CH 101/Keys 9

From WolfWikis

Exercises

Write the chemical formula for the following compounds and determine the oxidation states for all elements:

Of course you can only do this if you know what the names mean. On top of that you have to make sure the charges balance

• lanthanum sulfate

lanthanum is in group 3 of the periodic table and therefore always 3+ in a saltlike compound: La³⁺

sulfate is the oxoanion of sulfur in its highest oxidation state (-ate!): SO₄^2-

To make charges balance we need to multiply crosswise:

La³⁺₂(SO₄^2-)₃

This way we have 6 positive and 6 negative charges that cancel

So the compound is La₂(SO₄)₃

The oxidation state for La is obviously 3+

For (SO₄^2-) we compute

.... = -2

... + 4*(-2) = -2

x + -8 = -2

x= 8-2 =6

This is indeed the highest oxidation state for S because its valence configuration is 3s²3p⁴, so it can lose a maximum of 6 electrons.

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The formation of this salt would involve lanthanum hydroxide and sulfuric acid:

La(OH)₃ + H₂SO₄ ==> La₂(SO₄)₃ + H₂O

Notice that I have to write three OH^- groups for the lanthanum to achieve a neutral compound as La gives 3+ ions (third column in periodic table!)

Of course this is not balanced, but you should be able to fix that by now:

2La(OH)₃ + 3H₂SO₄ ==> La₂(SO₄)₃ + 6H₂O

• sodium bicarbonate

sodium is always +1 (except in the metallic element)

carbonate has 4+ for C (highest: -ate) and 3 oxygens in 2- state

As +4-3*2=-2 the ion should be CO₃^2-

Thus Na₂CO₃ would be sodium carbonate.

However bicarbonate is HCO₃^- and this requires only one sodium

NaHCO₃ is sodium bicarbonate

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The formation reactions for sodium carbonate and sodium bicarbonate give a clue to that funny name 'bi'carbonate.

Both can be considered formed by a reaction between sodium hydroxide NaOH and carbonic acid H₂CO₃

Notice that I can arrive at that acid real easy by simply replacing all Na by H! (Na₂CO₃ ==> H₂CO₃)

2NaOH + H₂CO₃ ==> Na₂CO₃ + 2H₂O

2NaOH + 2H₂CO₃ ==> 2NaHCO₃ + 2H₂O (I multiplied everything by 2 for better comparison)

As you can see you can get either carbonate or bicarbonate, but it depends on how much acid you add.

The formation of bicarbonate requires twice as much carbonic acid. (bi- means two!)

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The above reactions are a little theoretical because carbonic acid is very unstable and falls apart into the acidic oxide CO₂ very easily.

But if you dump some lye (NaOH) in water and bubble some carbon dioxide (the bubbles in your soda) through it you can do these reactions.

• calcium peroxide

Calcium is always 2+. Oxygen in peroxides is -1 and they usually occur in pairs: O₂^2-

Thus CaO₂.

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We could conceive of the following 'formation reaction'

Ca(OH)₂ + H₂O₂ ==> CaO₂ + 2H₂O

I must admit however that this is pure theoretical, because hydrogen peroxide H₂O₂ is a lousy acid.

The reverse reaction however is entirely possible. When you dunk the calcium peroxide in water you may well get hydrogen peroxide.

• lithium aluminium hydride

lithium is 1+ and Al 3+. Hydrogen is normally 1+ but in hydrides in combination with less electronegative elements is can be -1.

To achieve neutrality we need four H^-: LiAlH₄.

• rubidium ferrate

Ferrates are compounds of Fe(VI), the highest oxidation state that iron can achieve. The ion is FeO₄^2-. Rubidium is an alkalimetal and always K⁺. Thus K₂FeO₄.

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We could look upon this salt as based on KOH as a base and "H₂FeO₄" ("ferric acid") as an acid, but I do not think anybody has ever isolated such an acid. This is not unusual at all. There are many salts of non-existent hypothetical acids.

• scandium arsenide

Sc is exclusively 3+. Arsenide implies the lowest oxidation state: 3-. Thus ScAs

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The best way to make this compound is undoubtedly directly from the elements, but you could try:

Sc(OH)₃ + AsH₃ ==>ScAs + 3H₂O

Notice that I refuse to write the 'acidic' hydrogen to the left of the arsenic in arsene AsH₃

The reason is that that hydrogen is not very acidic at all and that arsene usually acts more like a weak base like NH₃.

Arsenic does not have a huge electronegativity and its oxidation number is not high at all. In fact it is negative...

Once again this is a case where the reaction may well run in reverse ScAs may very well hydrolize to form stinky and poisonous arsene.

• barium phosphate

Ba is exclusively 2+. There is a load of different phosphates (all 5+ species) but in the absence of further prefixes it is PO₄^3-.

By multiplying crosswise we find Ba₃(PO₄)₂

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This salt can very well be formed from barium hydroxide Ba(OH)₂ and phosphoric acid H₃PO₄

• calcium borate

Ca is 2+. Boron in highest oxidation state is 3+ and this produces BO₃^3-

Neutrality demands Ca₃(BO₃)₂

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We could postulate a formation involving Ca(OH)₂ calcium hydroxide and H₃BO₃ boric acid, but the latter is very weak.

• potassium chlorite

The -ite implies 2 notches below chlorate: 3+, thus ClO₂^- and the salt becomes KClO₂

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The formation would involve the base KOH (potassium hydroxide) and HClO₂ (chlorous acid) and yes both exist, although I'd keep the acid in dilute aqueous form if I were you. These chlorine based acids tend to go poof a bit in pure form.

• lithium fluoroborate

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Formation would involve LiOH (lithium hydroxide) and HBF₄ (fluoroboric acid), the latter can be formed by adding BF₃ to a solution of HF. So, BF₃ is the fluoride analog of an acidic oxide.

We shall see later that it is called a Lewis acid, in a different definition of the word acid, rather than calling it an "acidic fluoride". (Never seen that term anywhere...)

• cesium hydride

CsH

• vanadium(II)sulfate

VSO₄ (with S 6+)

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One can imagine this to be formed from V(OH)₂ (vanadium(II) hydroxide) and sulfuric acid.

• iron(II) disulfide

disulfides are like peroxides: S₂^2-, containing sulfur in the Ox= -1 rather than Ox= -2 state.

FeS₂ is a common mineral (pyrite), aka fools gold

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One can imagine this as formed from Fe(OH)₂ (iron(II)hydroxide) as base and H₂S₂ as acid, but neither of them are well known species, dihydrogendisulfide (the sulfur analog of hydrogen peroxide) would be rather unstable and a very weak acid, so the 'formation reaction' would be rather hypothetical. (However elemental sulfur (S₈) does dissolve in solutions containing the sulfide S^2- ion to form disulfide solutions.

• erbium selenate

Er is exclusively 3+. Selenate is like sulfate with Se 6+: SeO₄^2-

Neutrality demands Er₃(SeO₄)₂

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It would probably be feasible to form this from erbium hydroxide Er(OH)₃ and selenic acid H₂SeO₄. Neither are very common chemicals but they do exist. The selenic acid would be pretty strong too (compare sulfuric acid).

• sodium hypochlorite

hypochlorites are Cl-1+ species: ClO^-. Thus NaClO

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This is the active ingredient of bleach and it is made from chlorine and sodium hydroxide. One can imagine it formed from NaOH and HClO (hypochlorous acid), the latter is not very stable though.

• calcium hydrogen sulfate

Hydrogen sulfates are HSO₄^-.

Ca(HSO₄)₂

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A different (older) name for this stuff is calcium bisulfate. The same story holds here as for the bicarbonate above. Yes you can make this from calcium hydroxide Ca(OH)₂ and sulfuric acid in the right ratio, otherwise you get calcium sulfate (gypsum).

• indium chloride

There are actually two indium chlorides: InCl₃ and InCl. The indium (III) chloride is more common.

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InCl₃ can be formed from In(OH)₃ (indium(III) hydroxide) and hydrochloric acid HCl. Notice that chlorine has a rather low oxidation number in the latter. In fact it is negative (Ox= -1), but chlorine has a whopper of an electronegativity and HCl is a strong (non-oxo!) acid.

• thallium bromide

As with the previous there are two, but here the monovalent TlBr is more common.

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The corresponding bases would be TlOH and HBr. The latter is an even stronger acid than HCl. I'm not sure whether the first one exists. Thallium is terribly poisonous, although I suppose I am still alive to tell you I worked with it a bit. Hate that stuff.