Amphoteric compounds and their properties

The author of the article is Said Lutfullin

Chemistry is always a unity of opposites.

Consider the elements of the periodic system, the compounds of which exhibit amphoteric (opposite) properties.

Some elements, for example, compounds K (K2O – oxide, KOH – hydroxide) exhibit basic properties .

The main properties are interaction with acid oxides and acids.

Almost all metals exhibiting oxidation states +1 and +2) form basic oxides and hydroxides.

Some elements ( all non-metals and d-elements with oxidation states +5 and +6) form acidic compounds.

Acidic compounds are oxides and the corresponding oxygen-containing acids, they interact with basic oxides and bases, forming salts

And there are elements that form such oxides and hydroxides that exhibit both acidic and basic properties, that is, they are amphoteric compounds .

Most amphoteric oxides and hydroxides are solid (or gel-like) substances, slightly or insoluble in water.

What elements form amphoteric compounds?

There is a rule, a little conditional, but quite practical:

Elements lie on a conventionally drawn diagonal Be – At: the most common in the school curriculum are Be and Al

Amphoteric hydroxides and oxides are formed by metals – d-elements in an average oxidation state, for example

Cr 2 O 3 , Cr(OH) 3; Fe 2 O 3 , Fe (OH) 3

· And three exceptions: metals Zn, Pb, Sn form the following compounds, and amphoteric compounds.

The most common amphoteric oxides (and their corresponding hydroxides) are:

ZnO, Zn(OH) 2 , BeO, Be(OH) 2 , PbO, Pb(OH) 2 , SnO, Sn(OH) 2 , Al 2 O 3 , Al(OH) 3 , Fe 2 O 3 , Fe( OH) 3 , Cr 2 O 3 , Cr(OH) 3

The properties of amphoteric compounds are not difficult to remember: they interact with acids and alkalis .

with interaction with acids, everything is simple; in these reactions, amphoteric compounds behave like basic ones:

Oxides:

Al 2 O 3 + 6HCl → 2AlCl 3 + 3H 2 O

ZnO + H 2 SO 4 → ZnSO 4 + H 2 O

BeO + HNO 3 → Be(NO 3 ) 2 + H 2 O

Hydroxides react in the same way:

Fe(OH) 3 + 3HCl → FeCl 3 + 3H 2 O

Pb(OH) 2 + 2HCl → PbCl 2 + 2H 2 O

· With interaction with alkalis it is a little more difficult. In these reactions, amphoteric compounds behave like acids, and the reaction products can be different, it all depends on the conditions.

Either the reaction takes place in solution, or the reactants are taken as solids and fused.

· Interaction of basic compounds with amphoteric compounds during fusion.

Let’s take zinc hydroxide as an example. As mentioned earlier, amphoteric compounds interacting with basic ones behave like acids. So we write zinc hydroxide Zn (OH) 2 as an acid. The acid has hydrogen in front, let’s take it out: H 2 ZnO 2 . And the reaction of alkali with hydroxide will proceed as if it were an acid. “Acid residue” ZnO 2 2 -divalent:

2KOH (solid) + H 2 ZnO 2 (solid) (t, fusion) → K 2 ZnO 2 + 2H 2 O

The resulting substance K 2 ZnO 2 is called potassium metazincate (or simply potassium zincate). This substance is a salt of potassium and the hypothetical “zinc acid” H 2 ZnO 2 (it is not entirely correct to call such compounds salts, but for our own convenience we will forget about it). Only zinc hydroxide is written like this: H 2 ZnO 2 is not good. We write as usual Zn (OH) 2 , but we mean (for our own convenience) that this is an “acid”:

2KOH (solid) + Zn (OH) 2 (solid) (t, fusion) → K 2 ZnO 2 + 2H 2 O

With hydroxides, in which there are 2 OH groups, everything will be the same as with zinc:

Be (OH) 2 ( solid .) + 2NaOH ( solid .) (t, fusion) → 2H 2 O + Na 2 BeO 2 (sodium metaberyllate, or beryllate)

With amphoteric hydroxides with three OH groups (Al (OH) 3 , Cr (OH) 3 , Fe (OH) 3 ) it is a little different.

Let’s look at the example of aluminum hydroxide: Al (OH) 3 , write it in the form of an acid: H 3 AlO 3 , but we don’t leave it in this form, but take out the water from there:

H 3 AlO 3 – H 2 O → HAlO 2 + H 2 O.

Here we are working with this “acid” (HAlO 2 ):

HAlO 2 + KOH → H 2 O + KAlO 2 (potassium metaaluminate, or simply aluminate)

But aluminum hydroxide cannot be written like this HAlO 2 , we write it down as usual, but we mean “acid” there:

Al (OH) 3 ( solid .) + KOH ( solid .) (t, fusion) → 2H 2 O + KAlO 2 (potassium metaaluminate)

The same with chromium hydroxide: Cr(OH) 3 → H 3 CrO 3 → HCrO 2

Cr (OH) 3 (solid) + KOH (solid) (t, fusion) → 2H 2 O + KCrO 2 (potassium metachromate,

BUT NOT CHROMATE, chromates are salts of chromic acid).

The same principles as in the names of ordinary “salts”, the element in the highest degree of oxidation – the suffix AT, in the intermediate – IT.

These compounds are always formed when a strongly basic “world” (alkalis) and an amphoteric one (by fusion) come into contact. That is, just like amphoteric hydroxides with alkalis, amphoteric oxides will also react.

Interactions:

1. Amphoteric oxide with strong basic oxide:

ZnO (solid) + K 2 O (solid) (t, fusion) → K 2 ZnO 2 (potassium metazincate, or simply potassium zincate)

2. Amphoteric oxide with alkali:

ZnO (solid) + 2KOH (solid) (t, fusion) → K 2 ZnO 2 + H 2 O ↑

3. Amphoteric hydroxide with strong basic oxide:

Zn (OH) 2 (solid) + K 2 O (solid) (t, fusion) → K 2 ZnO 2 + H 2 O ↑

4. Amphoteric hydroxide with alkali:

Zn (OH) 2 (solid) + 2KOH (solid) (t, fusion) → K 2 ZnO 2 + 2H 2 O ↑

Remember, the reactions above take place during fusion .

· Interaction of amphoteric compounds with alkalis (here only alkalis) in solution.

In the Unified State Examination, this is called “the dissolution of aluminum hydroxide (zinc, beryllium, etc.) alkali.” This is due to the ability of metals in the composition of amphoteric hydroxides in the presence of an excess of hydroxide ions (in an alkaline medium) to attach these ions to themselves. A particle is formed with a metal (aluminum, beryllium, etc.) in the center, which is surrounded by hydroxide ions. This particle becomes negatively charged (anion) due to hydroxide ions, and this ion will be called hydroxoaluminate, hydroxozincate, hydroxoberyllate, etc.

Let us write down the abbreviated ionic equation of these processes:

Al(OH) 3 + OH → Al(OH) 4

The resulting ion is called “Tetrahydroxoaluminate ion”. The prefix “tetra” is added because there are four hydroxide ions. The tetrahydroxoaluminate ion has a – charge, since aluminum carries a 3+ charge, and four hydroxide ions 4-, in total it turns out -.

When alkali reacts with amphoteric hydroxide, a salt is formed in solution. The cation of which is an alkali cation, and the anion is a complex ion, the formation of which we considered earlier. The anion is enclosed in square brackets .

Al(OH) 3 + KOH → K[Al(OH) 4 ] (potassium tetrahydroxoaluminate)

Do not forget to ensure that all indexes are correctly affixed. Keep an eye on the charges, and keep in mind that they must sum to zero.

In addition to amphoteric hydroxides, amphoteric oxides react with alkalis. The product will be the same. Only if you write the reaction like this:

Al 2 O 3 + NaOH → Na[Al(OH) 4 ]

But this reaction is not balanced for you. It is necessary to add water to the left side, because interaction occurs in solution, there is enough water there, and everything will equalize:

Al 2 O 3 + 2NaOH + 3H 2 O → 2Na[Al(OH) 4 ]

In addition to amphoteric oxides and hydroxides, some especially active metals interact with alkali solutions, which form amphoteric compounds. Namely, it is: aluminum, zinc and beryllium. To equalize, the left also needs water. And, in addition, the main difference between these processes is the release of hydrogen:

2Al + 2NaOH + 6H 2 O → 2Na[Al(OH) 4 ] + 3H 2

2Al + 6NaOH + 6H 2 O → 2Na 3 [Al(OH) 6 ] + 3H 2

The table below shows the most common examples of the properties of amphoteric compounds in the exam:

Amphoteric substance Salt Salt name Reactions
Al Al 2 O 3 Al(OH) 3 Na[Al(OH) 4 ] Sodium tetrahydroxoaluminate Al(OH) 3 + NaOH → Na[Al(OH) 4 ] Al 2 O 3 + 2NaOH + 3H 2 O → 2Na[Al(OH) 4 ] 2Al + 2NaOH + 6H 2 O → 2Na[Al(OH) 4 ] + 3H 2
Zn ZnO Zn(OH) 2 K 2 [Zn(OH) 4 ] Sodium tetrahydroxozincate Zn(OH) 2 + 2NaOH → Na 2 [Zn(OH) 4 ] ZnO + 2NaOH + H 2 O → Na 2 [Zn(OH) 4 ] Zn + 2NaOH + 2H 2 O → Na 2 [Zn(OH) 4 ]+ H 2
Be BeO Be(OH) 2 Li 2 [Be(OH) 4 ] Lithium tetrahydroxoberyllate Be(OH) 2 + 2LiOH → Li 2 [Be(OH) 4 ] BeO + 2LiOH + H 2 O → Li 2 [Be(OH) 4 ] Be + 2LiOH + 2H 2 O → Li 2 [Be(OH) 4 ]+ H 2

The salts obtained in these interactions react with acids, forming two other salts (salts of a given acid and two metals):

2Na 3 [Al(OH) 6 ] + 6H 2 SO 4 → 3Na 2 SO 4 + Al 2 (SO 4 ) 3 + 12H 2 O

That’s all! Nothing complicated. The main thing is not to confuse, remember what is formed during fusion, what is in solution. Very often, tasks on this issue come across in Part B.

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