VCE Chemistry Question Thread

[scarletmoon]

This question is from page 43 on the 2015 chem checkpoints

House hold bleach contains the hypochlorite ion (ClO-) which in the presence of acid can be used to convert ferrous ion Fe 2+ to ferric ion 3+.

Write the partial ionic equation for the reduction equation?

[Cogglesnatch Cuttlefish]

This question is from page 43 on the 2015 chem checkpoints

House hold bleach contains the hypochlorite ion (ClO-) which in the presence of acid can be used to convert ferrous ion Fe 2+ to ferric ion 3+.

Write the partial ionic equation for the reduction equation?

So it seems like the ClO- is getting reduced to Cl- here, so:
ClO-   -----> Cl-
ClO- + 2H+ + 2e-    ------> Cl- + H20 should be the answer

EDIT: I should clarify that ClO- is reduced as the oxidation number of Cl is originally +1 when it is bonded with O- in this case and it becomes -1 after the reaction

[Eiffel]

got a question here

concordant titres would not normally be expected with this methods (back titration with acid base and redox). what is the only situation in which they would be expected?

[keltingmeith]

got a question here

concordant titres would not normally be expected with this methods (back titration with acid base and redox). what is the only situation in which they would be expected?

The situation where your experimental technique is perfect/pretty good. (who told you a back titration wouldn't give you concordant titres? Because if you're not getting concordant titres, then your experiment has no precision and is fairly useless... Why would we do a back titration if we can't get a precise answer out of it?)

[RazzMeTazz]

Would it be correct to say that a strong acid/base has a weak conjugate acid/base and vice versa?
If this is the case, I don't quite understand why NH3 (weak base) has a weak conjugate acid of NH4+ ?

Also, for acid base reactions I'm not quite sure when to use  reversible arrows in my equations.
I thought you would use it whenever a weak acid/base is involved to show that the reaction doesn't occur to completion, but what if you had a weak acid reacting with a strong base? Would you still include the reversible arrows in the chemical equation?

Finally, why would the back titration method be used when the substance being analysed is volatile? Are there any examples of this?

Thanks

[Eiffel]

The situation where your experimental technique is perfect/pretty good. (who told you a back titration wouldn't give you concordant titres? Because if you're not getting concordant titres, then your experiment has no precision and is fairly useless... Why would we do a back titration if we can't get a precise answer out of it?)

its question on the hw sheet.

can you give me some reasons when to expect/ when not to expect concordant results?


also what reaction can we see when something is dissolved in water vs reacted with water?

[keltingmeith]

Would it be correct to say that a strong acid/base has a weak conjugate acid/base and vice versa?

This is correct, because equilibria. You learn more about it in unit 4.

Also, for acid base reactions I'm not quite sure when to use  reversible arrows in my equations.
I thought you would use it whenever a weak acid/base is involved to show that the reaction doesn't occur to completion, but what if you had a weak acid reacting with a strong base? Would you still include the reversible arrows in the chemical equation?

The "reversible" arrows talks about equilibria - once again, a concern for unit 4.

Finally, why would the back titration method be used when the substance being analysed is volatile? Are there any examples of this?

If the product you want to measure is volatile, it will very quickly perform some kind of reaction. This means that not all of it will react in the initial titration, and so you won't be measuring all of it - hence why you instead use a different method that you can measure all of it with. There's a pretty good example given here.

[lzxnl]

Would it be correct to say that a strong acid/base has a weak conjugate acid/base and vice versa?
If this is the case, I don't quite understand why NH3 (weak base) has a weak conjugate acid of NH4+ ?

Also, for acid base reactions I'm not quite sure when to use  reversible arrows in my equations.
I thought you would use it whenever a weak acid/base is involved to show that the reaction doesn't occur to completion, but what if you had a weak acid reacting with a strong base? Would you still include the reversible arrows in the chemical equation?  if you have a weak acid reacting with a strong base, or vice-versa, it's essentially irreversible. Weak acid and weak base reactions may or may not be reversible depending on the relative acid/base strengths; VCAA won't be that specific. They'll only ask you to write acid-base reactions with water or if one of the acids/bases is strong

Finally, why would the back titration method be used when the substance being analysed is volatile? Are there any examples of this? Using an excess means you can quickly react with the volatile substance, hopefully stopping the volatile substance from disappearing

Thanks

Weak acids tend to have weak conjugate bases and vice versa. For you to have a strong base, you need a really negligible conjugate acid. Anything with weak acid-base properties will manage to have a conjugate that has acid-base properties, by definition.

[RazzMeTazz]

Thanks so much EulerFan101 and lzxnl for the help!

[lzxnl]

This is correct, because equilibria. You learn more about it in unit 4.

The "reversible" arrows talks about equilibria - once again, a concern for unit 4.

If the product you want to measure is volatile, it will very quickly perform some kind of reaction. This means that not all of it will react in the initial titration, and so you won't be measuring all of it - hence why you instead use a different method that you can measure all of it with. There's a pretty good example given here.

Actually, a strong acid doesn't have a weak base as a conjugate. Its conjugate essentially ISN'T a base. Do you really see chloride ion displaying basic properties? (Bronsted I mean)
Similarly, ammonia, the conjugate acid of the ridiculously strong base, the amide (NH2 -) ion, doesn't display acid properties at all.

Ammonia has a weak conjugate base of the ammonium ion because the only way ammonia can be weak is for its conjugate to dissociate. If its conjugate didn't dissociate, then ammonia's ionisation would go to completion and it wouldn't be weak.

[keltingmeith]

Actually, a strong acid doesn't have a weak base as a conjugate. Its conjugate essentially ISN'T a base. Do you really see chloride ion displaying basic properties? (Bronsted I mean)
Similarly, ammonia, the conjugate acid of the ridiculously strong base, the amide (NH2 -) ion, doesn't display acid properties at all.

Ammonia has a weak conjugate base of the ammonium ion because the only way ammonia can be weak is for its conjugate to dissociate. If its conjugate didn't dissociate, then ammonia's ionisation would go to completion and it wouldn't be weak.

Knew you'd be onto me.

It is true that a strong acid doesn't have a weak conjugate base - however, there is definitely a relativity there, as per the self-ionisation of water:



So, the weaker the acid, the stronger the conjugate base. The stronger the acid, the weaker the conjugate base.

Also, you can protonate a Chloride ion, so I do see it acting as a Lowry-Bronsted base, yes.

[RazzMeTazz]

For the purpose of VCE,  how much detail do we need to know about titration curves between the different combinations of strong/weak acids and strong/weak bases? Do we have to know about the buffer zone ?

Also, if you were determining the NH4+ content in fertiliser and you used the back titration method, reacting the fertiliser solution with excess NaOH to form NH3 and H2O, what would be the purpose of boiling this solution to cause the NH3 to evaporate, before you titrate the excess OH- ions with a strong acid such as HCl?

Is it because NH3 is a weak base and so it might interfere with the acid-base reaction between the excess NaOH and the HCl?

[lzxnl]

Knew you'd be onto me.

It is true that a strong acid doesn't have a weak conjugate base - however, there is definitely a relativity there, as per the self-ionisation of water:



So, the weaker the acid, the stronger the conjugate base. The stronger the acid, the weaker the conjugate base.

Also, you can protonate a Chloride ion, so I do see it acting as a Lowry-Bronsted base, yes.

Can you actually protonate a chloride ion in water? I'd have thought you'd protonate all the water first. The only time I can imagine HCl protonated is in another solvent. In like anhydrous HF or HSbF₆, you'd have H₂Cl⁺ but that's pretty extreme...

For the purpose of VCE,  how much detail do we need to know about titration curves between the different combinations of strong/weak acids and strong/weak bases? Do we have to know about the buffer zone ?

Also, if you were determining the NH4+ content in fertiliser and you used the back titration method, reacting the fertiliser solution with excess NaOH to form NH3 and H2O, what would be the purpose of boiling this solution to cause the NH3 to evaporate, before you titrate the excess OH- ions with a strong acid such as HCl?

Is it because NH3 is a weak base and so it might interfere with the acid-base reaction between the excess NaOH and the HCl?

Not the buffer zone, but you need to recognise that a strong acid/weak base titration has an equivalence point at an acidic pH.
The purpose of boiling the solution is because gases are less soluble in hotter temperatures (generally), because the enthalpy of dissolution is negative.
Let's say you didn't drive the gas off. When adding your acid, the acid would react with any excess sodium hydroxide AS WELL AS the ammonia present. You don't want the acid reacting with the ammonia present as that's an unknown.

[keltingmeith]

Can you actually protonate a chloride ion in water? I'd have thought you'd protonate all the water first. The only time I can imagine HCl protonated is in another solvent. In like anhydrous HF or HSbF₆, you'd have H₂Cl⁺ but that's pretty extreme...

You can protonate a chloride ion by putting in a strong acid, such as H2SO4. Not sure about HCl, though - my gut tells me that protonating it would be really hard and it wouldn't be very stable.

[lzxnl]

You can protonate a chloride ion by putting in a strong acid, such as H2SO4. Not sure about HCl, though - my gut tells me that protonating it would be really hard and it wouldn't be very stable.

Given that you're conducting this in an aqueous environment, wouldn't sulfuric acid just protonate the water? Water is a much stronger base than chloride ion.

Have you heard of HSbF₆? Also known as fluoroantimonic acid, it's one of the strongest (if not the strongest) acids known to humankind. Heck, it even protonates hydrocarbons. Read about it xD