Atmosphere - Chemistry

bonkysleuth SG

Which substance present in car exhaust does not have an effect on limestone?

1. CO

2.NO2

3.CO2

4.SO2

My answer is 3, CO2. I do not really know how to explain this but I'd try to. Please correct me where necessary. Mainly I think this is because catalytic converter convert some of the harmful gases into harmless ones and CO2 is part of the "harmless gases". On the other hand, the rest of the gases(air pollutants) are converted and come out as N2, H2O and CO2... (don't think this is a good explanation though)

2. Is unburnt hydrocarbons a primary pollutant? I think it is because it comes out directly from car exhausts into the atmosphere.

3. How is HNO3 (secondary pollutant) produced?

Is it through nitrogen oxides. If yes, then is there a need to elaborate from here?

4. Unburnt hydrocarbons react with NO2 in the air to form substances that cause tearing of the eyes. How do hydrocarbons get into atmosphere?

My answer: Incomplete combustion(burning without sufficient O2) will cause unburnt hydrocarbons to be produced, thus entering atmosphere.

This is not part of the question but I just want to know what are the products when unburnt hydrocarbons react with NO2? Is it the same as when hydrocarbons react with O2, giving CO2 and H2O and heat energy? What exactly are the substances that cause the tearing of the eye?

Thanks all.

dkcx SG

Which substance present in car exhaust does not have an effect on limestone?

1. CO

2.NO2

3.CO2

4.SO2

2. Is unburnt hydrocarbons a primary pollutant? I think it is because it comes out directly from car exhausts into the atmosphere.

3. How is HNO3 (secondary pollutant) produced?

Is it through nitrogen oxides. If yes, then is there a need to elaborate from here?

4. Unburnt hydrocarbons react with NO2 in the air to form substances that cause tearing of the eyes. How do hydrocarbons get into atmosphere?

My answer: Incomplete combustion(burning without sufficient O2) will cause unburnt hydrocarbons to be produced, thus entering atmosphere.

Thanks all.

Let me guess my way through...

1) If we are talking about reaction with limestone then my answer might be CO cos the other 3 gases are acidic in nature and acids would react with limestone. Since there is no mention about catalytic converters, i won't consider the redox reactions.

2) Fuels in car are mostly C8 compounds if i didn't remember wrongly and i don't remember C8 being classified as a pollutant but i'm unsure about this.

3) HNO3 is likely produced when oxides of nitrogen react with the water vapour in the air i assume.

4) NO2 might react with hydrocarbons to give CO2, N2 and H2O i think. What causes tearing in the eyes? Maybe acid which could be carbonic or nitric acid, nothing else comes to mind really.

Hydrocarbons can get into the air from industries, cars and natural gas is also a form of hydrocarbons.

Won't give much assurance to these answers since i am not free to really go research and double check but just taking a break from studying my sociology test tml :p

UltimaOnline SG

To add on to what dkcx has already said,

bonkysleuth, you should familiarize yourself with the acids that are produced when acidic (ie. non-metal) oxides dissolve in water.

sulfur dioxide + water --> sulfurous acid (aka sulfuric(IV) acid)

sulfur trioxide + water --> sulfuric acid (aka sulfuric(VI) acid)

nitrogen dioxide + water --> nitrous acid + nitric acid (aka nitric(III) acid and nitric(V) acid)

carbon dioxide + water --> carbonic acid

In fact, the following equilibrium equation (you should write it in the exams) will explain many phenomena, from why acids react with carbonates to liberate carbon dioxide gas, to why bubbling carbon dioxide into limewater will produce a white ppt.

CO2(g) + H2O(l) <---> H2CO3(aq) <---> 2H+(aq) + CO3 2-(aq)

Since this is an equilibrium reaction, you can look at it from the other direction :

2H+(aq) + CO3 2-(aq) <---> H2CO3(aq) <---> CO2(g) + H2O(l)

Note that because CO2(g) is a gas that leaves the reaction mixture, unless you contain the volume, the position of equilibrium will be pulled over to one side.

Carbon monoxide needs to be oxidized first, to form carbon dioxide, because it will react with basic oxides or alkalis. On it's own, CO(g) is not acidic, but is toxic due to it's high affinity to act as a strong ligand (in fact, to displace weaker oxygen ligands) with the iron in haemoglobin protein in red blood cells.

And yes, you should explain how nitric acid is produced, with these equation :

2NO2(g) + H2O(l) --> HNO2(aq) + HNO3(aq)

As for your question :

>>> This is not part of the question but I just want to know what are the products when unburnt hydrocarbons react with NO2? What exactly are the substances that cause the tearing of the eye? <<<

Glad you asked. In your upcoming 'A' levels, you will learn about free-radical substitution mechanism (you already came across it in 'O' levels, when you mix a halogen gas with alkanes, under UV light; you just didn't know it was called free-radical substitution).

Photochemical smog formation involves free radical substitution (initiation, propagation, termination). This is why it's called PHOTOchemical smog, because it involves UV radiation from sunlight allowing homolytic cleavage/fission, to generate highly reactive free-radicals.

Details on the chemistry of photochemical smog :
http://mtsu32.mtsu.edu:11233/Smog-Atm1.htm

About photochemical smog as a pollutant :
http://en.wikipedia.org/wiki/Photochemical_smog#Photochemical_smog

Garrick_3658 SG

@Q3:

I don't see how nitric acid alone is a pollutant. You should blatantly state to the marker/examiner/teacher that nitrogen dioxide in the air react with hydroxide ions (I haven't covered this topic yet, so pardon me if it is the hydrogen ions) in rainwater to form nitric acid, which in turn fall to Earth as acid rain.

UltimaOnline SG

@Q3:

It's a hydrolysis reaction, ie. water reacting with a species to form another species.

In this case, begin by drawing the Kekule structure of nitrogen dioxide (this is near impossible to draw for 'O' level students, and still challenging for many 'A' level students... a JC tuition student of mine said that she was the only one in the entire class who could successfully draw the structure required in a class test, thanks to my tuition).

Note that because nitrogen is in period 2 and does not have empty d-orbitals with which to expand its octet, the structure of nitrogen dioxide has a single bonded O with a -ve formal charge, a double bonded O with no charge, and an unpaired lone electron on N, which has a +ve formal charge. The molecular geometry is V shaped or bent or non-linear.

In the 1st step of the mechanism, two NO2 free-radical molecules form a (non-dative) covalent bond with their unpaired electrons, generating N2O4 (2 -ve formal charges on 2 singled bonded O atoms, 2 +ve formal charges on the 2 N atoms). Notice that this step confers upon both nitrogen atoms, a stable octet.

In the 2nd step, because oxygen is more electronegative than nitrogen (which would give nitrogen a partial +ve charge), and moreover because the nitrogen already has a positive formal charge, water acts as a nucleophile to attack the +ve formal charged nitrogen. A lone pair on the O (of the water molecule) forms a dative bond pair with the +ve formal charged nitrogen.

Simultaneously, to avoid violating the nitrogen's octet, the pi bond on that nitrogen becomes a lone pair on the oxygen, giving it a -ve formal charge.

The resultant +ve charge on the oxygen is removed by the loss of the proton (eg. to another water molecule to form a hydroxonium ion), such that the proton's bond pair becomes a lone pair on the oxygen.

In the 3rd step, a lone pair on one of the two single bonded -ve formal charged oxygens (that are bonded to the nitrogen that has just been attacked by water), forms a pi bond with the nitrogen. (This is the precursor for HNO3 nitric acid aka nitric(V) acid, with the loss of the leaving group described below).

Simultaneously, in order to prevent violation of its octet, the sigma bond between the two N atoms, cleaves to become a lone pair on the leaving group NO2-, which abstracts a proton (eg. from a hydroxonium ion; recall that one was formed in an earlier step) to form HNO2 nitrous acid aka nitric(III) acid.

Hence, in the final step, we obtain both acids HNO3(aq) and HNO2(aq).

UltimaOnline SG

@Q3:

Just to add : although rainwater is naturally slightly acidic due to carbon dioxide (forming carbonic acid), but in polluted areas with higher than usual concentrations of gaseous sulfur dioxide/trioxide and oxides of nitrogen, these further lower the pH of affected rainwater by formation of nitric acid (and nitrous acid) and sulfuric acid (and sulfurous acid).