Synoptic questions

These are a few questions from last years paper which I am struggling to do. If someone could over them with me and show me how to do them I would be happy.

1. Which one of the following represents a reaction that is feasible at all temperatures?
A P(S)--- Q(S) +R(G) Endothermic
B 2L(G) + M(G) --- 2N(G) Exothermic
C S(G)-----2T(G) Exothermic
D A(G)+B(G)---C(G) Endothermic

2. Which one of the following is not a redox reaction?
A Br2+ So2+ 2H20--- SO42- + 4H+ +2BR
B SNCL2 +HGCL2 ---HG+SNCL4
C CU20+H2S04---CUS04 +CU+H20
D 2CR042-+2H+---CR2072-+H20

3.Molecules which contain an element with an oxidation state of +5 include:
1. H2S03
2. NO2+
3.[CR(H20)4(CL2]+
4. CL03-

I am having most problems with finding out about redox reactions and oxidation states. They seem to crop up a lot and yet I just cant seem to crack them. I am really worried :frown:

Reply 1

Sycren

well number 3 is NO2+
to do it take the charge which is one plus and put it one side of an equals sign so;

=+1

and put oxidation state of oxygen and nitrogen (the thing you want to find on the other)

N -4=+1

(+4) simple algebra add 4 to both sides

N=+5

also CLO3-

Cl -6=-1
Cl=-1+6
Cl=5

so the synoptic was probably C which would be awnsers 2 and 4. Does that help you?

Reply 2

dimz

My only question to you is how is it possible to pick out the right element if you get me as for example in clo3-, why pick CL but not O?

Reply 3

Sycren

I suppose its just a skill/way of seeing things. Yo could take out oxygen if you wanted, the way that I remember is that I dont particularly know the oxidation state of nitrogen and so on and work it out from what oxygen is. Sorry this is really confusing to explain. Try and use the element where it is least there (exception is dont use hydrogen)

So Fe2O3 would be Fe would be +3

Reply 4

knocker

For question 1 you need think about what a spontaneous reaction means...

In this case it means that DG (delta G) is less than zero and remember that...

DG = DH - TDS

Therefore for DG to always be negative you want an exothermic reaction (DH < 0) and DS to be positive (DS > 0).

Therefore C is the correct answer.

Reply 5

knocker

Answer for q2 is D - looking at the oxidation states.

Oh and when trying to work out oxidation states in a compound always remember the elements that have fixed oxidation states and then go from there...

oxygen always -2 (except in peroxides)
hydrogen always +1 (except in hydrides)
group 1 always +1
group 2 always +2
group 7 always -1 (except when bonded to oxygen, eg ClO3(-), ClO(-) etc)

Most of the time the element you are looking for will be a d-block element or one of the main group elements beyond the 2nd period (S, N, Cl, etc)

Reply 6

Sycren

pick an element that you dont know the oxidation state of, If you had a transition metal in a compound then choose it as there is sometimes no way of telling what it could be as they have variable oxidation states.