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AQA - Unit 5 - Energetics, Redox and Inorganic Chemistry

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Reply 160
Original post by JONNYCMW
Why does covalent character in Al2O3 lower the melting point, yet covalent character in ZnSe increases the lattice enthalpy?


Hmm, I don't know lol.

In terms of lattice enthalpy, covalent bonding is said to be stronger. This could be because you're converting the compound into gaseous ions. In a completely ionic compound they're already ions, they just need to be separated. In ionic compounds with covalent character, the portions that are covalent don't have a charge but share an electron so the added energy of turning them into ions might mean more energy is needed.

In terms of melting point of the oxides, covalent bonding seems to be weaker. It could be because of the way the bonds are broken. If an ionic lattice is dissolved in water and then turned into gas, the ionic bonds are easy to break compared to covalent bonds (which do not break when dissolved), but to break ionic bonds without water (i.e. through heat) would make them more difficult to break so covalent bonding would be relatively weaker.

Part of what makes this question so complicated is that you're kind of comparing apples and oranges. The comparison in the properties would therefore be very difficult to explain and may have many factors and may be different under different circumstances.

A question is not going to come up comparing the melting point of MgO against Al2O3 or even SiO2 (a completely covalent macromolecule with an even lower melting point). The question will compare MgO (or MgO and Al2O3) against Na2O where you just have to talk about charges, or compare SiO2 with P4O10 or SO2 where you mention Van der Waals and many strong covalent bonds.

Basically, you can try and find out an answer if you want but i'm 90% sure that it will not be necessary to know for this exam. You just need to remember that covalent character increases the lattice enthalpy.

Hope that helped! :smile:
Reply 161
Original post by Sheldon
hey guys just thought I would post a list of some TM reactions that I made :smile:
If you find them useful I have other notes I made like that for periodicity and some other parts.


Doesnt [Cr(h20)6] 3+ also react with an excess of NH3?
to give [Cr(NH3)6] 3+, a purple/violet solution ?
Reply 162
Original post by Kirandeep Panesar
Have you got anything on chapter 14?


I dint have the AQA book by nelson thornes, which part is that about?
Original post by Sheldon
I dint have the AQA book by nelson thornes, which part is that about?


Redox equations and electrode potentials
Reply 164
Original post by Kirandeep Panesar
Redox equations and electrode potentials


No afraid not as the way i see it, there is not a whole lot of notes you can put on paper, maybe except Hydrogen Oxygen fuel cells. As its like the only section that is about technique and applying your knowledge not just remembering facts.
Reply 165
Original post by jimmy303
Hmm, I don't know lol.

In terms of lattice enthalpy, covalent bonding is said to be stronger. This could be because you're converting the compound into gaseous ions. In a completely ionic compound they're already ions, they just need to be separated. In ionic compounds with covalent character, the portions that are covalent don't have a charge but share an electron so the added energy of turning them into ions might mean more energy is needed.

In terms of melting point of the oxides, covalent bonding seems to be weaker. It could be because of the way the bonds are broken. If an ionic lattice is dissolved in water and then turned into gas, the ionic bonds are easy to break compared to covalent bonds (which do not break when dissolved), but to break ionic bonds without water (i.e. through heat) would make them more difficult to break so covalent bonding would be relatively weaker.

Part of what makes this question so complicated is that you're kind of comparing apples and oranges. The comparison in the properties would therefore be very difficult to explain and may have many factors and may be different under different circumstances.

A question is not going to come up comparing the melting point of MgO against Al2O3 or even SiO2 (a completely covalent macromolecule with an even lower melting point). The question will compare MgO (or MgO and Al2O3) against Na2O where you just have to talk about charges, or compare SiO2 with P4O10 or SO2 where you mention Van der Waals and many strong covalent bonds.

Basically, you can try and find out an answer if you want but i'm 90% sure that it will not be necessary to know for this exam. You just need to remember that covalent character increases the lattice enthalpy.

Hope that helped! :smile:


Thanks, these exceptions to the rule make chemistry rather interesting (+rep)
Reply 166
does anyone know all the redox reactions with h202? how do you oxidse it to oxygen gas? thanks?
Original post by soutioirsim
Noooo! I hate electrode potentials :afraid:


why do you hate it? its so easy!!!


Okay thankyou and you !!! :biggrin:
Reply 169
Hey,

Having a bit of a dilemma with the CGP revision guide for this module.

Working out total entropy change. I would normally do this for the reaction by working out products - reactants. This has always worked before.

THE CGP guide introduces a concept whereby you have to take into account the surroundings too. So the total entropy change = entropy change in the system (calculated like above) + entropy change in the surroundings (calculated by minus delta H over T).

We've never been taught this :/ Have any of you guys? We're trying to work out whether this would ever be used, we've done the mock exams and it's never asked for an entropy value in this way. And I can't work out how to distinguish which method to use. Enlighten me please!!!
Reply 170
Original post by adam1232
does anyone know all the redox reactions with h202? how do you oxidse it to oxygen gas? thanks?


I posted my equations earlier, but not sure if they're right.
I think they are.

OXIDATION
ACIDIC: h2o2 ------> 2e- + 2H+ + o2
ALKALINE: h2o2 + 2OH- ------> 2e- + 2h20 + o2

REDUCTION
ACIDIC: h2o2 + 2e- + 2H+ ------> 2h2o
ALKALINE: h2o2 + 2e- ------> 2OH-
Reply 171
Original post by rrelish
I posted my equations earlier, but not sure if they're right.
I think they are.

OXIDATION
ACIDIC: h2o2 ------> 2e- + 2H+ + o2
ALKALINE: h2o2 + 2OH- ------> 2e- + 2h20 + o2

REDUCTION
ACIDIC: h2o2 + 2e- + 2H+ ------> 2h2o
ALKALINE: h2o2 + 2e- ------> 2OH-


Thank you helped alot
Reply 172
Original post by kingjohno
Hey,

Having a bit of a dilemma with the CGP revision guide for this module.

Working out total entropy change. I would normally do this for the reaction by working out products - reactants. This has always worked before.

THE CGP guide introduces a concept whereby you have to take into account the surroundings too. So the total entropy change = entropy change in the system (calculated like above) + entropy change in the surroundings (calculated by minus delta H over T).

We've never been taught this :/ Have any of you guys? We're trying to work out whether this would ever be used, we've done the mock exams and it's never asked for an entropy value in this way. And I can't work out how to distinguish which method to use. Enlighten me please!!!


In short, no, you don't need to know that.

The specification says:

Be able to calculate entropy changes from absolute entropy value.

And

Understand that the balance between entropy and enthalpy determines the feasibility of a reaction; know that this is given by the relationship

Change in Free Energy = Change in Enthalpy - Change in Entropy x Temperature

(Derivation not required)


Thus for entropy you only need to be able to work out entropy change of a reaction using products - reactants (and plug it into the above formula to work out change in free energy or the temperature if change in free energy is 0) but nothing else.

The entropy of the system and the entropy of the surroundings calculations aren't needed for AQA (as stated by the bold bit), but are involved in the Entropy topic (so are worth knowing if you continue chemistry at uni) and may crop up for other exam boards, which is probably why they were included in your revision book.

Good luck! :smile:
(edited 12 years ago)
Reply 173
Original post by rrelish
I posted my equations earlier, but not sure if they're right.
I think they are.

OXIDATION
ACIDIC: h2o2 ------> 2e- + 2H+ + o2
ALKALINE: h2o2 + 2OH- ------> 2e- + 2h20 + o2

REDUCTION
ACIDIC: h2o2 + 2e- + 2H+ ------> 2h2o
ALKALINE: h2o2 + 2e- ------> 2OH-


why do we have to know this? what topic...?
Reply 174
How can you convert CrO4^2- to Cr(OH)6^3-??
Original post by M.A.
How can you convert CrO4^2- to Cr(OH)6^3-??


you normally go the other way using h2o2 in alkaline solution.
Reply 176
can ANYONE EXPLAIN BORN HABER CYCLES AND TITRATION STUFF

BIG THANK YOU, IM HAVING A HEART ATTACK OVER THIS
Reply 177
Original post by englishman129
you normally go the other way using h2o2 in alkaline solution.

yeah i know that, but I'm talking about the reverse. It is a question in my book
i absolutely detest chem5!
:frown:
the colours and transition metals and all the reactions make me feel dizzy!
its going to be my last exam
so hope i don't mess it up!
any revision tips especially for the colour memorising thing?? :s-smilie:
Original post by M.A.
yeah i know that, but I'm talking about the reverse. It is a question in my book


never seen that before, sorry.

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