Chemistry 3/4 2013 Thread

[xenial]

Okay thankyou!

So basically, it's like methods in that you shouldn't waste time making notes on the subject, you should just start doing prac exams and that'll cover any holes you have in your knowledge?

[Edward21]

With AAS, does it use both the Visible AND UV regions of the EM spectrum??

[Alwin]

With AAS, does it use both the Visible AND UV regions of the EM spectrum??

Remember that AAS uses a cathode lamp, with the metal specifically chosen So, if the metal absorbs strongly in the UV region of the EM spec then the lamp will also emit this specific wavelength.
However, the (monochrome) detector is usually only set to one wavelength that the cathode lamp emits strongly. So in theory, yes, AAS can use UV regions of the EM spec, but most metals (not all) emit a wavelength in the visible region, so the detector is set to detect this.

Hope it helps ^^

[Edward21]

Remember that AAS uses a cathode lamp, with the metal specifically chosen So, if the metal absorbs strongly in the UV region of the EM spec then the lamp will also emit this specific wavelength.
However, the (monochrome) detector is usually only set to one wavelength that the cathode lamp emits strongly. So in theory, yes, AAS can use UV regions of the EM spec, but most metals (not all) emit a wavelength in the visible region, so the detector is set to detect this.

Hope it helps ^^

Thanks, damn TSFX practice paper -_-

[teletubbies_95]

Hey guys !
I was doing vcaa 2012 , I was having trouble on determining what to include in my answer for question 3 (c) in the short answer?

[xenial]

Thanks!

Also how much do we need to know about batteries? Would we be able to need to know the differences between them - what different ones are used for? Be able to draw them etc.?

[Homer]

The following equations form part of the electrochemical series. They are ranked in the order shown.
Ag+(aq) + e– (Forward and back arrow) Ag(s)
Pb2+(aq) + 2e– (Forward and back arrow) Pb(s)
Fe2+(aq) + 2e– (Forward and back arrow) Fe(s)
Zn2+(aq) + 2e– (Forward and back arrow) Zn(s)
Mg2+(aq) + 2e– (Forward and back arrow) Mg(s)

Lead rods are placed in solutions of silver nitrate, iron(lI) sulfate and magnesium chloride. In which solutions would you expect to see a coating of another metal form on the lead rod? Explain

Coating? Isn't that similar to electrolysis? So for Lead, shouldn't something below it be used? Fe2+? since is is the stongest oxidant below solid lead?

ANS: Ag+ ions :s

[thushan]

Not on this occasion. This is a simple spontaneous redox reaction between a metal and a metal ion - there is no external current added here.

Ag+ is the only oxidant with the potential to oxidise the Pb (s) in the lead. In doing so, the Ag+ is reduced and you get an Ag deposit.

[Homer]

Not on this occasion. This is a simple spontaneous redox reaction between a metal and a metal ion - there is no external current added here.

Ag+ is the only oxidant with the potential to oxidise the Pb (s) in the lead. In doing so, the Ag+ is reduced and you get an Ag deposit.

but in the cell formed from Ag+/Ag and Pb/Pb2+ half cells, wouldn't the electron flow from Pb electrode to the Ag+/Ag half cells? So shouldn't the Pb electrode disintegrate over time instead of being coated by another metal?

Another question:
"lead would be able to reduce which ions from the electrochemical series.." What do they mean when they say lead would be able to reduce?"

[thushan]

but in the cell formed from Ag+/Ag and Pb/Pb2+ half cells, wouldn't the electron flow from Pb electrode to the Ag+/Ag half cells? So shouldn't the Pb electrode disintegrate over time instead of being coated by another metal?

Not exactly the half cell, but that's essentially what happens. The surface Pb is replaced with Ag. So it takes on the appearance of a coat.

[Jeggz]

For equilibrium, do we need to know what the concentration fraction or Qc is?

[lzxnl]

For equilibrium, do we need to know what the concentration fraction or Qc is?

What do you mean? You need to know how to work it out, that's for sure.

[Jeggz]

What's the difference between Qc and Kc though? I'm kinda confused :\

[thushan]

What's the difference between Qc and Kc though? I'm kinda confused :\

Qc is the concentration fraction at any point in time, Kc is the value that the concentration fraction would be if and only if the system were in equilibrium.

[Alwin]

You also can use Qc (sometimes also called CF, concentration fraction which is pretty dumb since my teacher uses CF to mean calibration factor too) to determine if the system is at equilibrium or not + which way it will shift. Also, some ppl write Keq for some reason unbeknownst to me. anways:

eg, Qc = Kc its at equilibrium. Qc > Kc, POE will move to the left and if Qc < Kc then POE moves to the right