VCE Chemistry Question Thread

[eagles]

Thanks!

I also read somewhere that the solubility of larger alcohols decreases rapidly with size because the longer hydrocarbon tails of the molecules get between the water molecules and break hydrogen bonds. These broken hydrogen bonds are only replaced by much weaker dispersion forces. Hence the less soluble a larger molecule.

Is this also a valid explanation?

[lzxnl]

Something like that, yes. The important bit is that water molecules can only bond a certain number of times. If they form bonds to non-polar molecules, the forces of attraction aren't that great and the broken hydrogen bonds aren't compensated for. The larger the alkanol is, the larger the non-polar end.

Example: something like methanoic acid is miscible in water (it will mix with water in all proportions), but huge acids like decanoic acid instead form dimers in water, where the hydrophilic acid functional groups bond to each other and the hydrophobic ends point in opposite directions. This way, the acid functional group is unable to bond to water and is thus insoluble.

[eagles]

Righto. Thanks

[Rishi97]

What is atomic absorption spectroscopy?
Easy explanation pls

[thushan]

What is atomic absorption spectroscopy?
Easy explanation pls

A technique where you determine the concentration of a metal ion by looking at how much light of a wavelength it absorbs. The more light it absorbs, the more concentrated the metal ion in solution.

[Rishi97]

Thanks thushan

That is an awesome explanation. It has helped me so much.

[eagles]

Can C₅H₁₀ have isomers such as 3-methyl,but-1-ene, and 3-methyl,but-2-ene?

Also, I have a question about significant figures. Do we need to consider the values from the periodic table? If all other values of a question use 4 significant figures but the elements we refer on the periodic table show 3, will this affect the significant figures of the final answer?

Cheers! xD

[Edward21]

Can C₅H₁₀ have isomers such as 3-methyl,but-1-ene, and 3-methyl,but-2-ene?

Also, I have a question about significant figures. Do we need to consider the values from the periodic table? If all other values of a question use 4 significant figures but the elements we refer on the periodic table show 3, will this affect the significant figures of the final answer?

Cheers! xD

Yup 3-methylbut-1-ene and 2-methylbut-2-ene (you wouldn't use 3-methyl as you can flip this molecule the other way and realise 2-methyl will work as the C=C bond is symmetrical).

I'm quite certain that your answer should be to 3, but some may disagree, I'd like to hear another opinion on this, I never really had this clarified for me..

[RKTR]

Also, I have a question about significant figures. Do we need to consider the values from the periodic table? If all other values of a question use 4 significant figures but the elements we refer on the periodic table show 3, will this affect the significant figures of the final answer?

Cheers! xD

Help With Sig Figs
THE FOOLPROOF GUIDE TO SIGNIFICANT FIGURES

[Limista]

Also, I have a question about significant figures. Do we need to consider the values from the periodic table? If all other values of a question use 4 significant figures but the elements we refer on the periodic table show 3, will this affect the significant figures of the final answer?

yeah you do need to consider the values from the periodic table.

when doing sig figs, you need to consider:
* what numerical values you are using from the worded question, and seeing how many sig figs they have. Let's assume in your case it is 4.
* very likely you are going to need molar masses from the periodic table. So now you're going to be dealing with numerical values that have 3 sig figs.
* if adding/subtracting a numerical value with 4 sig figs to a numerical value with 3 sig figs, the answer you get must be to 4 sig figs.
* if multiplying/dividing a numerical value with 4 sig figs to a numerical value with 3 sig figs, the answer you get must be to 3 sig figs.

EDIT: wanted to add some more... if the least number of sig figs in the worded question is 2, then your final answer must be to 2 sig figs. If the least number of sig figs in the worded question is 3, then your final answer must be to 3 sig figs. If the least number of sig figs in the worded question is 4, then your answer must still be to 3 sig figs PROVIDED that you used numerical values with 3 sig figs in your calculation (which would occur if you used molar masses from the periodic table). 

hope that makes sense 

[eagles]

Thanks for all your help 

[eagles]

Why does the equivalence point of a titration not necessarily going to be value 7?

Assuming that it is a neutralisation reaction, why, when the mole ratios of the 2 reactants are the same, may have a pH deviating from 7?

Thanks.

[Edward21]

Why does the equivalence point of a titration not necessarily going to be value 7?

Assuming that it is a neutralisation reaction, why, when the mole ratios of the 2 reactants are the same, may have a pH deviating from 7?

Thanks.

Well.. (this could take a while, hang in there!) at the equivalence point.. we have water being produced and a salt (if you react and acid and base) no acid nor base is in excess at this point (this is the point at which the reactants are present in their stoichiometric ratios, and are producing the products)

1st example. Strong acid + strong base. Let's try NaOH and HCl. We get out H2O and NaCl (this is a neutral salt) hence why at 25 degrees C, the pH is approximately 7.

2nd example. Strong acid + weak base. Let's try H2SO4 and NH3. We get out H2O and a salt of ammonium sulfate (NH4)2SO4. The NH4+ ion is a weak acid it has the ability to donate H+ and it does just that!! it increases the [H+] concentration of the solution at the equivalence point.. this is why the pH is lower than 7 at the equivalence point not because we've got a strong acid, but rather we're producing a conjugate acid, which is making the pH lower.

3rd example. Weak acid + strong base. Let's try CH3COOH and NaOH. We get H2O and CH3COONa, the CH3COO- (ethanoate ion) is the conjugate base of the ethanoic acid. This base can accept H+ from water, increasing the [OH-] concentration thus raising the pH. This is why the pH is above 7 at the equivalence point not because we've got a strong base, but rather we're producing a conjugate base from our weak acid, thus the pH is above 7 (when at 25 degrees celsius).

4th example. Weak acid and weak base. Let's try HOCl (hypochlorous acid) and NH3. We get H2O and ammonium hypochlorite. NH4OCl. The NH4+ is a weak acid, it donates H+ to the solution, but since we've got HOCl and NH3 in a 1:1 mol ratio, we also produce equal amounts of the hypochlorite ion OCl- which is a weak acid and will accept H+ from water producing OH- in solution, or taking up H+ already there. The result is a slightly lower pH from the NH4+ and a slightly higher pH from the OCl- base.. this produces an equivalence point that is relatively neutral at 25 degrees (7) because you're producing a conjugate acid and base at the same time.

The message of the story? The salt produced determines the pH

[lzxnl]

I have to disagree slightly. The acid strength does actually matter. With a strong acid weak base titration, as the acid is so much stronger, it protonates the weak base leaving a weak acid at equilibrium. However, with a weak acid and weak base titration, you really have to check the acidity constants of the weak acid and the conjugate acid of the weak base. Let's take sodium hydrogen sulfate and ammonia. Ammonia's conjugate acid, ammonium ion has Ka of around 5.6 times 10^-10, while sodium hydrogen sulfate itself has a Ka of around 0.01. This means that after reaction  we're left with ammonium sulfate. As sulfate is the conjugate of a stronger acid than ammonium is, sulfate ion is a far weaker base than ammonia. Similarly, ammonium ions are weaker acids than the hydrogen sulfate ion. Therefore, at equilibrium we will have mostly ammonium sulfate. As ammonium ions are more acidic than sulfate ions are basic, the solution would be slightly acidic at equivalence.

[Rishi97]

Which of the following best describes what occurs when a substance absorbs infrared radiation?
a) some of its bonds begin to vibrate
b) Vibrating bonds increase the intensity of their vibration

I have already eliminated c and d but I cant decide which one is right from a and b.
Pls provide a brief explanation. Thanks