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Chemistry Questions (3 Viewers)

Aysce

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theind1996

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Re: Reaction of alkanes question

Thanks for the site. Is this naming meant to be learnt in the HSC course? I swear I don't recall anything like this in the prelim course and I'm fairly certain that it isn't.
Not for hydroxyl groups.

For hydrocarbons - in Energy, section 2 or 3 somewhere iirc.
 

Aysce

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Production of LDPE & HDPE clarification

Here is the text:



Within these chemical equations, I notice that there is the catalyst Z within the resultant products. Although the problem for me is, I thought that the catalyst does not undergo permanent chemical change ie. it doesn't become a component of the resulting product(s). I'm a little confused here as to why it still remains in the products? :/
 

madharris

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Re: Production of LDPE & HDPE clarification

I think the text book is wrong. I think Z is meant to be an initiator not a catalyst as you're right, the catalyst doesn't change during the reaction however the initiator will

Someone correct me if I'm wrong :)
 

Aysce

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Re: Production of LDPE & HDPE clarification

My thoughts exactly, madharris. Although, the textbook should also explain how the catalyst is involved for the process of production for HDPE so I'm a little unsure.
 

Magical Kebab

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Re: Production of LDPE & HDPE clarification

The text is wrong, I'm pretty sure.

Catalysts should only speed the rate of the reaction, by lowering the activation energy required for the reaction to occur (they provide different mechanisms for the reaction to take place). They shouldn't be consumed. They may be part of a number of chemical transformations but again, never should they be part of the products i.e. they shouldn't be consumed by the reactants.
 

Aysce

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Re: Production of LDPE & HDPE clarification

Can I get some confirmation from Someth1ng/Deswa1/Nightweaver?
 

nightweaver066

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Re: Production of LDPE & HDPE clarification

Those equations are suitable for the production of LDPE.

I think the text book is wrong. I think Z is meant to be an initiator not a catalyst as you're right, the catalyst doesn't change during the reaction however the initiator will

Someone correct me if I'm wrong :)
Initiators react and become apart of the products. Catalysts can react but they don't end up in the products.

My thoughts exactly, madharris. Although, the textbook should also explain how the catalyst is involved for the process of production for HDPE so I'm a little unsure.
I don't think i've seen a clear cut explanation of how the production of HDPE works.

AFAIK, all you need to know in HSC chemistry is that the production of HDPE is through the Ziegler-Natta process (low temp. ~60C & low pressure ~20atm) using Ziegler-Natta catalysts (titanium chlorides or trialkyl aluminium compounds). This allows for surface polymerisation to occur, minimising the amount of branching, producing linear PE chains.

I don't recall learning any equations for this, but just that it follows the same "initiation, chain propagation then termination" process.

The text is wrong, I'm pretty sure.

Catalysts should only speed the rate of the reaction, by lowering the activation energy required for the reaction to occur (they provide different mechanisms for the reaction to take place). They shouldn't be consumed. They may be part of a number of chemical transformations but again, never should they be part of the products i.e. they shouldn't be consumed by the reactants.
Catalysts not only lower the activation energy, they also allow certain reactions to occur (e.g. fermentation, dehydration/hydration of ethanol/ethene, etc).
 

Aysce

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Re: Production of LDPE & HDPE clarification

Alright, thanks for the confirmation nightweaver.
 

someth1ng

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Re: Production of LDPE & HDPE clarification

AFAIK, all you need to know in HSC chemistry is that the production of HDPE is through the Ziegler-Natta process (low temp. ~60C & low pressure ~20atm) using Ziegler-Natta catalysts (titanium chlorides or trialkyl aluminium compounds). This allows for surface polymerisation to occur, minimising the amount of branching, producing linear PE chains.

I don't recall learning any equations for this, but just that it follows the same "initiation, chain propagation then termination" process.
Yeah, the explanation is quite dodgy but the actual *catalyst* is really the initiator to the reaction which leads to a more crystalline structure of polyethylene as there is less branching.
 

Aysce

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Salt bridge and migration of ions

The salt bridge is used as a means of migrating ions between the two half-cells in order to maintain electrical neutrality. Although, when referring to the process of how it does it between Kamal's notes and the Conquering chem textbook, I'm a little confused on which one to pick.

In the notes:

"The salt bridge is soaked in potassium nitrate solution: Thus, as the positive charge builds up in the left cell, NEGATIVE nitrate ions migrate towards the cell to neutralise the charge; as the negative charge builds up in the right cell, the POSITIVE potassium ions move towards the cell to neutralise it as well."

Okay so from here it's saying that the potassium and nitrate ions of the electrolyte solution in the salt bridge move into either of the two half-cells.

In the conquering chem textbook:

"Both positive and negative ions migrate through the salt bridge. In Figure 2.2 not only do nitrate ions migrate from the right-hand beaker into the left-hand one, but also positive ions migrate from left to right. Copper ions move into the salt bridge and ‘push’ potassium ions out into the silver nitrate solution. This migration preserves electrical neutrality in both beakers (half-cells)."

Here is figure 2.2:



From what I'm interpreting from the textbook, I see it that ions from the two half cells migrate through the salt bridge in which this also pushes out the necessary charged ions into the specific half cell to attain electrical neutrality.

TL;DR: Is it the ions alone in the electrolyte solution of the salt bridge that move into the half-cells to achieve electrical neutrality? Or is it the ions within the half cells as well as the ions within the electrolyte solution of the salt bridge that migrate to the specific half-cell to achieve electrical neutrality?

Sorry if this seems quite convoluted in expression, I tried to make it easy to understand :s
 

Galapagos

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Re: Salt bridge and migration of ions

Here's a good thread on this topic: http://forums.studentdoctor.net/archive/index.php/t-722358.html

"The salt in the solution can run out, but a salt bridge is the path for the anions to travel. For every anion that adds on one end, another falls off the other end. Just like the wire can't run out of electrons, because they are transferring across and not being consummed, the salt bridge can't run out of anions."

I think this kinda answers your question.

My understanding of it is that the cathode/anode will be neutralised by ions from the actual salt bridge, not from the cathode/anode solution; you will have salt bridge ions (i.e. K+ / NO3-) migrating into the half cell solutions, and cathode/anode ions attaching to the other end of the salt bridge... at least that's how I visualise it (somebody please correct me if I'm wrong).
 
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Aysce

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Re: Salt bridge and migration of ions

Here's a good thread on this topic: http://forums.studentdoctor.net/archive/index.php/t-722358.html

"The salt in the solution can run out, but a salt bridge is the path for the anions to travel. For every anion that adds on one end, another falls off the other end. Just like the wire can't run out of electrons, because they are transferring across and not being consummed, the salt bridge can't run out of anions."

I think this kinda answers your question.

My understanding of it is that the cathode/anode will be neutralised by ions from the actual salt bridge, not from the cathode/anode solution; you will have salt bridge ions (i.e. K+ / NO3-) migrating into the half cell solutions, and cathode/anode ions attaching to the other end of the salt bridge... at least that's how I visualise it (somebody please correct me if I'm wrong).
Thanks for the contribution Galapagos!

We'll have to confirm with others I suppose.
 

Galapagos

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Re: Salt bridge and migration of ions

see this diagram: http://www.splung.com/fields/images/batteries/CuZncell.gif

let's say electrons have just migrated to the cathode. The overall cell will be unequal in charge and so to neutralise this charge (SO4)2- ions from the cathode solution will be drawn to the salt bridge (which is negatively charged at this particular instant), which causes the negative charge to flow through the salt bridge and into the anode solution. Similarly, Zn2+ will be attracted to the salt bridge and will cause a positive charge to flow through the salt bridge. These changes occur as the system constantly works towards equilibrium, so even at equilibrium we have a flow of both anions and cations across the salt bridge.

The bolded section you noted is just kind of my way of imagining it. Whether or not the ions actually attach to the salt bridge, or whether they just sort of bump into it and hover, nobody really knows :p

I hope my explanation didn't confuse you further lol
 
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Aysce

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Re: Salt bridge and migration of ions

So from your example above, what I'm deducing is that the cation (Zn2+) and anion (SO42-) from the two half cells are the only ions used to maintain an electrical equilibrium. Are you certain that none of the ions in the salt bridge are actually used? Like for example, rather than using Zn2+, 2Na+ moves to the reduction half-cell.

Not confused, just still a little unsure haha.
 

Aysce

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Re: Salt bridge and migration of ions

I'll look further into what you proposed above. Appreciate the help a lot man :)

Although confirmation from others is always useful, just to ensure everything is legit.
 

Galapagos

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Re: Salt bridge and migration of ions

So from your example above, what I'm deducing is that the cation (Zn2+) and anion (SO42-) from the two half cells are the only ions used to maintain an electrical equilibrium. Are you certain that none of the ions in the salt bridge are actually used? Like for example, rather than using Zn2+, 2Na+ moves to the reduction half-cell.

Not confused, just still a little unsure haha.
Well all ions present have some role to play in carrying their respective charges (obviously). The salt bridge just acts as a wire, or a highway for charges to flow through, without the two half cell solutions having direct contact with one another.

Anyway, hope I helped. If not, just ignore me. I'm certainly confused. I think the concepts are simple, but the technicalities (such as the one you have brought up) can be complex.

Here are some more websites that explain it (1000x better than I could ever do):

http://www.wiredchemist.com/chemist...-material-for-chemistry/chapter-18/animations

http://www.chembio.uoguelph.ca/educmat/chm19105/galvanic/galvanic2.htm

http://www.chemicalforums.com/index.php?topic=59251.0

Fark. going to bed.
 
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Galapagos

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Re: Salt bridge and migration of ions

In that last link, I found something interesting:

"Yes, salt bridges have a limited life span - they loose their ions to the half cells solutions. We usually select ions in salt bridges in such a way they are inert for the researched process, so their presence can be ignored."
 

Aysce

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Re: Salt bridge and migration of ions

Of course you have helped. I appreciate that you have gone to the trouble of searching for websites for me to read - I'll be sure to read them tomorrow. Right now...my mind is dead :haha:
 

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