HSC Chemistry Question Thread

[HighTide]

I would like some clarification on this as well! Someone told me that Na+ is extremely weak because it's the conjugate acid of NaOH, a strong base, is this right?

Sup,
Not sure if this has been answered before, but thinking about it like this should help:
I'll just take a strong acid HA as the example. So HA + water --> A- and hydronium. For HA to be a strong acid, it must ionize to completion in water. Hence if you set up an acidity constant Ka will be very high.

Now consider, the reverse of the equation, so: A- + hydronium --> HA + water. In this case, if A- was a strong conjugate base, then, it would accept the hydrogen and form HA, and hence its basicity constant would be high. So the problem with this is that you can't have a high Ka for one reaction, and a high Kb for its backwards reaction, because then you're not having complete ionization. Hence it should be: strong acid --> weak conjugate base, strong base--> weak conjugate acid, weak base --> strong conjugate acid, weak acid --> strong conjugate base.

So in the case of NaOH, which is a very strong base, we will have a very weak conjugate acid Na+. It's basically an inverse relationship. The higher the acidity, the lower the basicity. eg. HCl is a strong acid, Cl- is a weak conjugate base.

Hope this helps

[ch3ntastic]

For the "Standard Potentials" sheet supplied in the exams. Potassium is at the top and fluorine is at the bottom. Which one is the stronger oxidant/reductant? I get really confused between the whole OIL-RIG and its agents. Thanks

[armtistic]

Hey Guys.

I've noticed when it comes to the equations for the batteries i.e. dry cell or silver-button cell in POM the equations vary widely between sources.

I was wondering someone could point out which exactly would be most desirable to use in the HSC and also how do you even remember such complex equations?

Also how important are the solubility rules?

[kiwiberry]

For the "Standard Potentials" sheet supplied in the exams. Potassium is at the top and fluorine is at the bottom. Which one is the stronger oxidant/reductant? I get really confused between the whole OIL-RIG and its agents. Thanks

As you go up the standard potentials sheet, ease of oxidation increases (as does reactivity), so K_(s) is the strongest reductant. This means that K⁺ is the weakest oxidant. All the species on the right are reducing agents, and all the ones on the left are oxidising agents

[jakesilove]

Hey Guys.

I've noticed when it comes to the equations for the batteries i.e. dry cell or silver-button cell in POM the equations vary widely between sources.

I was wondering someone could point out which exactly would be most desirable to use in the HSC and also how do you even remember such complex equations?

Also how important are the solubility rules?

Hey! It's important to have SOME kind of equation, but as long as it balances, it won't really matter what it is. As you've said, there are plenty of variations on the internet. Pick the easiest one, and learn it off by heart!

Solubility rules are vital. You need to know them, as you could have up to 8 marks in an HSC exam relying on these principles. They're tough to remember, but with enough practice, you'll do it!

[J.B]

Hi,
I was just wondering why is sodium ethanoate written as CH3COONa, if the Na is the cation and the ethanoate the anion? As I would have automaticallly swapped them around?
Thanks.

[jakesilove]

Hi,
I was just wondering why is sodium ethanoate written as CH3COONa, if the Na is the cation and the ethanoate the anion? As I would have automaticallly swapped them around?
Thanks.

Hey! It's really only written like that to indicate how the bonding has occurred. The Sodium atom is bonded to one of the Oxygen atoms (the one that isn't double bonded), as suggested by the empirical formula. Make sense?

[J.B]

Hey! It's really only written like that to indicate how the bonding has occurred. The Sodium atom is bonded to one of the Oxygen atoms (the one that isn't double bonded), as suggested by the empirical formula. Make sense?

Ok thankyou, so in an exam is it best to write it like this?

[jakesilove]

Ok thankyou, so in an exam is it best to write it like this?

I reckon so! It isn't a big deal, but I think that form is just easier to work with Also, it makes it easier for you to remember the ionisation (ie. the Na just popping off).

[Kle123]

200mL of a same concentrated sulfuric acid (18M) was dropped by accident.
Possible neutralising substances available: 300mL of 2M NaOH(aq) solution, 200g packet of solid sodium hydrogen carbonate (NaHCO3) or 100 g of NaOH pellets
Question: which substance would be most appropriate clean up neutralising agent and why?

[Newbalance101]

How do you add half equations together? I'm really bad at it :/

[kiwiberry]

200mL of a same concentrated sulfuric acid (18M) was dropped by accident.
Possible neutralising substances available: 300mL of 2M NaOH(aq) solution, 200g packet of solid sodium hydrogen carbonate (NaHCO3) or 100 g of NaOH pellets
Question: which substance would be most appropriate clean up neutralising agent and why?

NaOH isn't a good neutralising agent in general because it's a strong base, so if excess is used by accident, the spill would become dangerous again. This leaves NaHCO3, which is good neutralising agent because it is weak, bubbles during neutralisation so you're able to see when the reaction has finished, and because it's solid, it helps prevent the spill from spreading. Not relevant here, but NaHCO3 is also amphiprotic which is beneficial if the spilt substance is unknown because it can neutralise both acids and bases

[kiwiberry]

How do you add half equations together? I'm really bad at it :/

Balance the half equations first so there's the same number of electrons in both. After that, all you have to do is add the two equations together (combine everything on the reactants side and products side) and cancel the electrons on both sides

[beau77bro]

Hey how does one write net ionic equations? I'm confused with the whole different sets of equations used, so there's half, complete, nuclear and net, are there any others I need to know?
thankyou

[jakesilove]

Hey how does one write net ionic equations? I'm confused with the whole different sets of equations used, so there's half, complete, nuclear and net, are there any others I need to know?
thankyou

Hey! Yep, those are the equations you'll need to be able to figure out. Net ionic equations essentially leave out spectator ions; other than that, it's just the complete equation. Half equations break the reaction up into reduction and oxidation halves (so one side should be adding electrons, one side should be losing electrons). Not sure whether you're looking for clarification or actual help, so if there's a specific equation that you're struggling with post it up and we can help you out!