The question tells you hydrogen peroxide is converted into O2 so that gives you the half equation:

H2O2 --> O2 +2H+ + 2e-
(remember that the oxidation state of oxygen in H2O2 is only -1)
Oxygen's oxidation state changes from -1 to 0 and there are two so two electrons are given off.

You are supposed to be able to remember the half reaction for the reduction of MnO4- (and Cr2O7(2-) in other questions), which is:

MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O
Manganese's oxidation state changes from +7 to +2 so five electrons are added.

Make the number of electrons on both reactions the same so the oxidation of H2O2 is multiplied by 5 and the reduction of MnO4- is multiplied by 2 (making 10 electrons each).

5H2O2 --> 5O2 +10H+ + 10e-
2MnO4- + 16H+ + 10e- --> 2Mn2+ + 8H2O

Cancel out things that appear on both sides of the equation (in this case H+ and e-) and merge.

5H202 + 2MnO4- + 6H+ ---> 5O2 + 2Mn2+ + 8H2O

Thanks for that :smile:

Reply 725

InItToWinItGetIt?

Original post by BethBeth

Both CGP and Nelson Thornes says pale pink, just consulted them :smile:

Best to stick with that methinks :cool:

No idea why someone negged you for this? :confused:

Ah ok, if another book says it as well then it should be alright. Thanks :smile:

lol meh, rep is very low down my priority list atm :tongue:

Reply 726

sam_xo

Original post by strawberry_cake

Were there no chem5 papers for jan06? I can't find any :s-smilie:

Oh I just realised you said Jan 06. Lol sorry, not got them!

Reply 727

Wish I Could Change This

Can someone please write out the equation of Co2+ with HCl?

Reply 728

NRican

What colour is [Cr(NH3)6)3+ I am pretty sure the NT book has it wrong?

Reply 729

NRican

Original post by Wish I Could Change This

Can someone please write out the equation of Co2+ with HCl?

[Co(H20)6)2+ +4Cl- = [CoCl4]2- + 6H20

Reply 730

Raimu

Original post by Wish I Could Change This

Can someone please write out the equation of Co2+ with HCl?

Co(H2O)6 + 4Cl- ----> Co(Cl4)2- + 6H2O

Reply 731

cheeseisnice

Original post by NRican

What colour is [Cr(NH3)6)3+ I am pretty sure the NT book has it wrong?

I thought it was purple. What does the NT book say?

Reply 732

ChloeElizabeth

Don't know if anyone's asked this already (toooo many pages) but in the specification it says - "be able to explain why values from mean bond enthalpy calculations differ from those determined from enthalpy cycles".

But don't enthalpy cycles use mean bond enthalpies? Or do they mean Born-Haber cycles which use enthalpies of electron affinity and things like that rather than bonds broken vs bonds made??

Reply 733

xX.Sabeel.Xx

Original post by starburst92

anyone?

yeah we do:

Reduction: O2(g) + 2H2O(l)+ 4e- > 4OH-(aq)

Oxidation: 2H2(g) + 4OH-(aq) > 4H2O(l) + 4e-

Overall: 2H2(g) + O2(g) > 2H2O(l)

Reply 734

BethBeth

Original post by teddyWS

would it also be the same as:
1)enthalpy of sublimation of Na
2)bond dissociation of Cl

I don't like enthalpy of sublimation - never heard of it referred to as that, stick with enthalpy of atomisation and you will definitely get the marks..

I suppose it is bond dissociation in the case of 1/2Cl2(g) ---> Cl(g)
But if you got this in a Born-Haber Cycle of MgCl2 for instance you would have to times enthalpy of atomisation by two.

Enthalpy of atomisation = Enthalpy change attending the formation of one mole of gaseous atoms from the element in it's standard state.
Best be careful and stick to one definition. :smile:

Reply 735

BethBeth

Original post by InItToWinItGetIt?

Ah ok, if another book says it as well then it should be alright. Thanks :smile:

lol meh, rep is very low down my priority list atm :tongue:

True, true! :biggrin:

I don't get it sometimes though :tongue:

Might lose sleep on it after the exam :wink:

Reply 736

laura123

What synoptic stuff do we need to know from the other units?

Reply 737

jamest92

Original post by NRican

What colour is [Cr(NH3)6)3+ I am pretty sure the NT book has it wrong?

Its purple i think

(edited 12 years ago)

Reply 738

adam1232

hey could anyone help me with redox equations im just not sure when you include only one reagent and product for example in the specimen paper,

3 (a)An aqueous solution of sulfur dioxide was reacted in separate experiments as follows.

Reaction 1 with HgO
H2O + SO2 + HgO ? H2SO4 + Hg

Reaction 2 with chlorine
2H2O + SO2 + Cl2 ? H2SO4 + 2HCl

(ii) Show, by writing a half-equation, that this oxidising agent in reaction 1 is an
electron acceptor.
................................ ................................ ................................ ................................ ..
(iii) Write a half-equation for the oxidation process occurring in reaction 2.
................................ ................................ ................................ ................................ ..
(iv) Write a half-equation for the reduction process occurring in reaction 2.
................................ ................................ ................................ ................................ ..

the answer for ii is Hg 2+ + 2e- -------> Hg

the answer for iii is 2H2O + SO2 ? H2SO4 + 2e-

why dosnt the oxygen on the Hg come into the half equation?

Thanks
pleaseeeeeeeeeee help meeeeeeeeeeeeeee

Reply 739

HMcVicar

Has anyone bothered going over unit two stuff like halides and extraction of metals ect?