Aqa chem 4/ chem 5 june 2016 thread

you know when they ask you to draw the repeating unit of a poly(alkene). does the monomer need to have brackets at the end ??

Can someone please help me with this question?

The concentration of iron(II) ions in aqueous solution can be determined by titrating the solution, after acidification, with a standard solution of potassium manganate(VII).

(i) Explain, by reference to the data given in the table above, why hydrochloric acid should not be used to acidify the solution containing iron(II) ions.

(ii) Explain, by reference to the data given in the table above, why nitric acid should not be used to acidify the solution containing iron(II) ions.

So the answers are:
i) MnO4 -/Mn2+ has a more positive Eο value than Cl2/Cl– and will oxidise Cl– or change Cl– to Cl2

Allow converse answers

(ii) NO3-/HNO3 has a more positive Eο value than Fe3+/Fe2+ and will oxidise Fe2+ or changeFe2+ to Fe3+

BUT what I'm confused on is can't the answers be the other way round too since the electrode potential for Cl2/cl- is greater than that of Fe3+/Fe2+ so could have the same answer as part ii, and similarly Mno4-/mn2+ has a great a potential than No3-/HNO3 so could have the same answer to part i?

If anyone can help me out i'd greatly appreciate it, so confused about this right now

help with this please @ P4O10 (phosphorus oxide) can also take the form P2O5 but why is it written as it P4O10?

help with this please @ P4O10 (phosphorus oxide) can also take the form P2O5 but why is it written as it P4O10?

I think it is because P4O10 is more stable - perhaps because it is a larger molecule?

What paper is this from?

I think it is because P4O10 is more stable - perhaps because it is a larger molecule?

It is stable and it also is what is generally stated as the most common general oxide of phosphorus.

reference to stability won't get you the marks for this question, it'll be ignored on MS.

Then what should we refer to then? Bonds?

Then what should we refer to then? Bonds?

It's literally by definition, or "one molecule contains 4 P atoms and 10 oxygen atoms so molecular formula is p4o10" or because scientists say it is etc.

Molecular formula instead of empirical

Oh and btw, who here, is great at teaching me Electrochemistry (Electrode Potentials & Electrochemical Series & Batteries)?

That is the only part of Unit 5 I am trying to figure out but if somebody can explain some parts more clearly, will be muchly appreciated boyos and girlos

Highest electrode potential = best oxidising agent.
Most negative electrode potential when putting into an equation, you flip it around.
Overall electrode potential = most +ve - most -ve
Most negative goes on left hand side of electrode written diagram thing. (Apart from the standard hydrogen electrode)
All solutions must be one molar with standard temperature and pressure.
One vertical Line means change of state, 2 vertical lines means different side of the electrode thing.
Salt bridge has to be able to carry ions and must be inert..
Hope that's covered some of it up?


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I think you may have been confused that the highest most positive electrode potential is going to be the best reducing agent and the most negative electrode potential will be the best oxidising agent.

E(cell) = E(Rcell)-E(Lcell)

Solutions are fine

A vertical line represents a phase change, so (e.g. from a solid to a liquid) and the double vertical lines represents a salt bridge in the conventional cell representations.

Salt bridges must be ionic (e.g. KNO3, KCL) that can have the ability to transfer electrons around in a circuit, as ionic solutions allow for charges to be carried.

All that is fine, its just when you are given questions on about why one oxidises another or one reduces another and how are we meant to write an answer, in terms of the equation with their respective potentials?

Compared to acyl chlorides why are acid anhydrides less prone to hydrolysis?

Ahh fair enough, hope the examiners have chill whilst marking this year's CHEM4 papers lol



You sure about that buddy?