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Equilibria, Energetics and Elements (F325) - June 2011 Exam.

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Reply 1760

sheep_go_baa

Original post by sportycricketer

Just to verify, at which electrode does Red and Ox take place?

Is it Red at Cathode?

Is it Ox at Anode?

If the cathode is the positive terminal and the anaode the negative, then yes.

Reply 1761

Pandit Bandit

can anyone tell me the ligand shapes please!?

Reply 1762

susan23

is Chromium a transition element because it has 3d5 so its partially filled? whats so special about Chrominum and Copper...I know coppers not a transition metal cause its full d-subshell???????????/

Reply 1763

Sereni

Original post by Pandit Bandit

which complexes have trigonal planar shapes guys?

None of the do

Octahedral is when there are 6 ligands and tetrahedral is when there are 4, or square planar with cis platin like ****.

Reply 1764

wbataylor

Original post by arvin_infinity

Its all over the past papers! but if you say so!

DO I need know cr(h2o)6 colour?

Its cus the syllabus has changed in recent years, uses and colours will not be tested anymore - except obviously for the precipitation reaction etc.

and im sure you only need to know the colour and colour changes of copper and cobalt complex ions?

Reply 1765

yesioo

can anyone tell me the ligand shapes please!?

Trigonal Planar, Octahedral, Square planar. Only Octahedral and Square undergo cis/trans

Reply 1766

KnuckleheadNinja

how do you calculate standard enthalpy change from a list of enthalpy of formations?
because i was going to do bonds broken-bonds formed. but thats not right is it?

(edited 12 years ago)

Reply 1767

Rob16

Original post by susan23

is Chromium a transition element because it has 3d5 so its partially filled? whats so special about Chrominum and Copper...I know coppers not a transition metal cause its full d-subshell???????????/

Copper is a transition metal as it forms the 2+ ion with an incomplete d subshell.

Chromium and copper special because they both lose an electron from the 4s subshell so that they can have 5 (or 10) electrons in the 3d subshell, which makes it more stable

Reply 1768

DrDr

Original post by sheep_go_baa

If the cathode is the positive terminal and the anaode the negative, then yes.

Don't be difficult.

Anode is always positive, remember guys.

Reply 1769

Rob16

Original post by yesioo

Trigonal Planar, Octahedral, Square planar. Only Octahedral and Square undergo cis/trans

Isn't it tetrahedral not trigonal planar?

Reply 1770

Flux_Pav

we don't need to memorise examples of a complex ion do we? unless they tell us the formula only then we can draw it

Reply 1771

DrDr

Original post by susan23

is Chromium a transition element because it has 3d5 so its partially filled? whats so special about Chrominum and Copper...I know coppers not a transition metal cause its full d-subshell???????????/

Scandium and Zinc aren't transition metals, Copper and chromium are.

What is special about them is that they gain an electron in their 3d orbital (5 and 10) before their 2nd 4s orbital.

Reply 1772

susan23

Original post by DrDr

Scandium and Zinc aren't transition metals, Copper and chromium are.

What is special about them is that they gain an electron in their 3d orbital (5 and 10) before their 2nd 4s orbital.

oh never mind i hget it

(edited 12 years ago)

Reply 1773

DrDr

Original post by susan23

WHATTTT SCANDIUM HAS APRTIALLY FILLED D SUBSHELL??? u mean zinc and copper!!!! RIGHT>????

Scandium and zinc are d-block elements, but they only form one ion each: Zn2+ and Sc3+ which have empty and full orbitals respectively and therefore are not transition metals (which, remember, have multiple oxidation states and partially filled d-orbitals)

Try to calm down, stressing is your biggest enemy at a time like this :smile:

Reply 1774

Sereni

Original post by Flux_Pav

we don't need to memorise examples of a complex ion do we? unless they tell us the formula only then we can draw it

You need to know the copper and cobalt ones

Reply 1775

Kalamari Dave

Original post by Ayostunner

Can someone please explain the concept of the feasibility of a reaction, i cant seem to get my head round it

Two aspects to this, the Gibbs free energy and cell potentials.

1. Gibbs free energy.

?G = ?H - T?S
gibbs free energy = enthalpy change of reaction - (temperature reaction is carried out*change in entropy)
If ?G is less than or equal to zero, that reaction is said to be feasible or spontaneous at that temperature (i.e. it happens. Example: petrol going to CO2 and H2O in the presence of oxygen. ?G < 0 only at a high temperature, which is why you need a hot spark in order to raise the temperature, increast T?S so that overall ?G is negative, to let the reaction happen)

2. Electode potentials.

It dictates which substances are oxidised/reduced in a reaction.

e.g.
Zn^2+ + 2e? ? Zn E°cell = - 0.76
Cu^2+ + 2e? ? Cu E°cell = + 0.34

The more positive half cell will Oxidise the other one. The oxidising one will go the forward reaction, and the oxidised one will go backwards

Copper half cell more positive than zinc, so it will go forwards, and zinc will go backwards.

Zn ? Zn^2+ + 2e?

Cu^2+ + 2e? ? Cu

Therefore overall reaction is Zn + Cu2+ ? Zn2+ + Cu when the copper and zinc half cells are connected.

This means that when the reactants on the left hand side, Zn and Cu2+ are mixed, a reaction will occur.

When you do it the other way round, say add Zn2+ to Cu, and expect the reverse direction, Zn2+ + Cu ? Zn + Cu2+, you change the direction of the half cells and reversing the sign of Ecell.

Zn ? Zn^2+ + 2e? E°cell = + 0.76
Cu ? Cu^2+ + 2e? E°cell = - 0.34

The more positive half cell will Oxidise the other one. The oxidising cell will go forwards, the oxidised cell will go backwards

We see here that Zn2+ will be the oxidiser, and so will go the forward reaction, and so Cu will go the backward.

Zn ? Zn^2+ + 2e?
Cu^2+ + 2e? ? Cu
overall: Zn + Cu2+ ? Zn2+ + Cu

Look here, the reaction is exactly the same as before! Since the products are already present in the reaction, its "already finished".

So, the reaction between Zn2+ and Cu isn't feasible, because it doesn't happen.

(it makes sense if you think about it: Zinc2+ won't take Copper's crummy electrons, its a "more reactive" metal so it prefers to stay ionic.)

Hope that answers your question :smile:

Reply 1776

Kalamari Dave