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AQA - Unit 5 - Energetics, Redox and Inorganic Chemistry

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Hey how did you work out the equation for this question?

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

(edited 12 years ago)

Reply 701

BethBeth

Original post by teddyWS

could someone please help me label the following steps in the BH cycle:
1)Na(s) -->Na(g)
2) 1/2 Cl2 (g)--> Cl (g)

1) Enthalpy of atomisation of Na
2) Enthalpy of atomisation of Cl

Reply 702

Crowned Temmy

Do we have to know the colours of ions such as vanadium ions, manganese ions? Because I wonder why people have a list of 50 colours.

Reply 703

hey_its_nay

does anyone have the jan 11 paper and markscheme?

Reply 704

xX.Sabeel.Xx

Original post by SashaLuLu

ionic bonds are always stronger than covalent bonds. this is due to the strong electrostatic forces between oppositely charged ions. It is easier to break the covalent bonds. It will only be incredulity strong if it is macromolecular. Silicon dioxide is macromolecular.

not always, Na2O is giant ionic, ionic bonds need to be broken to melt it, but it has lower melting point than SiO2 which is a macromolecular, which needs many covalent bonds to be broken.

I thought covalent bonds were stronger than ionic in general?

Reply 705

strawberry_cake

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

Reply 706

BethBeth

Original post by Raimu

Original post by jimmy303

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

In case you miss this Raimu :smile:

Reply 707

Wish I Could Change This

Original post by Luke0011

Page 80, look at the example. :smile:

the acylation page? it says nothing about cobalt :s-smilie:

Reply 708

xX.Sabeel.Xx

Original post by Crowned Temmy

Do we have to know the colours of ions such as vanadium ions, manganese ions? Because I wonder why people have a list of 50 colours.

colours for vanadium has dropped off the syllabus but we can be given questions on its reactions

theres only 2 we need to knowfor manganese... that is:
MnO4- is purple
Mn2+ Is colourless

Reply 709

InItToWinItGetIt?

Original post by xX.Sabeel.Xx

colours for vanadium has dropped off the syllabus but we can be given questions on its reactions

theres only 2 we need to knowfor manganese... that is:
MnO4- is purple
Mn2+ Is colourless

I thought Mn 2+ is pale pink :s-smilie:

Reply 710

Sparkly-Star

I've just gotta finish revising chapter 16, then 13, past papers and review it all again. :smile:

Reply 711

BethBeth

Original post by InItToWinItGetIt?

I thought Mn 2+ is pale pink :s-smilie:

Very pale pink, and colourless in low concentrations, is what my tutor told me.

Reply 712

Crowned Temmy

Original post by xX.Sabeel.Xx

colours for vanadium has dropped off the syllabus but we can be given questions on its reactions

theres only 2 we need to knowfor manganese... that is:
MnO4- is purple
Mn2+ Is colourless

Oh thanks.

Reply 713

jamest92

Original post by strawberry_cake