Year 12s: what will be the first way you research your future options? >>

OCR A2 CHEMISTRY F324 and F325- 14th and 22nd June 2016- OFFICIAL THREAD

Scroll to see replies

Reply 3040

jerseyalevel

Original post by M0n1

In electrode potential
does reduction happen on the positive or negative terminal?

Oxidation produces electrons (which are negative) so oxidation occurs on the negative / reduction on the positive

Reply 3041

ranz

Original post by Rust Cohle

https://i.gyazo.com/49069f767bf5ea95d7e870c6a979a5e2.png
There is question. Since we know that the Chromium gains mass its going from aqeous ---> solid. So write that half equation:

Cr -----> Cr3+ + 3e-

The X electrode loses mass so it goes from solid ----> aqeous. In order to to help you form the half equation, we're told that the aqeous is XSO4 meaning oxidation state of X = +2.

X2+ + 2e- -----> X

-----------------------------------

Now simply create a full equation with the two half equations:

2Cr3+ + 3X ----> 3X2+ + 2Cr

-------------------------------

Moles of Cr we can find since we know its mass (1.456g) and the mr 52. n = m/Mr = 0.028

Apply this to the full equation and you'll find that the moles of X = 0.042 using stoichiometry (molar ratios)

We can know work Mr of X since we have the mass of 1.021g. Mr = m/n = 24.3 = Magnesium

just a q how come u divided mole by cr only and nit by cr2so4

Posted from TSR Mobile

Reply 3042

username2640523

Original post by ranz

just a q how come u divided mole by cr only and nit by cr2so4

Posted from TSR Mobile

Cr is formed as a solid thats why

Posted from TSR Mobile

Reply 3043

ranz

Original post by tcameron

I still don't really understand the June 2015 42iii question on the adding of the Mg to the buffer :/

i remember that q, so hard.
baically its apparently prettty simple my friends helped me.
just calc how many moles of mg added(0.25), minus 1mol of the acid by that, thats whats left kf the acid. 0.25+1 mole of conjucate base is produced as orevious page wrote that conjucate base had 1moldm3 so that + mole of mg 0.25 then sub values into kc eq and find H+. its confusing hope this helped

Posted from TSR Mobile

Reply 3044

mechanism

Original post by Dinasaurus

surely by losing a Hydrogen it has changed oxidation state.

I don't think so

Reply 3045

FluffyCommie

Original post by Dinasaurus

surely by losing a Hydrogen it has changed oxidation state.

It's becuase, in the prescence of Ca(OH)2 the Pyruvic acid is completely dissociated. Therefore it can be considered to be completely dissociated BEFORE the reaction takes place and therefore is a spectator ion for the reaction:

2C3H3O3- + 2H+ + Ca2+ +2OH- --> Ca2+ +2C3H3O3- + 2H20

The ions cancel on each side to give:

2H+ +2OH- --> 2H2O

Reply 3046

LewisE-Y

Original post by ScarletRose

Anyone got the mark scheme to the 2015 paper?

http://www.chemhume.co.uk/A2CHEM/ExamsMSA2/Mark%20schemes%20new%20A2/F325_MS_June15.pdf

Reply 3047

mechanism

Can anyone please explain Jan 2013 Q8)C)ii ? I've worked it out before but I really don't remember how it's done.

Reply 3048

Dinasaurus

What ratio of acid to acidoate should we use in a buffer?

Am I supposed to always say 2:1?

Reply 3049

smartalan73

Hey can anyone quickly explain why the concentration of HCOOH is 0.24 and not 0.64 (3.2x0.2) in this buffer question please?

Reply 3050

victoria98

Original post by smartalan73

Hey can anyone quickly explain why the concentration of HCOOH is 0.24 and not 0.64 (3.2x0.2) in this buffer question please?

Acid is in excess. So it'll react with all of the NaOH and you'll have some left over. You've got 0.64 moles of HCOOH and 0.4 moles of NaOH so 0.64-0.4=0.24 moles HCOOH left.

(edited 7 years ago)

Reply 3051

lai812matthew

Original post by smartalan73

Hey can anyone quickly explain why the concentration of HCOOH is 0.24 and not 0.64 (3.2x0.2) in this buffer question please?

0.64-0.4 quick

Reply 3052

EllieO

Original post by thad33

Does anyone have a nice way of explaining cathodes and anodes? I keep mixing them up. I know the direction that the electrons flow in but everywhere I look to help explain the terminals, they contradict each other.

CATHODES are positive terminals that attract ANIONS and ANODES are negative terminals that attract CATIONS. They only was I can remember it is by thinking that cats make me feel positive! (I don't know if this is what you were looking for?)

In cell potentials it's the half cell with most negative electrode potential that loses its electrons, therefore it acts as the ANODE/negative terminal!

Reply 3053

ranz

Original post by EllieO

so oxidation happens at negative terminal?

Posted from TSR Mobile

Reply 3054

lukemurp890

Original post by Dinasaurus

For writing ionic equations of say Ca(OH)2 + 2Pyruvic Acid ----> (Pyruvate)2Ca2+ + 2H2O

The ionic equation is apparently just H+ and OH- ----> H2O

Can someone explain how, I don't get why the Pyruvic Acid to Pyruvate isn't also in the ionic equation if it lost a Hydrogen?

On the left hand side 2 Pyruvic acid breaks down into 2 Pyruvate and 2 H+.

On the right, (Pyruvate)2Ca2 breaks down to 2 Pyruvate and Ca2 2+.

The 2 Pyruvates on the left and right cancel each other out. So they are not present in the ionic equation.

Reply 3055

AqsaMx

CO + 2H2 > CH3OH

How could CH3OH be removed? Is it by decreasing pressure shifting eq to the left?

Reply 3056

Dinasaurus

Guys how do I calculate the pH of a buffer solution of a week acid, is it not Ka=[H]^2 / (HA)

So you do -log of the sqrt[HA]Ka

Reply 3057

AqsaMx

Could anyone explain how you would describe an experiment for enthaky change of solution?

Reply 3058

HJS14

When we get those crazy equations to balance (usually near the end) how do you know whether to use H+ ions, OH-, H20, or electrons? EG last Q on 2014 paper