverized metals with an excess of sulfur and with protection from the air They
cannot be precipitated by H 2 S from solution, they hydrolyze when they come in
contact with water
3 Acid.<,a/ts suchasNaHCO 3 KH 2 PO 4 , or K 2 HPO 4 Calculate the pH of the
solution you wish to make (see p 362) Choose an indicator (Table 23-2) whose
K t is close to the calculated [H
+ ] in this solution (see p 366) Titrate the acid to
the color change, then stop and evaporate off the water An alternative method
for NaHCO 3 NaHSO 3 and NaHS is simply to saturate an NaOH solution with
the appropriate gas (CO 2 , SO 2 , or H 2 S), the resulting solution will contain the
desired salt
4 Metasalts andp\iosalts Many solid acid salts serve as starting materials
for the preparation of were/salts and /r>>osalts because they lose water when
heated vigorously For example dihydrogen salts will yield wctasalts, such as
sodium nu /aarsenate
NaH 2 AsO 4 -^ NaAsO, + H 2 Ot