HSC Chemistry Question Thread

[wesadora]

What is meant by a carboxylic acid functional group (COOH) having 'two sites for hydrogen bonding'. Is it literally just because there's the hydrogen bonding between the O and H in the OH, and another possible site for hydrogen bonding due to the double bonded H?
--> and thus alkanoic acids have a higher BP/MP than alkanols/alkanes due to these stronger IMF

[RuiAce]

What is meant by a carboxylic acid functional group (COOH) having 'two sites for hydrogen bonding'. Is it literally just because there's the hydrogen bonding between the O and H in the OH, and another possible site for hydrogen bonding due to the double bonded H?
--> and thus alkanoic acids have a higher BP/MP than alkanols/alkanes due to these stronger IMF

Recall that -COOH looks like
     O
C<
     OH

The -OH site is one site for hydrogen bonding as it is clear that the -OH site is similar to the hydroxyl functional group.

The =O site is yet another site for hydrogen bonding. It is clear that this oxygen with its double bond is going to be extremely polar, allowing for very strong intermolecular forces to form

I.e. One of the sites is the OH site, whereas the other site is the extra O's site


Edit: Upon rechecking I noticed you said double bonded H. If this was a typo for double bonded O, then yes.

[wesadora]

Recall that -COOH looks like
     O
C<
     OH

The -OH site is one site for hydrogen bonding as it is clear that the -OH site is similar to the hydroxyl functional group.

The =O site is yet another site for hydrogen bonding. It is clear that this oxygen with its double bond is going to be extremely polar, allowing for very strong intermolecular forces to form

I.e. One of the sites is the OH site, whereas the other site is the extra O's site

simple! thanks rui

[wesadora]

do you write states for radicals? e.g. O^. or O^._(g) ?

[massive]

guy for part b of the question attached. the order of reactivity (increasing) is X<Z<Y. I was just confused tho, ik that X produces the least voltage however compared to copper it undergoes oxidation (as its negative electrode) and Y and Z undergo reduction (positive electrodes), doesn't that mean that X is more reactive than Y and Z??

[massive]

do you write states for radicals? e.g. O^. or O^._(g) ?

I'm pretty sure you don't write states for radicals

[wesadora]

I'm pretty sure you don't write states for radicals

yeah that's what i thought some ppl do though so i got confused :|

[kimmie]

Hey for Qs like these, how do we know that which is the positive or negative terminal?  And also which is the cathode and anode?

[Cindy2k16]

Hey for Qs like these, how do we know that which is the positive or negative terminal?  And also which is the cathode and anode?

I'm not sure, but you might be able to use the standard potentials table to figure out which material in the lead acid cell is oxidised/reduced and therefore the cathode anode.

The material that is reduced is the cathode-
The material that is oxidised is the anode-
I remember it as RED CAT, AN OX. RED(uction) CAT(hode). AN(ode) OX(idation)

Otherwise, you should know one of the two batteries pictured since you are required to memorise information about one of them for one of the syllabus dotpoints. Using your knowledge of just one of them you can establish what the anode and cathode is an thus what 1 and 2 is.

Also, in a galvanic cell cathode is positive and anode is negative. I remember this by thinking of how cats are great haha. Cats --> cathode. Good -->positive.
Hope this helps

[RuiAce]

do you write states for radicals? e.g. O^. or O^._(g) ?

I'm pretty sure you don't write states for radicals

yeah that's what i thought some ppl do though so i got confused :|

Technically that one is a gas but they care less most of the time for radicals because, being honest radicals are things without a full outer shell so they're extremely reactive and they're gonna become something else soon enough.

[RuiAce]

guy for part b of the question attached. the order of reactivity (increasing) is X<Z<Y. I was just confused tho, ik that X produces the least voltage however compared to copper it undergoes oxidation (as its negative electrode) and Y and Z undergo reduction (positive electrodes), doesn't that mean that X is more reactive than Y and Z??

The polarity of copper tells us which electrode copper is. Here, copper is the negative electrode (i.e. anode) in the cell with X, and the positive electrode (i.e. cathode) in the cell with Y.

Recall RED-CAT AN-OX, i.e. copper is oxidised with X but reduced with Y and Z. Copper is the one undergoing these reactions, not the other three.

So since copper oxidises, X gets reduced, and hence X is less reactive.
and etc.

[RuiAce]

Hey for Qs like these, how do we know that which is the positive or negative terminal?  And also which is the cathode and anode?

These are just the dry cell and the lead-acid cell. They are the cells you studied.

Whilst you aren't expected to replicate that diagram, if you knew your cell well then you would know which is which and thus be able to infer it from your own knowledge.

Or you can use common sense, because a dry cell is literally a BATTERY and we all know what a battery looks like (cylinder with a head poking out on the +'ve terminal)


Also, by definition the positive terminal represents the cathode, and the negative represents the anode.

[massive]

The polarity of copper tells us which electrode copper is. Here, copper is the negative electrode (i.e. anode) in the cell with X, and the positive electrode (i.e. cathode) in the cell with Y.

Recall RED-CAT AN-OX, i.e. copper is oxidised with X but reduced with Y and Z. Copper is the one undergoing these reactions, not the other three.

So since copper oxidises, X gets reduced, and hence X is less reactive.
and etc.

wait so you're saying a positive polarity corresponds to a negative terminal ? :S

[RuiAce]

wait so you're saying a positive polarity corresponds to a negative terminal ? :S

Where exactly did I say that?

[massive]

Here, copper is the negative electrode (i.e. anode) in the cell with X, and the positive electrode (i.e. cathode) in the cell with Y.

over here :S