I always think about this question. Is it due to attraction or repulsion or is there something else that makes one of them a gas and the other a liquid (water)?
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$\begingroup$ related chemistry.stackexchange.com/questions/63380/… $\endgroup$– MithoronNov 28, 2017 at 20:28
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3 Answers
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The reason is hydrogen bonding.
The relatively large electronegativity difference between oxygen and hydrogen (about 2.3) is the reason why the partially positively charged (try to think why they are partially positively charged) hydrogens are attracted to the lone pairs on the oxygen atoms of other water molecules.
The relatively smaller electronegativity difference between sulphur and hydrogen (about 0.5) is too small to polarise the covalent bond to a required extent. Secondly, the lone pairs on the sulphur atom are delocalised in the sp3 orbitals and the d orbitals - an option unavailable to oxygen. So, the “negative charge” of the lone pairs is also distributed and the interaction is lowered.
So, the hydrogen bonding in water causes the water molecules to be associated with each other due to intermolecular forces of attraction, whereas in the case of hydrogen sulphide, these forces of attraction area much weaker.
So, lesser energy is required to overcome the forces of interaction between the hydrogen sulphide molecules than those between water molecules. This energy is available at room temperature and hence, hydrogen sulphide is a gas, while water is still a liquid.
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In water molecule the more electronegative oxygen atom has small size and is present as a central atom, so that it forms the intermolecular hydrogen bonding strongly.
However, in $\ce{H2S}$ molecule the central atom sulfur is less electronegative and is bigger in size, so that it is unable to form the intermolecular hydrogen bonding.
Therefore $\ce{H2S}$ molecule can not exist in liquid form.
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1$\begingroup$ This is pretty much the same answer as the earlier one with the essential details stripped off. $\endgroup$– andselisk ♦Dec 18, 2017 at 5:29
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Well, first of all depends on what temperature you are considering. But let’s take room temperature (20 degrees). In Water, the oxygen atom is highly electronegative and can polarize (partially) the hydrogen atoms, thus hydrogen-hydrogen bonds between the H2O molecules can be formed creating a very high boiling point. In H2S those bonds don’t exist, because sulfur is much less electronegative. Secondly, the lone pairs on the sulphur atom are delocalised in the sp3 orbitals and the d orbitals - an option unavailable to oxygen. So, the “negative charge” of the lone pairs is also distributed and the interaction is lowered.
So, the hydrogen bonding in water causes the water molecules to be associated with each other due to intermolecular forces of attraction, whereas in the case of hydrogen sulphide, these forces of attraction area much weaker.
So, lesser energy is required to overcome the forces of interaction between the hydrogen sulphide molecules than those between water molecules. This energy is available at room temperature and hence, hydrogen sulphide is a gas, while water is still a liquid.
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$\begingroup$ "In H2S does bonds don’t exist, because sulfur is much electro negative. " That doesn't make any sense: $\ce{O}$ is more electronegative than $\ce{S}$. $\endgroup$– GertNov 28, 2017 at 19:21
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$\begingroup$ @Gert true, I might have a moment there where I skipped the thought $\endgroup$ Nov 28, 2017 at 19:24
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$\begingroup$ Not "hydrogen-hydrogen bonds". They,re just hydrogen bonds", said once, and they are between hydrogen and a much more electronegative element with compact atoms (typically N, O, or F). $\endgroup$ Nov 28, 2017 at 19:30