Rod's Chemistry 3/4 Questions Thread

[Rod]

if you balance the Cr, you'll have +6 charge on the right hand side of the equation, so you would need 6e-(instead of 9e-) on the left hand side to balance the equation

Ohh I see. Thank you homer and lzxnl !

[Rod]

Hey guys just an easy question here,

For a redox reaction to occur, must all elements in the reaction change in oxidation number? Or at least one?

Thank you

[lzxnl]

Hey guys just an easy question here,

For a redox reaction to occur, must all elements in the reaction change in oxidation number? Or at least one?

Thank you

Just at least one. As an example, consider the reaction you gave with dichromate. The hydrogen ions, when forming water, still have oxidation number +1.

[Rod]

Hey guys, just needed some help on this question;

Q22.
The thermite process can be used to weld lengths of railway track together. A mould
is placed over the two ends of rails to be joined and it is filled with a ‘charge’ of
aluminium powder and iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron which joins the rails together.
c Write the overall equation for the thermite process.

So lets start with the half equations;

Fe03  +2e-  Fe2+  3O2-

What would the other half equation be? Aluminium powder was used...

Thank you

[Rod]

I was trying to do an arrow after 2e- but I'm not sure what I did. Anyway I tried to write Fe03 + 2e0 arrow Fe2+ + 3o2-

[Rod]

Another question here sorry, in this one I'm confused as fk

I won't type up the whole question, it's pretty long but it's about titrating solid iron with potassium premanganate to produce Mn2+ and Fe3+.

So after doing the whole, long question, I got everything wrong because my equation was wrong; here is the equation I did:

2Fe + Kmn04+ 8H+ ------- Fe3+ + Mn2+ + 4h20

Here is the answers:

5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(aq)

So as you can see that Mn04- stuffed me up. Where does that Mn04 come from? Why did they just suddenly take out the potassium out of it? Where did the potassium go?

Thanks guys, sorry again for all the questions and confusion

[SocialRhubarb]

Potassium is merely a spectator ion in the reaction. It doesn't react with any other species and hence can be omitted from the equation. There's no problem with putting the potassium in, but you'll just have potassium ions as one of your products, which you're missing in your equation.

You also seem to have 2 Fe on the left hand side of your equation, but only one Fe on the right hand side of your equation.

Also, the answers suggest you're not titrating solid iron, which makes sense as it is difficult to obtain accurate measurements from a titration of a solid.

[Rod]

Hey guys this isn't a chemistry question but I just need some advice.

I've been doing many chemistry and physics questions these holidays, but something new that I've been doing is that I've kept the solutions in front of me on my laptop (although not visible). It's not like I look at the solutions before doing the question, I do the question first, show my working and answer, and then IMMEDIATELY look at the solutions. I've noticed that I've stopped double checking my answer like I did before to check if I've done any silly mistakes. Will this new habit cause me any damage to my studies? Should I stop? Should I only look at it if I am completely stuck? Any advice appreciated.

Thank you

[Yacoubb]

Hey guys this isn't a chemistry question but I just need some advice.

I've been doing many chemistry and physics questions these holidays, but something new that I've been doing is that I've kept the solutions in front of me on my laptop (although not visible). It's not like I look at the solutions before doing the question, I do the question first, show my working and answer, and then IMMEDIATELY look at the solutions. I've noticed that I've stopped double checking my answer like I did before to check if I've done any silly mistakes. Will this new habit cause me any damage to my studies? Should I stop? Should I only look at it if I am completely stuck? Any advice appreciated.

Thank you

I think maybe only look at solutions if you're stuck. Finish the questions, and then check your answers, using the worked solutions if you've gotten something wrong and you don't know how to correctly solve it.

[Rod]

I think maybe only look at solutions if you're stuck. Finish the questions, and then check your answers, using the worked solutions if you've gotten something wrong and you don't know how to correctly solve it.

That sounds good. Will be doing that from now on.

Thank you Yacoubb !

[Rod]

Hey guys!

I've been going through the spectroscopic techniques (Flame tests, AES, AAS, UV, IR, NMR) and I was looking at the questions, it seems as if VCAA like combining many techniques into one question, making us read of several graphs usually for finding the structure of a compound.

I've been doing alright in that, but I am struggling to fully cement all the theory of these techniques.

So firstly, I was fine with flame test and AES, struggling in AAS, can someone please help me?

Here is what I know;

So we use AAS as it is more versatile and can be used for several elements. Unlike the first two techniques, where light is emitted, this technique focusses on the absorption of light. We know that different elements absorb different wavelengths and energies of light in order for their electrons to become excited, this is because of differing electron arrangements and different numbers of protons and neutrons. So THEREFORE, we can shine light onto an element, see how much it absorbs, and by looking at the amount it absorbs we can find how much and what the element is. Here is an anology I have created, I have friends named Mary and Matthew. They both look identical so I can't distinguish. Mary likes eating dog food, while Matthew hates it, Matthew likes chicken but Mary is vegan so would never eat chicken. So one day I through a piece of chicken on the floor, one of them pick it up and eats it, that person must be Matthew and the other person cannot be Matthew.

So the steps in AAS:
1. Element is placed in lamp
2. Element is vaporised WHY DO WE VAPORIZE IT DONT UNDERSTANT
3. A specific wavelength of light is shone onto element
4. Remaining light goes into monochromator and I have no idea what the purpose of this monochromator is
5. A detector counts the light left. And by taking this away from the original, we know how much has been absorbed and thus can find out WHAT the element is and also.. apparently how much - how the heck do they now how much??

THANK YOU!!

[Edward21]

Your very own thread eh? 

In the flame the solution that the metal ions are in is evaporated and the ions become atomised, so reduction occurs for example. Na+(aq) + e- -> Na(g) I never really understood how it becomes vaporised in the gaseous state, BUT the function is to allow the atoms to maximise their exposure to the characteristic electromagnetic radiation, think about a block of metal or little pieces and then shine a light, the little pieces will more accurately absorb what's being emitted. Have a look on chem guide uk for explanations on everything!

Mono(one)chromator(colour) it selects one wavelength, usually the strongest one possible for accuracy

You can't find out what you have (if that's what you were mentioning not sure?) because you need to know what's in the solution to put in the right cathode lamp made of the same metal!!

You construct a calibration curve (it's pretty much a linear relation, but deal with curve for now) with absorption vs. concentration. You put known standard solutions of that metal ion in and record the absorbance, from there you can plot the corresponding points on your graph (the absorbance and concentration_, then you put in your unknown sample, you get an absorbance reading and from there you can see the resulting concentration. The calibration line establishes the relationship between absorbance and concentration, they're unrelated variables until you connect them like this drawing up a calibration curve, or reading one is key for most spectroscopic techniques  You'll see heaps of these in the textbook

[Rod]

Your very own thread eh? 

In the flame the solution that the metal ions are in is evaporated and the ions become atomised, so reduction occurs for example. Na+(aq) + e- -> Na(g) I never really understood how it becomes vaporised in the gaseous state, BUT the function is to allow the atoms to maximise their exposure to the characteristic electromagnetic radiation, think about a block of metal or little pieces and then shine a light, the little pieces will more accurately absorb what's being emitted. Have a look on chem guide uk for explanations on everything!

Mono(one)chromator(colour) it selects one wavelength, usually the strongest one possible for accuracy

You can't find out what you have (if that's what you were mentioning not sure?) because you need to know what's in the solution to put in the right cathode lamp made of the same metal!!

You construct a calibration curve (it's pretty much a linear relation, but deal with curve for now) with absorption vs. concentration. You put known standard solutions of that metal ion in and record the absorbance, from there you can plot the corresponding points on your graph (the absorbance and concentration_, then you put in your unknown sample, you get an absorbance reading and from there you can see the resulting concentration. The calibration line establishes the relationship between absorbance and concentration, they're unrelated variables until you connect them like this drawing up a calibration curve, or reading one is key for most spectroscopic techniques  You'll see heaps of these in the textbook

Thank you so much Edward !!

I've understood everything you have said! Now with that all out of the way,  a little more questions have popped up unfortunately . My first question is that I see how we can use this method quantitatively, but how can we use it qualitatively? Also, what's the point of using and learning flame tests + AES when we have AAS, which is basically 100x better than both combined.

Thanks again Ed!

[Edward21]

Well I don't really think it is qualitative?? I mean to use it, you have to have the cathode lamp fitted with the same metal you want to analyse, unless you want to put a solution in there and see a green flame from Cu2+ ions haha. I've never really considered it qualitatively due to the specificity of the technique.

Well AES is a glorified flame test to start with this is just basic qualitative technique, you'd use to it try and guess/observe the flame colour so if you saw green fire for a sec then you'd go OK there's Cu2+ probably, so you fit the AAS with the copper lamp and go from there.

I guess AES is just a cheap qualitative technique then you go to AAS; the expensive, specific quantitative one.

[Rod]

Well I don't really think it is qualitative?? I mean to use it, you have to have the cathode lamp fitted with the same metal you want to analyse, unless you want to put a solution in there and see a green flame from Cu2+ ions haha. I've never really considered it qualitatively due to the specificity of the technique.

Well AES is a glorified flame test to start with this is just basic qualitative technique, you'd use to it try and guess/observe the flame colour so if you saw green fire for a sec then you'd go OK there's Cu2+ probably, so you fit the AAS with the copper lamp and go from there.

I guess AES is just a cheap qualitative technique then you go to AAS; the expensive, specific quantitative one.

Understood! Pretty stupid question haha! Thank you