ocr a f325 revision thread

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Reply 2100

otrivine

can someone tell me why in hydrogen fuel cell, the equations differ when in alkali ?

Reply 2101

DudeBoy

Original post by otrivine

correct

No effect on Kc as Kc only changes by/ is affected by temperature.

1) Balance the following equation using oxidation numbers

Pt + HNO3 + HCl -----> H2ptCl6 + NO2 + H2O

Not sure how I'd do this, run through it for me :colondollar:

Reply 2102

chignesh10

Original post by DudeBoy

Using quadratic equation to find pH

Ka=[H+][A-]/[AH]
Ka=[H+]^2/[AH-H+]
Ka[AH-H+]=[H+]^2
[H+]^2-Ka[-H+]-Ka[AH]
Use quadratic :smile:

I doubt it will come up but you never know

It won't come up but if you do use it then it is OK

Reply 2103

überambitious_ox

50cm cubed of 0.25 mol per dm cubed butanoic acid with 50cm cubed of 0.05 mol per dm cubed sodium hydroxide
A buffer solution forms

Calculate the pH of the buffer solution.

The Ka of butanoic acid is 0.0000151

Please could someome explain the method for this to me. Just had a little freak out when I got the beginning part of this question wrong.

Thanks a lot

Reply 2104

otrivine

Original post by DudeBoy

Not sure how I'd do this, run through it for me :colondollar:

first work out which is oxidised and which is reduced

Reply 2105

otrivine

http://www.ocr.org.uk/Images/65869-question-paper-unit-f325-equilibria-energetics-and-elements.pdf

question 8, the calculation, how did they know the relationship between S2o3- and Cu is 1:1?

Reply 2106

BrightStar57

Original post by otrivine

http://www.ocr.org.uk/Images/65869-question-paper-unit-f325-equilibria-energetics-and-elements.pdf

question 8, the calculation, how did they know the relationship between S2o3- and Cu is 1:1?

check this link it explains the question quite well
http://www.youtube.com/watch?v=OAkFX-7n8iU&list=PLSOeDyfBcTw034_XLjzHdP3yQ8cbIo4De

Reply 2107

otrivine

Original post by BrightStar57

check this link it explains the question quite well
http://www.youtube.com/watch?v=OAkFX-7n8iU&list=PLSOeDyfBcTw034_XLjzHdP3yQ8cbIo4De

Thank you,

why is that the rate constant does not increase with temp?

Reply 2108

meg0001

Is it worth it doing the legacy papers?

Reply 2109

Pride

Original post by überambitious_ox

I think I have an answer of 4.22, but I'm not sure if that's right, have you got a markscheme to check? I was thinking then I can go through the method?

First off though, you definitely work out number of moles of the acid and the alkali, and from that, work out the number of moles of butanoate, and take that away from the number of moles of acid, to find how much is left over.

edit: I can't see any problems, so I'll just put it down :smile:

number of moles of acid = 0.0125 - 2.5x10^-3 = 0.01mol
no. of moles of butanoate = 2.5x10^-3 mol

then divide both by 0.1dm^3, that's the combined amount of solution
put them into the Ka expression, find [H+], to get 6.04x10^-5 moldm^3
then -log[H+]

(edited 10 years ago)

Anyone wanna help?

does anyone know if we would ever be required to do Ka and all that with a dibasic acid or will it definitely always be mono basic?

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Reply 2112

Pride

Original post by DudeBoy

Anyone wanna help?

ok, personally I'd write down the ionic equation, I got
HCO3-(aq) + H+(aq) --> CO2(g) + H2O(l)
Then I'd explain that adding water allows dissolves the sodium hydrogencarbonate (it says it's 0.5g, so it's solid), allowing the movement of ions in solution, so that the reaction between the H+ in citric acid and the hydrogen carbonate ion can take place.

any thoughts?

Reply 2113

DudeBoy

Original post by Pride

This the mark scheme, quite a hard question

Reply 2114

amin666

Original post by Pride

just remember, Kc=[H+]^2/[HA]
4.22 is correct

Reply 2115

Poffertje

Quick question - you know when you have cis/trans with 4 coordinate complex, are you allowed to put your 2 same groups (eg Cl-) on the bottom or on the actual sides ie vertically? Are you allowed either way as long as you do cis and trans?

Thanks :smile:

Reply 2116

DudeBoy

Just a heads up, I think catalysts are gonna come up.
Haber process, Fe
Hydrogenation of alkenes, Ni
Contact process, V2O5
Decomposition of H2O2, MnO2

Reply 2117

DudeBoy

Original post by Myda

I think that would be fine IMO, but just check a marksheme and see what it says :smile:

Reply 2118

Poffertje

Original post by DudeBoy

I think that would be fine IMO, but just check a marksheme and see what it says :smile:

Alright thanks. I couldn't find my papers to check for a question but I've got them now

Reply 2119

Better

Original post by PythianLegume

What do you want to know about them?

Basically, E=mcT is based around how much energy you need to heat a substance. The energy needed to raise by 1K is the same at all temperatures, and is equal to the constant c. Given that most reactions at A-Level occur in heavily diluted solutions, we tend to only focus on the energy needed to heat the water.

Initial rates can be calculated in a number of ways. Either a concentration/time graph can be drawn, and a tangent drawn at t=0. The gradient of the line is the rate, because gradient = conc/time = rate. In 'clock reactions', the initial concentration is approximated by using 1/t, where t is the time for the reaction to complete. This isn't exact, it's just an approximation.

Not sure about the Difference between a CLock Reaction and a Conc time graph? Specifically in the context of the 4 marker on January 2013.

For E=mcT

When you work out H=E/n

I don't know what to choose as the number of moles i.e.

NaOH + HCl ---> H2O + NaCL

What is the number of moles I choose???