OCR 21C C7 help! Past paper question

Can someone please help me on how to answer this question? How do the balanced equations show how strong/weak an acid is?

(edited 7 years ago)

Reply 1

Username1502

Perhaps a neutralisation titration reaction? So react the methanoic acid with sodium hydroxide perhaps to create salt and water and this proves it is an acid as it neutralised the alkali. Then do the titration with hydrochloric acid and see if that requires less volume of acid to neutralise the same mass/volume of alkali.

If you use less hydrochloric acid than methanoic acid to neutralise the same amount of alkali it is clear that HCl is the stronger acid!

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Reply 2

abbigm1

Oh I saw this question too, as far as I'm aware this question is from the specimen paper which is for the pre 2012 course. The spec has changed since then.

Reply 3

niamhdoesmaths

Titration seems like the best way to go about it, as someone else said
The more alkali used to neutralise the acid, the more acidic it is. So you would expect to use more alkali for the hydrochloric acid that for the methanoic.
Balanced equations don't tell you how acidic something is, but they do tell you whether or not something is an acid. I guess they just want you to use them to demonstrate your understanding really.

Reply 4

KomradeKorbyn

Original post by NiamhM1801

I'm confused, I get the first bit (showing it's an acid), but if equations don't tell you how acidic something is, how do you prove one is a strong acid and one is a weak acid?

You're supposed to pick an alkali (sodium hydroxide is the easiest one), and write an equation for it with methanoic acid (HCOOH + NaOH = HCOONa + H2O). This shows it's an acid because it's a neutralisation reaction, acid + alkali = salt + water.

You then do the same thing for hydrochloric acid - HCl + NaOH = NaCl + H2O. But from here, how do you show that one of the acids is stronger? I expected one of the equations to need more alkali (NaOH), showing that the acid must be stronger, but they both need equal amounts of alkali (1 x NaOH), so surely they're equal strength? Where have I gone wrong?

Anybody have a mark scheme?

Reply 5

username2452153

React methanoic acid and HCl with chemical with weak alkaline property, say Na2So4. Strong acids should be able to emit SO2 upon reacting with metal sulphates while HCOOH cant. "Hope you know how to detect SO2 gas emitted." This should show you HCOOH is less acidic

Reply 6

niamhdoesmaths

Original post by KomradeKorbyn

Don't get me wrong - I'm confused about it too!
Yeah exactly, I know how to prove they're both acids, however the concentration is harder.
I think what you'd have to do is just use the symbol equations to show that they're both acids, including possibly an ionic equation between H+ and OH-, and then describe how the amount of alkali needed to neutralise each acid will be different - I don't know how you'd know that without being told though. I certainly couldn't work it out, but I am hopeless at chemistry.

Yeah, how old is this paper? I'm pretty sure you wouldn't get anything like this now. It seems old-spec to me. I would also like to see the mark scheme though.

Reply 7

KomradeKorbyn

Original post by NiamhM1801

I thought I had it all figured out for C7, and then this sort of thing comes along :redface:

Still, if it's old spec then hopefully we won't get anything like this - C7 and P7 do seem to be the only ones where they want us to have to think about figuring a problem out a bit more, rather than just memorising a bunch of stuff (although there's a lot of that too).

The answer above you by @tangotangopapa2 makes some sense though, but I have no idea about the SO2 thing, maybe it's something they only taught you on the old spec?

Or maybe the answer is because HCOOH has two hydrogen ions, acid, reacting with one OH ion, alkali, whereas HCl has one hydrogen ion, acid, reacting with one OH ion, alkali, so HCOOH is weaker as it has a 2:1 ratio of acid ions to alkali ions? I don't know if this is actually how acidity works though, and it might just be completely wrong :s-smilie:

No idea haha

Reply 8

niamhdoesmaths

Original post by KomradeKorbyn

I thought I had it all figured out for C7, and then this sort of thing comes along :redface:

No idea haha

Haha, I'm screwed for C7 even without this stuff :redface:

Yeah we shouldn't do - especially if it's old spec stuff. I certainly haven't been taught anything to do with it so I'd hope we wouldn't get asked. (This is where it comes up on the paper isn't it :s-smilie:

😂).

Yeah it does a bit, although I wouldn't have known it. Yeah perhaps it was just an old spec thing (only old spec kids remember this 😂)

Yeah your answer would make sense too, if that how acids work that is. I have no idea. Surely the more hydrogen ions given off the more acidic it must be? It makes sense to me at least. Maybe not to a chemist, but to me :tongue:

Reply 9

KomradeKorbyn

Original post by NiamhM1801

Haha, I'm screwed for C7 even without this stuff :redface:

:wink:

.

Yeah it does a bit, although I wouldn't have known it. Yeah perhaps it was just an old spec thing (only old spec kids remember this :wink:

Yeah your answer would make sense too, if that how acids work that is. I have no idea. Surely the more hydrogen ions given off the more acidic it must be? It makes sense to me at least. Maybe not to a chemist, but to me :tongue:

tbh I just hope I can bluff my way through this exam enough to come out okay, and not bring my other chem exam grades down as they went fairly well, good luck tomorrow :smile:

Reply 10

niamhdoesmaths

Original post by KomradeKorbyn

tbh I just hope I can bluff my way through this exam enough to come out okay, and not bring my other chem exam grades down as they went fairly well, good luck tomorrow :smile:

Me too really, although it'll definitely be my hardest science exam! As long as I don't pull my grade down I'm ok, like you said.
Good luck to you too :smile: