Chemistry Questions

last SG · 2009-04-02T19:16:41+00:00

The OH group donates electrons to the benzene ring through resonance, therefore increasing electron density of the benzene ring. Since benzene is the nucleop...

d3sT1nY SG

Ok. Kind-hearted souls out there, please give your guidance to me.

Here we go:

1. Which of these structural formulae does not represent an isomer of pentane?

a) Pentane (straight chain)

b) 2-methylpentane (branched chain) -> I suppose is 6 carbons already My answer

c) 2,2-methylypentane (a cross shaped molecule with 5 carbons)

d) Cyclopentane (ring shaped, 5 carbons) -> The book's answer

2. Which of the following compounds has the empirical formula, CH2O?

a) Methyl ethanoate

b) Methanoic acid

c) Methanol

d) Methane

3. Cracking of hydrocardon molecules produces __________

a) polymers

b) larger hydrocarbon molecules

c) less volatile hydrocarbon molecules

d) more volatile hydrocarbon molecules

Edited:

4. Is there a difference in the products formed between electrolysis of molten NaCl and concentrated NaCl?

dkcx SG

1) a b c are all alkanes with the formula CnH2n+2 but d has the formula of an alkene which is CnH2n thus its different from the other 3. However i would say the word isomer is misleading.

For your info, c has 7 C not 5 cos pentane is a 5 carbon straight chain with 1 carbon on top n 1 below the 2nd carbon

2) You can just write out all their chemical formula and all 4 seems wrong.

3) D. Cracking gives you smaller molecules and thus they are more volatile as they have lower boiling points. A and B is definitely wrong

4) Yes. Moltan electrolysis will get u Cl2 and Na metal. Concentrated NaCl will give you H2 and O2 as products of electrolysis.

UltimaOnline SG

dkcx SG

Didn't that that in concentrated solutions the ions can 'outnumber' the OH- n H+ ions since its always perceived that your solvent is always more than your solute even in highly soluble compounds.

d3sT1nY SG

Alright thanks.

Will come back with more questions later on.

UltimaOnline SG

Didn't that that in concentrated solutions the ions can 'outnumber' the OH- n H+ ions since its always perceived that your solvent is always more than your solute even in highly soluble compounds.

The molarity of the solvent is of course certainly much greater (by definition of "solute" and "solvent") than the molarity of the solute, but the molarity of H+ and OH- ions from the auto-ionization or auto-dissociation of water remains very low at room temperature; Kw = [H+][OH-] = 1.00 x 10^-14

Ultimately, the issue is how concentrated is "concentrated"?

Also, dkcx, as one of your personal principles of "importance of practicality over theoreticality", theoretical concepts are only useful when applied to explain practical experimental observations.

And I vividly remember carrying out this electrolysis in my lab - the wonderful smell of molecular chlorine from the aqueous sodium chloride (undergoing electrolysis) was distinctly unmistakable.

d3sT1nY SG

Ultimately, the issue is how concentrated is "concentrated"?

Also, dkcx, as one of your personal principles of "importance of practicality over theoreticality", theoretical concepts are only useful when applied to explain practical experimental observations.

And I vividly remember carrying out this electrolysis in my lab - the wonderful smell of molecular chlorine from the aqueous sodium chloride (undergoing electrolysis) was distinctly unmistakable.

Sorry, regarding the bolded text. The aqueous NaCl, in this case, means the controversial "concentrated" NaCl? Because diluted NaCl will not produce Cl gas.

UltimaOnline SG

Sorry, regarding the bolded text. The aqueous NaCl, in this case, means the controversial "concentrated" NaCl? Because diluted NaCl will not produce Cl gas.

Aqueous means "dissolved in water".

The "concentrated" aqueous NaCl(aq) will produce H2 gas at cathode (reduction of protons) and Cl2 gas at anode (oxidation of chloride anions).

The "dilute" aqueous NaCl(aq) will produce H2 gas at cathode (reduction of protons) and O2 gas at anode (oxidation of hydroxide anions).

d3sT1nY SG

5. Which of the following aqueous solutions contains an electrolyte?

a) 0.1M glucose

b) 0.2M ethanol

c) 1M ethanoic acid

d) 1.5M CH3OCH3 (Ether but I don't know the name)

6. Which half reaction can occur at the anode in a voltaic cell?

a) Zn --> Zn²⁺ + 2e^-

b) Fe³⁺ --> Fe²⁺ + e^-

UltimaOnline SG

5. Which of the following aqueous solutions contains an electrolyte?

a) 0.1M glucose

b) 0.2M ethanol

c) 1M ethanoic acid

d) 1.5M CH3OCH3 (Ether but I don't know the name)

6. Which half reaction can occur at the anode in a voltaic cell?

a) Zn --> Zn²⁺ + 2e^-

b) Fe³⁺ --> Fe²⁺ + e^-

Hint for Q5)

Only one of these dissociate significantly to form ions, required for conduction of electricity.

Hints for Q6)

Red Cat riding An Ox. Reduction occurs at the Cathode. At the Anode, Oxidation occurs.

Fe3+ is reduced to Fe2+

Reduction : Fe3+ + e- --> Fe2+

Fe 2+ is oxidized to Fe3+

Oxidation : Fe2+ --> Fe3+ + e-

d3sT1nY SG

5. So I suppose carboxylic acid is more acidic than alcohol.

Let's see, because of the oxygen (highly electronegative) that is double bonded to the carbon, increase the extent of the negative charge of the carboxylate ion, thus stabilizing the ion in aqueous state.

Alcohol has a ethyl group attached directly to the oxygen, dispersing the negative charge, thus destabilizing the alcohol ion.

6. Oh, crap, I see wrongly. The half equation I typed out is not possible.

UltimaOnline SG

5. So I suppose carboxylic acid is more acidic than alcohol.

Alcohol has a ethyl group attached directly to the oxygen, dispersing the negative charge, thus destabilizing the alcohol ion.

6. Oh, crap, I see wrongly. The half equation I typed out is not possible.

The carboxylate anion is stabilized by both resonance and induction - the negative ionic charge on the conjugate base is spread out (by resonance and induction) over 2 electronegative oxygen atoms.

basmoth SG

Sorry to ask but what sort of isomerism (of pentane) do these two exhibit?

b) 2-methylpentane (branched chain) and c) 2,2-dimethylpentane (a cross shaped molecule with 5 carbons).

They can't be functional group, structural or positional isomers, can they?

Additionally, 'Alcohol has a ethyl group attached directly to the oxygen, dispersing the negative charge, thus destabilizing the alcohol ion.' is wrong as the positive inductive effect of the electron donating alkyl group will actually intensify the negative charge on the O atom, making the conjugate base ion for ethanol less stable. I think. :p

dkcx SG

2-methylpentane and 2,2-dimethylpentane are not really isomers of pentance since they differ in the number of carbons and that by itself does not make them isomers. Rather, they are just different types of alkanes.

The answer as given in the book d which is a cycloalkane has the general formula simiar to that of an alkene which is in a way something totally different from alkanes and the more odd 1 out in the 4.

As mentioned, i do not agree with the use of the word isomers in the question since that is wrong to me.

d3sT1nY SG

Actually I'm sorry for a mistake. It's actually not 2,2-dimethylpentane BUT 2,2-dimethylpropane. So that would be 5 carbons.

dkcx SG

Ok but is the 2 methyl really pentane or butane?

d3sT1nY SG

Deleted picture.

7. Which compound contains both ionic and covalent bonds?

a) Aluminium chloride

b) Ethyl propanoate

c) Titanium oxide

d) Diamond

8. What is the mass of chromium, Cr, that can be extracted from 76g of chromium(III) oxide?

9. Which of the following underlined substance has been reduced?

a) Ag⁺Cl^- --> AgCl

b) N₂ --> NH₃

Ok, your help is greatly appreciated. I don't know whether is it the book I'm using has so many mistakes or is it my chemistry knowledge has gone rusty.

dkcx SG

Deleted picture.

7. Which compound contains both ionic and covalent bonds?

a) Aluminium chloride

b) Ethyl propanoate

c) Titanium oxide

d) Diamond

8. What is the mass of chromium, Cr, that can be extracted from 76g of chromium(III) oxide?

9. Which of the following underlined substance has been reduced?

a) Ag⁺Cl^- --> AgCl

b) N₂ --> NH₃

Ok, your help is greatly appreciated. I don't know whether is it the book I'm using has so many mistakes or is it my chemistry knowledge has gone rusty.

7) Erm not sure cos none seems possible to me since most of the compounds that have both that i know are ceramics.

8) Find the number of moles of Cr2O3 in 76g and multiply it by the Mr of Cr. Should be an easy question

9) Reduction is a loss of oxygen, gain of hydrogen or decrease in oxidation number.

Ag(0) - Ag (+1)
N(0) - > N (-3)

Should be clear i hope :)

Maybe you should get a new book since this seems to be loaded with a number of errors in their questions itself.

UltimaOnline SG

Aluminium chloride is a well known example of a compound that is predominantly covalent but with ionic character, due to the relatively small electronegativity difference between aluminium and chloride. (In contrast, aluminium oxide is predominantly ionic with covalent character, which explains its amphoteric properties; 'A' level students can attempt drawing out the hydrolysis mechanisms of covalent vs ionic oxides).

Oxides of titanium, are a much more interesting matter. Titanium has an electronegativity rating relatively similar to aluminium, to be precise - Al (1.5) is slightly more electronegative than Ti (1.3). After electronegativity, there is secondary factor* that determines the extent of covalent vs ionic character in such compounds - its actual structure, in turn due to the valencies or stiochiometries of the compound.

(*'A' level H2/H3/H1 students will only be examined on the primary factor of electronegativity difference. Anything beyond this is for your own further reading only.)

Oxides of titanium illustrate this principle rather nicely.

From :

http://adsabs.harvard.edu/abs/1994PhRvB..5013974S

>>> The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, Ti₂O₃, and TiO₂ series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, TiO₂ is better viewed as a rather covalent oxide. <<<

Wikipedia on

TiO - http://en.wikipedia.org/wiki/Titanium(II)_oxide

Ti2O3 - http://en.wikipedia.org/wiki/Titanium(III)_oxide

TiO2 - http://en.wikipedia.org/wiki/Titanium(IV)_oxide

PubChem's (structural diagrams) on :

Ti2O3 - http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=123111

TiO2 - http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=26042

d3sT1nY SG

7) Erm not sure cos none seems possible to me since most of the compounds that have both that i know are ceramics.

Should be clear i hope :)

8) Find the number of moles of Cr2O3 in 76g and multiply it by the Mr of Cr. Should be an easy question

I got 52 but there's no 52 in the multiple choices answer. Relatively easy but the answer are really driving me crazy and making me doubtful of my knowledge.

9) Reduction is a loss of oxygen, gain of hydrogen or decrease in oxidation number.

Ag(0) - Ag (+1) So why is it Ag⁺ 0 instead of +1
N(0) - > N (-3)

Maybe you should get a new book since this seems to be loaded with a number of errors in their questions itself.

Then I wouldn't recommend this book published by EPH Pte Ltd, author is Bob Ryan - O Level Chemistry guide (Orange in colour).

I thought EPH books should be quite good.