brightsky's Chem Thread

[lzxnl]

Okay so im presuming vcaa is just being dodgy....

Also with mass spec, when im asked to give the formula of the fragment that gives rise to a particular peak, im never sure whether to include the . Indicating presence of unpaired electron or not. Is there a quick and foolproof way of checking whether a fragment ion is a radical?

Foolproof, here's a suggestion:
Add up ALL of the valence electrons of the atoms. If it's an odd number, you definitely have a radical.
For most VCAA questions, radicals will be stuff with like one less hydrogen than a familiar molecule, like CH3.

[brightsky]

Oh no i mean lets say we have a molecule x and are given the mass spectrum of it. Lets say the question asks us to deduce what fragment ion produced a certain peak. The formula is always easy to work out, but how do we determine whether the fragment ion is a radical not?

[lzxnl]

Yeah, you have the molecular formula, so then count up the number of valence electrons in this fragment ion. What I said was referring to a ion.
Note. This counting treats a +1 charge as minus one electrons, so CH3+ has 6 valence electrons. If the electron count is even, you may have to work out a structure yourself unfortunately. If you can't, chances are you have a radical. Like NO.

[brightsky]

Ah i see so just to clarify: we determine the moleculsr formula of the cation, count up the valence electrons, and then take 1 away. If the result is even, then no dot required. If the result is odd, then insert a dot along with the positive sign. Yay?

[lzxnl]

Yeah, pretty much.

[brightsky]

Electrolysis is used to coat an iron badge with copper metal. Accidentally, the circuit was connected so that the direction of current flow was reversed. Describe what you expect to happen at the badge surface.

[lzxnl]

Electrolysis is used to coat an iron badge with copper metal. Accidentally, the circuit was connected so that the direction of current flow was reversed. Describe what you expect to happen at the badge surface.

So normally...you're meant to have the iron as the negative cathode where the copper ions in solution are reduced to copper metal. This negative cathode is only a negative cathode because the external voltage makes it a cathode.
If you reverse the current flow, the external voltage supports the spontaneous oxidation of iron and the reduction of copper ions. You'll plate the copper metal with iron.
As an extra bonus, you'll end up heating stuff up as the external voltage isn't actually necessary for the galvanic cell to work (assuming standard conditions).

[brightsky]

what is one undesirable effect that the production of ammonia has on the environment? does production of CO and CO2 gases count? the undesirable effects of ammonia production are less conspicuous than those of sulphuric acid/nitric acid production...

[lzxnl]

Well CO can kill you. I'd presume CO emissions count as harmful.
CO2 isn't as dangerous but still a substance to be avoided.

[brightsky]

might be a stupid question, but why aren't there any endothermic spontaneous redox reactions?

[lzxnl]

might be a stupid question, but why aren't there any endothermic spontaneous redox reactions?

There are. Remember the ammonia synthesis? Well, let's take the decomposition of ammonia into nitrogen and hydrogen gas. This is always endothermic. Modify the reaction conditions and you can make this spontaneous. It's a spontaneous endothermic redox reaction.

VCE at times blurs the distinction between spontaneity and enthalpy. The two are related, but not directly. It is true that reactions with larger negative enthalpy changes tend to be more spontaneous. That does not mean that they are always spontaneous; spontaneity is given by the sign of the Gibbs free energy change, or whether the reaction quotient Q is greater or smaller than K.

I'll give you two simple examples of spontaneous endothermic reactions.
1. Evaporation of water at 101.3 kPa, 373 K. As more energy is put into the system, the water will spontaneously evaporate, which is endothermic.

2. Dissolution of some salts. Ammonium nitrate dissolves in water quite well, but its dissolution is endothermic. Problem?

[brightsky]

If there is such a thing as a spontaneous endothermic redox reaction, then what happens when we form a galvanic cell out of the reactants? Surely, if the reaction is energy-requiring, there would not be any chemical energy to convert into electrical energy?

[lzxnl]

Technically, delta H isn't the energy from the reaction; the Gibbs free energy change is, delta G, = delta H - T delta S (entropy), which is actually the (negative of the) maximum amount of useful non-expansion work that can be done. AKA work done on an electron through a potential difference.

The Gibbs energy change actually determines whether something is spontaneous or not, and it is this that determines whether we're recharging or discharging. Enthalpy, as you can see, is a component of the Gibbs energy change, but it's not everything.

[Aurelian]

If there is such a thing as a spontaneous endothermic redox reaction, then what happens when we form a galvanic cell out of the reactants? Surely, if the reaction is energy-requiring, there would not be any chemical energy to convert into electrical energy?

Yes. As nliu indicated, something called the Gibbs free energy change (∆G) of a reaction determines whether or not that reaction is spontaneous. ∆G must be negative for the reaction (as written) to be spontaneous. A reaction can have a negative ∆G but a positive ∆H, since ∆G = ∆H - T∆S for constant temperature, where "∆S" is the change in another quantity, entropy, and T is temperature.

E^o values actually reflect Gibbs free energy values, given by the relation ∆G^o = -nFE^o where n is the number of transferred electrons in the reaction. Hence although you can have a spontaneous redox reaction which is endothermic, you can't have a spontaneous redox reaction with a positive E_react.

[lzxnl]

Yes. As nliu indicated, something called the Gibbs free energy change (∆G) of a reaction determines whether or not that reaction is spontaneous. ∆G must be negative for the reaction as written, and a reaction can have a negative ∆G but a positive ∆H, since ∆G = ∆H - T∆S for constant temperature, where "∆S" is the change in another quantity, entropy, and T is temperature.

E^o values actually reflect Gibbs free energy values, given by the relation ∆G = -nFE^o. Hence although you can have a spontaneous redox reaction which is endothermic, you can't have a spontaneous redox reaction with a positive E_react.

I'm going to be VERY picky here...
Your relation with delta G and electrode potentials is slightly inconsistent. Either the Gibbs energy change is the standard change, or the electrode potential is not the standard electrode potential. I don't think we can have a mix