Need help with chem questions

Audi SG

I have some questions which popped up in my head as I did some chem questions.

1a. If we have 2 compounds which have the same molecular formula, different structural formula, but one of them have a C=C bond, does it make them isomers?

1b. My teacher writes in the notes that isomers of compounds MUST have at least 3 C atoms. However when I do the TYS, there’s this question which asked me to identify isomers and i reaize that the compound has got only 2 C atoms. Who’s correct? My teacher or the question?

2. The organic compound used to make plastics is an alkene, mainly ethene right? (i figure i should be right since polyethene is plastic) Just refreshing my memory, so you guys can confirm this for me.

3. H2O2 decomposes according to the equation
2H2O2 (l) -> 2H2O (l) + O2(g)
what would be the effect on the rate of decomposition and the vol. of O2 produced if equal vol. of water was added to the H2OO2 before the reaction began?

With water added, H2O2 should be less concentrated so rate of reaction drops but final vol. of O2 remains the same?

4. which compound will eliminate acid in the soil but does not react with ammonium fertilizers?

A CaCO3 B Ca(OH)2 C Ca(NO3)2 D CaO
I wrote CaCO3 (because I did such a question in the TYS before). Ca(OH)2 is out since it’s an alkali and reacts with ammonium salts. But how about CaO? I chose CaCO3 because acid will react with carbonate (so no more acid in soil), but CaO is a base not alkali so it will also not react with ammonium fertilizer…

5. Salt PQ is to be prepared by reacting the carbonate P with the acid HQ by titration. What are the solubilities of the carbonate, acid and salt?

I wrote all are soluble.

6. Is there any solution which contains H+ OR OH- ions COMPLETELY? i think no, it’s always either high amount of H+ /OH- ions or low amt of them

7.How many grams of hydrated MgSO4.7H2O are required to prepare 500cm3 of 0.4mol/dm3 solution?
I got 24 g. (not sure if correct cuz the teacher only gave us questions w/o answers)

8. There are 5 different atoms F, G, H, I and J
Insiide the nuclei of

Atom F 1 Proton 2 neutrons
Atom G 2 Protons 1 neutron
Atom H 3 Protons 3 Neutrons
Atom I 5 Protons 5 neutrons
Atom J 5 protons 6 neutrons

which of the statement is correct?
F and G are isomers as they have the same nucleon no.
F and H are in same group as they have 1 valence electrons*
I and J gain the same no. of electrons to attrain stable structure
H and I are in the same period as they have the same no. of electronic shells.**

Not sure out of the 2 possible options with asterisk, which one if a better choice. I choice the second option.

9. One last question.

if you want to determine whether a reaction is endo or exo, can you just look at the state of the compound?

eg. C30H62 in solid form changes to C30H62 in liquid form. Since changing to liquid involves bond breaking, that means the reaction is endo?

How about this C30H62 (g) -> 30CO2 (g) + 31 H2O (g)

In such a case, how do I know if it’s bond breaking or bond forming?

Apologize for bombarding you all with sooo much questions cuz i amassed all the questions after doing hundreds of them.

Thanks!

UltimaOnline SG

Q1.
Reason why you’re having difficulty with these, is because you need a minimum of ‘A’ levels or diploma to understand all the different types of isomers possible. Your teacher and the TYS qn are both correct, because they’re talking about DIFFERENT types of isomers. For ‘O’ levels, any two compounds with the same molecular formula but different structural formula are called isomers. See Wikipedia for more info :
http://en.wikipedia.org/wiki/Isomers

Q2.
Polyethene is a common plastic. Not all plastics consist of alkene monomrs. See Wikipedia for more info :
http://en.wikipedia.org/wiki/Plastics

Q3.
You’re right.

Q4.
The oxide ion of CaO will accept protons from water to form hydroxide ions, and also accept protons directly from ammonium cations to form ammonia gas.

Q5.
You’re right.

Q6.
If you mean “only” when you say “completely”, then of course it’s not possible since all charges need to be balanced. Presence of counterbalancing ions (even if only spectator) are required.

Q7.
Molarity x Volume = No. of Moles.
No. of moles x molar mass (of HYDRATED salt) = sample mass.

Q8.
Boron is a metalloid, and doesn’t gain electrons to form anions.

Q9.
For a covalent compound changing from solid to liquid, or liquid to gas, you’re overcoming (‘breaking&#8217 Wink intermolecular forces, not breaking covalent bonds (if you write ‘bonds’ the marker will assume you’re referring to covalent bonds and you’ll lose marks). And yes, it’s still endothermic.

In the other equation you gave, there is obviously both bond forming and bond breaking, so there’s no way to tell for sure without further relevant enthalpy information. However, by considering the stability of the reactants vs products, you can make a reasonable deduction. CO2 and H2O are obviously very stable compounds, hence the reaction is likely exothermic.

yiha093 SG

qn 8 ans is D. i have that paper too =)

im so happy after reading the forum, makes me think that my chem isn't bad ... >.<

Audi SG

Q2.
Polyethene is a common plastic. Not all plastics consist of alkene monomrs. See Wikipedia for more info :
http://en.wikipedia.org/wiki/Plastics

Q3.
You’re right.

Q4.
The oxide ion of CaO will accept protons from water to form hydroxide ions, and also accept protons directly from ammonium cations to form ammonia gas.

Q5.
You’re right.

Q7.
Molarity x Volume = No. of Moles.
No. of moles x molar mass (of HYDRATED salt) = sample mass.

Q8.
Boron is a metalloid, and doesn’t gain electrons to form anions.

Thank UltimaOnline for clearing all my doubts!

May I ask if we want to obtain hydrogen for haber process, do we get it from fractional distillation of air or cracking of petroleum? It just so happens that in 2 Paper 1 questions we did in school, my teacher said there's little H2 in air so we must get from cracking of petroleum. But in my prelims, I wrote that and the answer turns out to be obtaining H2 from air. The marking is confusing me a lot.

UltimaOnline SG

Neither. Both are lousy methods.

The standard industrial way to obtain hydrogen is from methane. See Wikipedia :
http://en.wikipedia.org/wiki/Hydrogen_production

However, for the ‘O’ and ‘A’ level syllabuses, which does not teach the abovestated industrial process, you could always offer electrolysis of water (eg. as aqueous sodium chloride) to generate the required hydrogen gas.

yiha093 SG

Neither. Both are lousy methods.

The standard industrial way to obtain hydrogen is from methane. See Wikipedia :
http://en.wikipedia.org/wiki/Hydrogen_production

In tb , N2 is from fractional distill of air while H2 is from cracking.

UltimaOnline SG

That’s coz ‘O’ levels are kept simple (but not necessarily accurately reflective of industrial practices in the real world). Since at ‘O’ levels, you’re briefly exposed to the idea of alkane cracking (the chemistry of which is really considerably more complex than is implied at ‘O’ levels), which does generate small amounts of hydrogen, so at ‘O’ levels it’s ok to say “cracking is used to generate hydrogen”.

Theoretically, you could crack all larger alkanes until you obtain methane, which is still the main industrial source for hydrogen. However, cracking is an inefficient source of hydrogen, and more feasible industrial methods of obtaining methane are used. See Wikipedia :
http://en.wikipedia.org/wiki/Methane