ocr a f325 revision thread

Does anyone know where the comment is that the person put on with all the legacy past papers? I can't find it anywhere.

Reply 1984

Original post by ofudge

Why does an increase in rate not lead to an increase in the rate constant?

The rate constant increases with temperature, because of particles exceeding activation energy and hence, more frequency of collision increases

Reply 1985

Original post by ofudge

Why does an increase in rate not lead to an increase in the rate constant?

because it's a constant. K literally means the constant, so remains constant whatever you do to the reactants as you cannot change it. It's the combination of the value of K and the molecules in the rate equation and to what power they're raised that determines the rate. The only thing that changes it is increasing temperature.

(edited 10 years ago)

Reply 1986

Gulzar

2

Original post by ofudge

Why does an increase in rate not lead to an increase in the rate constant?

It does increase k but only if temp is increased. Otherwise k remains constant.

Reply 1987

ofudge

Original post by bmpink

because it's a constant. K literally means the constant, so remains constant whatever you do to the reactants as you cannot change it. It's the combination of the value of K and the molecules in the rate equation and to what power they're raised that determines the rate.

Thank you very much. So even with increased/decreased temperature it remains the same?

Are there any other "constants" in this exam that we need to know stay the same?

Thanks a lot for your help.

Reply 1988

does anyone want to revise asking each other some hard questions as pracTISE?

Reply 1989

Original post by ofudge

Thank you very much. So even with increased/decreased temperature it remains the same?

Are there any other "constants" in this exam that we need to know stay the same?

Thanks a lot for your help.

That's ok! The rate constant only depends on the temperature. That's the only thing that can change it, because of increasing number of collisions per second. YES! There's Kc, equilibrium constant, which only changes with temperature and not pressure or anything else, Ka the acid dissociation constant, and Kstab the stability constant. I'm pretty sure that's all of them. Oh and Kw the ionic product of water, which increases with increasing temperature(I think! double check, there is a question on Kw in Jan 2011 I think)

Reply 1990

During the analysis of brass, 1.65 g of the alloy was reacted with concentrated nitric acid. The resulting solution was neutralised, transferred to a volumetric flask and made up to 250 cm3 using distilled water.
An excess of aqueous potassium iodide was added to a 25.0 cm3 portion of the solution from the volumetric flask and the liberated iodine was titrated with 0.100 mol dm–3 sodium thiosulphate. 20.0 cm3 of aqueous sodium thiosulphate were required to remove the iodine.

The reactions taking place in this titration may be summarised as follows.
2Cu2+ + 4I–  2CuI + I2
I2 + 2S2O32–  2I– + S4O62–
(i) Calculate the amount, in moles, of sodium thiosulphate in 20.0 cm3 of solution.

for this I get 0.0025 moles and mark scheme gets 0.002 ?

Reply 1991

_HabibaH_

8

Original post by otrivine

During the analysis of brass, 1.65 g of the alloy was reacted with concentrated nitric acid. The resulting solution was neutralised, transferred to a volumetric flask and made up to 250 cm3 using distilled water.
An excess of aqueous potassium iodide was added to a 25.0 cm3 portion of the solution from the volumetric flask and the liberated iodine was titrated with 0.100 mol dm–3 sodium thiosulphate. 20.0 cm3 of aqueous sodium thiosulphate were required to remove the iodine.

The reactions taking place in this titration may be summarised as follows.
2Cu2+ + 4I–  2CuI + I2
I2 + 2S2O32–  2I– + S4O62–
(i) Calculate the amount, in moles, of sodium thiosulphate in 20.0 cm3 of solution.

for this I get 0.0025 moles and mark scheme gets 0.002 ?

The MS is right as (20 x 0.10)/1000 = 0.002

You used 25cm3 instead of 20cm3

Reply 1992

ofudge

Original post by bmpink

That's ok! The rate constant only depends on the temperature. That's the only thing that can change it, because of increasing number of collisions per second. YES! There's Kc, equilibrium constant, which only changes with temperature and not pressure or anything else, Ka the acid dissociation constant, and Kstab the stability constant. I'm pretty sure that's all of them. Oh and Kw the ionic product of water, which increases with increasing temperature(I think! double check, there is a question on Kw in Jan 2011 I think)

Ah that's great thanks :smile:

I knew what they were but didnt realise they were constants! oops :L
Need 115/150 in this exam to get my A... pressures on! good luck to you.

Reply 1993

Original post by _HabibaH_

The MS is right as (20 x 0.10)/1000 = 0.002

You used 25cm3 instead of 20cm3

opps sorry made , what about these two

For every one mole of Cu2+ ions present in solution, deduce the amount, in moles, of S2O32– ions needed for the titration.

answer ......................... mol
[1]
(iii) What is the amount, in moles, of Cu2+ ions present in 25.00 cm3 of solution?

Reply 1994

Original post by otrivine

During the analysis of brass, 1.65 g of the alloy was reacted with concentrated nitric acid. The resulting solution was neutralised, transferred to a volumetric flask and made up to 250 cm3 using distilled water.
An excess of aqueous potassium iodide was added to a 25.0 cm3 portion of the solution from the volumetric flask and the liberated iodine was titrated with 0.100 mol dm–3 sodium thiosulphate. 20.0 cm3 of aqueous sodium thiosulphate were required to remove the iodine.

The reactions taking place in this titration may be summarised as follows.
2Cu2+ + 4I–  2CuI + I2
I2 + 2S2O32–  2I– + S4O62–
(i) Calculate the amount, in moles, of sodium thiosulphate in 20.0 cm3 of solution.

for this I get 0.0025 moles and mark scheme gets 0.002 ?

where did you get this paper?

Reply 1995

the colour between

Fe2+ and Mno4 -- > Mn2+ + Fe3+ e.t.c

does the colour change from purple, to almost pink(colourless)

Reply 1996

_HabibaH_

8

Original post by otrivine

opps sorry made , what about these two

For every one mole of Cu2+ ions present in solution, deduce the amount, in moles, of S2O32– ions needed for the titration.

answer ......................... mol
[1]
(iii) What is the amount, in moles, of Cu2+ ions present in 25.00 cm3 of solution?

0.002 moles?

Reply 1997

Original post by _HabibaH_

0.002 moles?

they put 1 ?

Reply 1998