OCR Chemistry A F325 Equilibria, Energetics and Elements Wed 13 June 2012

are u being lenient cause i see like in examiner reports they say if you give more than 2 points they give u 0 marks :confused:

true say pal,

sorry rob 0 marks :frown:

Reply 981

otrivine

14

Original post by chemicalX

lets turn up the heattttt :biggrin:

ok

-mg has a 2+ charge and is smaller than sodium and so has a higher charge density
-sodium has a 1+ charge and is larger than mg2+
- therefore mg2+ has a stronger attraction towards cl- and so forms stronger ionic bonds

how do endothermic reactions take place spontaneously? (2)

brilliant 3/3 and mention as well more energy needed to overcome bonds :smile:

G<0 and therefore deta H and deta S are both positive and temperature has to be at high temperatures for the reaction to be spontaneous

Reply 982

hockeyjoe

15

could anyone help me with this?

A shampoo is buffered by the addition of a mixture of methanoic acid and sodium methanoate.

The pH of this shampoo is 4.9. Calculate the hydrogen ion concentration in the shampoo, and hence the retio of methanoate ions to methanoic acid.

Reply 983

chemicalX

1

Original post by otrivine

brilliant 3/3 and mention as well more energy needed to overcome bonds :smile:

G<0 and therefore deta H and deta S are both positive and temperature has to be at high temperatures for the reaction to be spontaneous

yh ill give you the Benefit of the doubt and give you all marks,

im assuming you meant that TdeltaS > delta H
and deltaS must be +ve and the temp must high enough.
well done :smile:

your turn

Reply 984

otrivine

14

Original post by chemicalX

yh ill give you the Benefit of the doubt and give you all marks,

im assuming you meant that TdeltaS > delta H
and deltaS must be +ve and the temp must high enough.
well done :smile:

your turn

no mate i am not going to accept that mark just give me 0 and what should be the correct answer :smile:

Define: standard electrode potential and state what e.m.f stands for (4)

Reply 985

otrivine

14

Original post by hockeyjoe

could anyone help me with this?

A shampoo is buffered by the addition of a mixture of methanoic acid and sodium methanoate.

The pH of this shampoo is 4.9. Calculate the hydrogen ion concentration in the shampoo, and hence the retio of methanoate ions to methanoic acid.

is the hydorgen concentration 1.258925 x 10-4?

Reply 986

hockeyjoe

15

Original post by otrivine

can you give me formula of sodium methanoate please :wink:

HCOO-Na+

Reply 987

hockeyjoe

15

Original post by otrivine

is the hydorgen concentration 1.258925 x 10-4?

lmao the answer isn't 12.7:1 if that's what you end up at. Yer conc of H+ = 10^-4.9

Reply 988

Robpattinsonxxx

11

Original post by otrivine

no mate i am not going to accept that mark just give me 0 and what should be the correct answer :smile:

Define: standard electrode potential and state what e.m.f stands for (4)

The standard electrode potential is the electromotive force of a half cell compared with a standard hydrogen half cell measured at 298k with solution conventrations of 1.0moldm-3 and a gas pressure of 1atm

Define standard conditions...

Reply 989

chemicalX

1

Original post by otrivine

no mate i am not going to accept that mark just give me 0 and what should be the correct answer :smile:

Define: standard electrode potential and state what e.m.f stands for (4)

the marks are what my assumptions where.

ok and standard electrode potential - is the measure of electrons passing along the voltmeter when one of the half cells is a standard hydrogen half cell???

and e.m.f. is electron motive force.

ok ive got to go and do that past paper now :biggrin:

good luck with your Jan 11 paper :biggrin:

Reply 990

otrivine

14

Original post by hockeyjoe

lmao the answer isn't 12.7:1 if that's what you end up at. Yer conc of H+ = 10^-4.9

ok so now we got the conc. let me work out the ratio :wink:

and how many marks was this by the way

Reply 991

otrivine

14

Original post by chemicalX

the marks are what my assumptions where.

ok and standard electrode potential - is the measure of electrons passing along the voltmeter when one of the half cells is a standard hydrogen half cell???

and e.m.f. is electron motive force.

ok ive got to go and do that past paper now :biggrin:

good luck with your Jan 11 paper :biggrin:

yes close you said it is the enthalpy change when the emf of a standard half cell is compared with the half cell of hydrogen under standard conditions and yes its right :wink:

so you got 3/4
when will you be back tell me what you got later

Reply 992

otrivine

14

Original post by Robpattinsonxxx

The standard electrode potential is the electromotive force of a half cell compared with a standard hydrogen half cell measured at 298k with solution conventrations of 1.0moldm-3 and a gas pressure of 1atm

Define standard conditions...

yep correct
standard conditions are 298k , 100kpa and 1 mol dm-3

Reply 993

otrivine

14

Original post by hockeyjoe

lmao the answer isn't 12.7:1 if that's what you end up at. Yer conc of H+ = 10^-4.9

sorry

can you write the whole equation as in question and are we given Ka value?

Reply 994

Robpattinsonxxx

11

So are standard conditions for say lattice enthalpy the same as standard conditions for standard electrode potential?

Reply 995

niaghez

Original post by LifeIsGood

Exactly!

Do you understand the Enthalpy of Solution question in Jan11, somebody kindly explained but I still don't understand it :mad:

Jan11 Q6d

Basically, correct me if i am wrong, rubidium is larger than potassium so the nuclear attraction is reduced. This means it has less attraction to fluoride ions than potassium ions. Remember K+ has a greater attraction for h20 so more energy is needed to separate K+ and h20. Also, remember that enthalpy change of solution is affected more by lattice enthalpy than by enthalpy of hydration. :biggrin:

Reply 996

chemicalX

1

Original post by otrivine

yes close you said it is the enthalpy change when the emf of a standard half cell is compared with the half cell of hydrogen under standard conditions and yes its right :wink:

so you got 3/4
when will you be back tell me what you got later

awesome!

ok ill be back at about 5-6 ish as im going to get some food and then do the paper and mark etc so about 5-6ish :biggrin:

ttyl

Reply 997

niaghez

At 25 °C, the acid dissociation constant Ka for ethanoic acid has the value
1.75 × 10–5 mol dm–3.
2 (c) (i) Calculate the pH of the solution formed when 10.0 cm3 of 0.154 mol dm–3 potassium
hydroxide are added to 20.0 cm3 of 0.154 mol dm–3 ethanoic acid at 25 °C.

HOW DO I DO THIS :frown:

Reply 998

otrivine

14

Original post by Robpattinsonxxx

So are standard conditions for say lattice enthalpy the same as standard conditions for standard electrode potential?