AQA A Level Chemistry Paper 3 20th June 2018 Unofficial Markscheme

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Method for 6 marker
20cm3 of B 20 cm3 of C measured using a volumetric pipette. 2cm3 of X in a Conical flask on a white tile.
Add 10 cm3 of 0.05moldm-3 A to the flask and immediately start a stopwatch. Measure time taken for first colour change (light blue).
Repeat twice more then mean. Repeat with 0.10->1.00moldm-3 concentrations of A at 0.05 intervals ensuring volume is constant.
Rate=K[a]^x and therefore Log(rate)= xLog(a)+Log(k) and as rate is directly proportional to 1/t, Log(1/t)= xLog(a)+Log(k)
Plot log(1/t) against log(A concentration) Gradient is rate order Y intercept is a log(k), 10^Yintercept is = K
This value * (Intial A Conc) = Intial rate
But not actually intial rate a value proportional to it, which is okay as the question only wants a measure of it

Reply 41

HeartBlink

CsF for the one that has the biggest difference between born haber and date book?

Reply 42

Lalalozzaland

Was it me or was one of the multiple choice questions wrong. It was the ligand substitution one, it asked for which one was correct but I thought 3 of them seemed correct and one was incorrect?

Reply 43

eliza154

Original post by Jadejay

I thought it asked for the temperature at minute 4 not the temperature change
For rate I got 4.2x10^-4 but it was probably because our graphs were different and I think the calorimetry one needs a negative sign

you're right about the calorimetry one, I'll change that :smile:

I thought it asked for the change because then we had to calculate the uncertainty of the change? and there'll definitely be a range for the rate one so hopefully we're both right

Reply 44

Jadejay

Original post by london256145

guys please help i don't know why but on the last question i got like -2.6x10³and i don't know why ?? :bawling:

How did you do it? I got -21 but I don’t think it was right

Reply 45

TheSwyn

Original post by brendonurine

Same, did you say stop stop watch when blue starts to appear or when turns dark blue?

I might be completely wrong but isn’t that average rate? Not initial. I said to plot how amount of E reacted therefore how the amount of A changes over time and and then gradient at t=0. Then repeat for a different start concentration of A

The question specified it must measure initial rate

(edited 5 years ago)

Reply 46

Sweetie08

Original post by LunaCat

HNO3 acting as a base?

i did that as i was sure it wasn't any of the other ones but still dont know how its a base

Reply 47

london256145

Original post by Sweetie08

calorimetry q?

yeahhhh

Reply 48

Hdheimdnbf

Original post by Sweetie08

i did that as i was sure it wasn't any of the other ones but still dont know how its a base

Same

Reply 49

Sweetie08

Original post by Hdheimdnbf

That’s probs why then so how many marks do you think I get

probs 3 out of 5 as you lose an answer mark and mark for number of mols of water produced

Reply 50

VetTheChemist

Original post by LunaCat

CsF for the one that has the biggest difference between born haber and date book?

I put LiI

Reply 51

tan_pz

HNO3 acts as a reducing agent?

Reply 52

HeartBlink

Original post by Sweetie08

i did that as i was sure it wasn't any of the other ones but still dont know how its a base

Base accept a proton, HNO3 accept a protom to become a H2NO3+ which then dissociates to form NO2+ and H20

But then bases are also electron acceptor, so i wasn't sure if it was that one...

Reply 53

Hdheimdnbf

Original post by Sweetie08

probs 3 out of 5 as you lose an answer mark and mark for number of mols of water produced

And a mark for wrong equation so 2/5 rip 😭

Reply 54

HeartBlink

Original post by Vetahanj

I put LiI

Isn't it biggest metal and smallest non-metal that have the largest covalent character?

Reply 55

HeartBlink

Ion that won't catalyse reaction if I- etc, Fe3+?

Reply 56

Jadejay

Original post by eliza154

you're right about the calorimetry one, I'll change that :smile:

I thought it asked for the change because then we had to calculate the uncertainty of the change? and there'll definitely be a range for the rate one so hopefully we're both right

Yeah I worked out the change for the other part but for that question I’m sure it mentioned the 4th minute and I thought it meant the temperature at that time.. did I misunderstand?

Reply 57

VetTheChemist

Original post by Sweetie08

i did that as i was sure it wasn't any of the other ones but still dont know how its a base

Hno3 is basically NO3+ and it goes to NO2+, therefore, it has been reduced by a value of 2. Reduction is the gain of an electron and therefore if it gains 2 electrons it has basically accepted a lone pair which is a Lewis base