HSC Chemistry Question Thread

[RuiAce]

A conjugate base doesn't necessarily have to be basic in water in it's own right.

Consider the massively long sequence something like phosphoric acid goes through:

H₃PO₄ <-> H₂PO₄^- + H⁺ <-> HPO₄^2- + 2 H⁺ + PO₄^3- + 3 H⁺

You can say that dihydrogen phosphate is the conjugate base of phosphoric acid. But at the same time, dihydrogen is the conjugate acid of hydrogen phosphate. So what do you reckon? Is dihydrogen phosphate an acid or a base?

The conclusion is that conjugate bases and acids are not enough to simply determine whether your substance is acidic or basic. Conjugate acids only relate B-L theory between a specific acid and a base, not on a generic scale. An amphiprotic substance can easily enough be either.


Your example is, however, unique. HSO₄^- exerts acidic properties in water. This is one of which you must know.

[katherine123]

for questions involving the production of sulfuric acid process
do i need to mention the different temperature conditions and % conversion ie initially 1000 degrees, 550 degrees (97% conversion), 400 degrees (99.7% conversion)   or is there a specific compromise temperature

[RuiAce]

for questions involving the production of sulfuric acid process
do i need to mention the different temperature conditions and % conversion ie initially 1000 degrees, 550 degrees (97% conversion), 400 degrees (99.7% conversion)   or is there a specific compromise temperature

Well if you want to maximise your marks you would try to remember all three parts to as accurate as you can.

With temperature you'd want to remember that they're different for each of the three parts. Initially 1000 degrees yes but that's just for part 1.

Part 2 is where all the equilibrium principles come in - catalytic oxidation of sulfur dioxide into sulfur trioxide. I just memorised somewhere around 450 degrees C though I think. (But I did mention the first bed giving a 70% yield and the last giving a 99.7%)

It's not specific either I don't think. But it shouldn't be too far out from 450. 400 and 550 look acceptable.

[zoeh]

Hi,
Why are cyclohexene and cyclohexane used instead of just hexane and hexene when comparing reactivity of alkenes and alkanes?
Thankyouu

[jakesilove]

Hi,
Why are cyclohexene and cyclohexane used instead of just hexane and hexene when comparing reactivity of alkenes and alkanes?
Thankyouu

Hey!

This is a really good question. It's important to understand this, because I've seen multiple HSC questions asking about it!

Basically, we just need an Alkane and an Alkene to test, right? Unfortunately, most Alkenes have EXTREMELY low boiling points. Therefore, it is different to directly compare an Alkane and corresponding Alkene (because it's fairly difficult to test a gas. We want two liquids)! Cyclohexane/ene, due to its cyclical structure, 'sticks' together and remains a liquid much more easily. Additionally, it is non-reactive (won't explode upon testing) and fairly non-toxic.

I hope that answers your question!

[zoeh]

Hey just another question,
In the shipwrecks option, why do acidic environments accelerate corrosion?

[RuiAce]

Hi,
Why are cyclohexene and cyclohexane used instead of just hexane and hexene when comparing reactivity of alkenes and alkanes?
Thankyouu

May be of interest - I actually did the experiment with BOTH cyclohexene/cyclohexane and hex-1-ene/hexane. BOTH produced the same results you'd anticipate.

In fact, I don't know if Jake will disagree but I reckon BOTH are acceptable from my first-hand experience.

What we CANNOT use is something like ethane/ethylene. For reasons he pointed out.

[jakesilove]

May be of interest - I actually did the experiment with BOTH cyclohexene/cyclohexane and hex-1-ene/hexane. BOTH produced the same results you'd anticipate.

In fact, I don't know if Jake will disagree but I reckon BOTH are acceptable from my first-hand experience.

What we CANNOT use is something like ethane/ethylene. For reasons he pointed out.

I would generally stick to Cyclohexane/ene, because I've seen questions outlining the specific benefit of using that chemical.

[jakesilove]

Hey just another question,
In the shipwrecks option, why do acidic environments accelerate corrosion?

Whilst I don't have the specifics right now, take a look at the Chemical equations for the formation of rust. On the right hand side (the products, including rust), there is also a Hydroxide group. If you add acid, by Le Chatelier's principle, the equilibrium will shift towards the side with the base (the products) to neutralise the added acid. This also increases the yield of rust!

Additionally, I think there are factors to do with bacteria propagation, having an area for the reaction to move forward etc. Sorry, I don't remember much from this topic!

Jake

[zoeh]

Whilst I don't have the specifics right now, take a look at the Chemical equations for the formation of rust. On the right hand side (the products, including rust), there is also a Hydroxide group. If you add acid, by Le Chatelier's principle, the equilibrium will shift towards the side with the base (the products) to neutralise the added acid. This also increases the yield of rust!

Additionally, I think there are factors to do with bacteria propagation, having an area for the reaction to move forward etc. Sorry, I don't remember much from this topic!

Jake

Thankyou!

[amandali]

queS: an electrochemical cell was constructed using Mg anode and Pt cathode. the cell potential was measured to be 3.8V. Which would be the suitable electrolyte for cathode half cell?
a)HCl
b)Mg(NO3)2
c)PtI2  platinum iodide
d) KNO3

not sure why its A

[RuiAce]

queS: an electrochemical cell was constructed using Mg anode and Pt cathode. the cell potential was measured to be 3.8V. Which would be the suitable electrolyte for cathode half cell?
a)HCl
b)Mg(NO3)2
c)PtI2  platinum iodide
d) KNO3

not sure why its A

If the anode was Mg metal then the anolyte must have been Mg²⁺.

Oxidation occurs at the anode. Magnesium metal is converted into magnesium ions. According to the data sheet, this has an E^o value of 2.36V

So all we have to do is subtraction.

3.8-2.36=1.44

Which is approximately equal to 1.36V which is the value of the chlorine gas/chloride ion reduction's E^o value.
______________

Alternatively, this can be done through process of elimination.

The cathode is Pt. Pt is an inert metal, and used whenever there are gases involved in the equation. So recalling that chlorine gas gets reduced into chloride ions, A has to be correct.

B is wrong because Mg²⁺ is the anolyte, not the catholyte.

C is the weird one. If the cathode is Pt and the catholyte is PtI₂, then platinum must be taking part in the reaction. Logically, this is fine because magnesium is going to be heaps more reactive than platinum. However that's the thing; platinum is so unreactive to the point this probably just won't occur at all. Also, it was probably intended to be a dud option as Pt does not appear on the standard reduction potentials on the data sheet

D is wrong because K is more reactive than Mg. If the cell was constructed with potassium ions then the magnesium metal would have to be the cathode.

[conic curve]

Could someone help me with this question I attached below

[RuiAce]

Could someone help me with this question I attached below

Saturated means that no more of a substance can dissolve. What happens when it doesn't dissolve? It precipitates.

What is the precipitate.

[katherine123]

If the anode was Mg metal then the anolyte must have been Mg²⁺.

Oxidation occurs at the anode. Magnesium metal is converted into magnesium ions. According to the data sheet, this has an E^o value of 2.36V

So all we have to do is subtraction.

3.8-2.36=1.44

Which is approximately equal to 1.36V which is the value of the chlorine gas/chloride ion reduction's E^o value.
______________

Alternatively, this can be done through process of elimination.

The cathode is Pt. Pt is an inert metal, and used whenever there are gases involved in the equation. So recalling that chlorine gas gets reduced into chloride ions, A has to be correct.

B is wrong because Mg²⁺ is the anolyte, not the catholyte.

C is the weird one. If the cathode is Pt and the catholyte is PtI₂, then platinum must be taking part in the reaction. Logically, this is fine because magnesium is going to be heaps more reactive than platinum. However that's the thing; platinum is so unreactive to the point this probably just won't occur at all. Also, it was probably intended to be a dud option as Pt does not appear on the standard reduction potentials on the data sheet

D is wrong because K is more reactive than Mg. If the cell was constructed with potassium ions then the magnesium metal would have to be the cathode.

so any inert metal can have any type of gas but it depends on what the electrolyte is? eg. Pt metal reducing Hydrogen ions to hydrogen gas