chemistry questions

bonkysleuth SG

1.Selenium oxide is a white solid that dissolves in water to give an acid in which selenium has an oxidation of +4.

a. Deduce the formula of the oxide.

I'm not sure what this question is talking about and asking for. The oxidation of selenium of +4 is after it dissolves in water or before? But whatever the case, I wrote the answer as Se4+. Please rectify me where and whenever necessary.

b.Write the chemical formula of the acid made by dissolving this oxide in water.

SeO + H2O -> ??I'm not sure whether I got the first part of the formula correct. >.<

2. Suggest why the melting and boiling point of beryllium are much higher than those of the other elements in the same group.

I wrote that it is less reactive than the other elements down the group whcih are more metallic (metallic properties increase as we go down the group in the periodic table) and melts and boils at a lower temperature. A little confused with my explanation too. Metals have high boiling and melting boils after all.

Please help. thanks.

^tamago^ SG

1. SeO₂
Key: Selenium should be chemically similar to Sulphur since they are in the same Group (IV).
Selenous acid is H₂SeO₃ (which is sometimes written (OH)₂SeO). Does it look like sulphurous acid H₂SO₃ to you?

2. SeO₂ + H₂O ⇌ H⁺ + (HSeO₃)^-

Selenous acid can be crystallised, but in solution it is a diprotic acid:
H₂SeO₃ ⇌ H⁺ + (HSeO₃)^- ; pK_a = 2.62
(HSeO₃)^- ⇌ H⁺ + (SeO₃)^2- ; pK_a = 8.32

^tamago^ SG

3. Go read up on metallic bonds (and how the number of valence electrons affect the bonding) in your textbook.

Roughly, as we move up along Group II, the atoms will have a smaller radius, and so the delocalised electrons are closer to the nuclei, thus the Be atoms form a stronger bond between themselves.

bonkysleuth SG

Selenium oxide is a white solid that dissolves in water to give an acid in which selenium has an oxidation of +4.

1. SeO₂

Selenous acid is H₂SeO₃ (which is sometimes written (OH)₂SeO)

2. SeO₂ + H₂O ⇌ H⁺ + (HSeO₃)^-
* brackets not needed for HSeO₃, just for formatting purposes cos ^- is not displayed correctly.

I dont understand what it means by 'oxidation' and 'oxide'. why is it SeO2?? selenium has a charge of -2 and oxygen is also -2. so when you reduce it, you'll get SeO. Then why is it SeO2 in this case? thanks for your help. ;p

^tamago^ SG

Oxidation state.

Oxide is simply the compound formed only with the element in question, and oxygen.

It is chemically similar to Sulphur. SO₂.

Selenium has a dominant oxidation state of +4 anyway.

Which level are you studying? 'A' Levels or 'O' Levels?

Darkness_hacker99 SG

O Level Question.

bonkysleuth SG

But the oxidation of an element is sometimes that of its group number. Say that of those in group IV. We take carbon as the element and the oxidation state is CH4. This is what is written on my tb. So I was thinking if the oxidation is +4 then shouldn't the formula be SeO4. Sorry but I really don't get the main concept behind all this. rofl. hahaha. thanks for spending your time to share the info anyway. and yea, its o levels.

^tamago^ SG

So I was thinking if the oxidation is +4 then shouldn't the formula be SeO₄.

zhun bo? explain why.

^tamago^ SG

O Level Question.

guess i gave him too much information then. zzzz.

UltimaOnline SG

Tamago gave the correct required level of information to understand this question properly, not “too much” at all. It’s just ‘too bad’ the ‘O’ level teacher decided to ‘zhen’ his students with such a question. It’s good for the student to understand in greater depth of a particular topic, even if such understanding is not explicitly required (ie. tested) for the immediate (eg. ‘O’ level) exam syllabus. This is one aspect of how IVY LEET schools prepare their students to snatch up all available distinction places in the bell curve – KiaSuism the Singapore way.

In reply to the TS/OP’s qn about why Selenium’s oxidation state is +4 in both its acid and its oxide, formulae H2SeO3 and SeO2 respectively :

Oxygen is more electronegative than Selenium, and hence :

In the case of SeO2 (O=Se=O), the electrons in the bond pairs (btwn Se and O) are polarized towards the (more electronegative) oxygen nuclei, thus each oxygen has a -2 oxidation state (O.S.) and this is why Se has a +4 O.S. (since it has 4 bond pairs, and it ‘loses’ (ie. kena polarized away) the electron it ‘invested’ into each bond pair).

In the case of H2SeO3,
HO - Se = O
HO -

It’s similar to the oxide, insofar as Selenium has 4 bond pairs with (the more electronegative oxygen), and hence has an O.S. of +4.

Also note that Se being Grp VI has 6 valence electrons, which are (in considering its formal charge and O.S.) in both the oxide and the acid :
1 lone pair (ie. 2e-) + 4 bond pairs.

The lone pair remains (relatively) closer to the Se nucleus, but (as described earlier) the bond pairs are polarized towards the more electronegative oxygen, hence Se has ‘lost’ 4 electrons (one per bond pair) in its bonding with oxygen, and therefore, has an O.S. of +4 in these compounds (SeO2 and H2SeO3).

Enjoy your ‘O’ levels in the few months you have left; a lot more fun awaits you at ‘A’ levels and University!