VCE Chemistry Question Thread

[zhen]

Increasing the concentration of your electrolyte would decrease its resistance, yes. Since the electrolyte is a crucial part of the circuit, this will result in a larger current flowing through -> more electrons -> more mass at the cathode.

Thanks. I was wondering if I can relate collision theory to redox reactions involving electron transfers and say the higher concentration results in more redox reactions from collision theory.

[zsteve]

Hmmm... for a galvanic cell? In the instance of a Cu/Zn cell higher CuSO4 solution concentration would mean more collisions between Cu2+ and Zn(s) electrode which could have an effect. I'm not sure - I think you could say that it is a factor, but I'm pretty sure the gains in current are the main thing here.

[Bri MT]

I'm not sure about the concentration causing increased reaction rate for an electrolytic cell, wouldn't the number of electrons be a limiting factor?

[lzxnl]

I'm actually confused about why there is an issue.
The reaction here is forcing nickel ions to react with metallic copper to form nickel metal and copper ions. Then, the mass of what you electroplate is a stoichiometry problem. If you have a higher concentration of electrolyte solution, you have more nickel ions available to react, and assuming your copper is still in excess, that means you have more limiting reagent, which means the reaction proceeds further.

[zhen]

I'm actually confused about why there is an issue.
The reaction here is forcing nickel ions to react with metallic copper to form nickel metal and copper ions. Then, the mass of what you electroplate is a stoichiometry problem. If you have a higher concentration of electrolyte solution, you have more nickel ions available to react, and assuming your copper is still in excess, that means you have more limiting reagent, which means the reaction proceeds further.

Thanks. I realised that the results are what was expected based on the theory.

[peanut]

Would you write 2,2-dimethyl-3-ethylpentane or 3-ethyl-2,2-dimethylpentane? "dimethyl" comes before "ethyl" alphabetically, but "ethyl" comes before "methyl" alphabetically. In other words, do we ignore or take into account prefixes when sorting alphabetically?

[sweetiepi]

Would you write 2,2-dimethyl-3-ethylpentane or 3-ethyl-2,2-dimethylpentane? "dimethyl" comes before "ethyl" alphabetically, but "ethyl" comes before "methyl" alphabetically. In other words, do we ignore or take into account prefixes when sorting alphabetically?

Hey there!
A set of IUPAC rules I found online suggest that when there are two or more substituents (methyl, ethyl, etc.) are in a compound, we ignore the di-, tri-, etc. prefixes.

(Happy to be corrected- I'm just going by what I know/can find )

[pha0015]

Hi, could you provide examples of tetrahedral molecules in the form of A1B2C2 (subscript) and explain why they're polar?
Thanks

[keltingmeith]

Hi, could you provide examples of tetrahedral molecules in the form of A1B2C2 (subscript) and explain why they're polar?
Thanks

One example I can think of is dichloromethane, CCl2H2. Why do you think this might be polar? What is your current understanding of polarity?

[pha0015]

One example I can think of is dichloromethane, CCl2H2. Why do you think this might be polar? What is your current understanding of polarity?

I know that the covalent bonds are polar, but the overall shape is somewhat symmetrical (at first glance), which, if that were the case, would mean that it's non-polar. I'm not too sure why it's asymmetrical though. Is it because one atom is more polar than the other? If so, how would you be able to tell, especially if they are close to each other on the periodic table, without an electronegativity table?

[pha0015]

Why causes something to be polar?

A molecule is polar when one side/end of the molecule has an opposite charge to the other.
Examples of Polar Tetrahedrals

Ammonia NH3
(Image removed from quote.)
Nitrogen is one the top-right of the periodic table, thus has a very high electronegativity compared to hydrogen. This causes nitrogen to pull hydrogen's electrons closer to the top, making the molecule more negative on one side.
Also, nitrogen has a lone pair on the very apex of the tetrahedral structure, thus making the molecule even more negative on that end. However, bottom of the tetrahedral molecule is only hydrogen positive protons because their sole electron is used to bond at the top. HENCE, the top is negative, and the bottom is positive; polar.

HCFC's + Halons
(Image removed from quote.)
CH3Cl
If you look at the one with the bottom-right with chlorine, chlorine has almost a full orbital of electrons, whereas the hydrogens at the bottom have their only electron being used in the middle with the carbon. Chlorine is also highly electronegative and will pull most of the electrons closer towards it (the top). Hence, the top is negative, the bottom is positive; polar.

Because of this, these polar molecules, like ammonia, can dissolve in water since it is also polar.

If you need further help, feel free to ask! Hope this helps

I already have a good understanding of those types of tetrahedrals (AB3C), but thanks anyways

[keltingmeith]

I know that the covalent bonds are polar, but the overall shape is somewhat symmetrical (at first glance), which, if that were the case, would mean that it's non-polar. I'm not too sure why it's asymmetrical though. Could you explain that?

I think the issue is because you're thinking of this thing like a square. The term symmetrical is kind of a weird one to be using in chemistry - in this instance, I would definitely call this symmetrical, but it's probably not symmetrical in the way you're used to. In this sense, I'd probably avoid using the word "symmetry", unless they're more informative/specific about it (is it rotational symmetry? Mirrored symmetry? etc.).

What you need to think about is how this thing looks in 3D space - as a tetrahedron. Then, think about how the dipole exists between single molecules, and add them all up. I'm assuming you're familiar with the case of AB4? Say, CCl4? In this case, there is a dipole between each carbon and chlorine atom, however each of these dipoles in cancelled out. It's not even a case of each dipole has an alternate pair - instead, to remove each dipole, the other three removes just a little bit of it.

I recommend reading up on force vectors and how to add/subtract them to try and wrap your head around this. Particularly useful if you plan on doing spec.

[geminii]

Hey everyone,

So this redox thing is confusing me a bit... I'm trying to write the oxidation half-equation for the following reaction:
2K + Cl2 -> 2K+ + 2Cl-
I'm not sure whether the coefficient before the element plays a role in the number of electrons I need to add. For example:
One possibility in my mind is 2K -> 2K+ + e- (ignoring the coefficients)
Second possibility: 2K -> 2K+ + 2e- (adding two electrons because there are two K+'s)

Which one is correct? :/

Thanks!

[julijulib]

Hey everyone,

So this redox thing is confusing me a bit... I'm trying to write the oxidation half-equation for the following reaction:
2K + Cl2 -> 2K+ + 2Cl-
I'm not sure whether the coefficient before the element plays a role in the number of electrons I need to add. For example:
One possibility in my mind is 2K -> 2K+ + e- (ignoring the coefficients)
Second possibility: 2K -> 2K+ + 2e- (adding two electrons because there are two K+'s)

Which one is correct? :/

Thanks!

The second option - charges need to be balanced (the left side of the equation is 0, since K is neutral, so the right side must also be 0, and since you have 2 positive charges due to 2 K+, you also need 2 electrons )

[MisterNeo]

Hey everyone,

So this redox thing is confusing me a bit... I'm trying to write the oxidation half-equation for the following reaction:
2K + Cl2 -> 2K+ + 2Cl-
I'm not sure whether the coefficient before the element plays a role in the number of electrons I need to add. For example:
One possibility in my mind is 2K -> 2K+ + e- (ignoring the coefficients)
Second possibility: 2K -> 2K+ + 2e- (adding two electrons because there are two K+'s)

Which one is correct? :/

Thanks!

For the redox half equations, you include all the co-efficients balanced.

Net Ionic Equation

Half Equation: Oxidation

Half Equation: Reduction

Hence,

You have to include the coefficients for the entire balanced equation. So your second option is the correct one because 2 potassium give 2 electrons.
Hope this helps