AS Chemistry- helping each other out!

Can someone explain the elimination reaction and what each arrow is showing in the image I attached. Thanks :smile:

Original post by Super199

Can someone explain the elimination reaction and what each arrow is showing in the image I attached. Thanks :smile:

If you look at the Br, you know it's more electronegative than the carbon, so the electrons in the bonding pair will be more so towards the Br than Carbon, the Br takes the electrons which leaves the carbon atom with three bonds. But, you know that carbon has 4 bonds so the bond on the hydrogen (the one that's attacked by the OH-) is placed as a double bond, which means that the H+ is removed and is accepted by the OH-

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Reply 822

Georgiam247

Original post by ainura.km

Hey everyone :smile:

Can someone explain me why the units used for solubility are moles/100g water and not g/100g of water? :smile:

I guess its because moles is more universal, and I'm pretty sure the unit for solubility can also be moles/L of water or g/100g water, the ratio/concentration of solvent to solute is the important thing :redface:

Reply 823

ilovecake123

11

Hi guys
I'm doing edexcel chemistry and my pratical assessment is tomorrow!
people have done it before me but there were a variety of results.
does hydrogen turn universal indicator red? others wrote that this was the ammonium ion?
thanks
any help would be appreciated

Reply 824

username1426305

Original post by ilovecake123

Hi guys
I'm doing edexcel chemistry and my pratical assessment is tomorrow!
people have done it before me but there were a variety of results.
does hydrogen turn universal indicator red? others wrote that this was the ammonium ion?
thanks
any help would be appreciated

If there are more hydrogen ions in solution, it's acidic so the universal indicator would be red, it depends on the strength though, how strongly acidic it is. Ammonium ions are basic so if you find them in solution the indicator would be blue/purple depending on how strongly alkaline the solution is.

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Reply 825

kek6969

AQA chemistry here, I'm gonna fail hard

Reply 826

Dylann

21

Original post by thatgr

AQA chemistry here, I'm gonna fail hard

No you won't, I won't let that happen :biggrin:

! What's your strongest/weakest topic so far?

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Reply 827

kek6969

Entertainment Forum Helper

Original post by Dylann

No you won't, I won't let that happen :biggrin:

! What's your strongest/weakest topic so far?

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Not really sure about strongest topics, overall I'm strong in general in section 1, apart from some more complex amounts of substance questions, I find the first unit is a lot more memory based which I'm good at :tongue:

I'm struggling more in unit 2 especially in the energetics

Reply 828

jerseyalevel

6

Hi guys,
Could anyone doing OCR AS chemistry that has already done the 2015 evaluative task on 'enthalpy of combustion' please share some of the questions with me? I really need to improve this one so!
thanks !

Reply 829

Dylann

21

Original post by thatgr

Not really sure about strongest topics, overall I'm strong in general in section 1, apart from some more complex amounts of substance questions, I find the first unit is a lot more memory based which I'm good at :tongue:

I'm struggling more in unit 2 especially in the energetics

So if you're strong in unit 1 there's no way you're gonna fail! :biggrin:

see we're already making progress. I would recommend using this guy:

www.youtube.com/user/NewcastleChemistry to help! He should be able to help you with energetics unit 2. Make notes and once you think you've understood head over to:

www.a-levelchemistry.co.uk and select AQA AS and do some of the energetics exercises! This should help reinforce what you've learned.

For the substance questions, I understand some of them can be pretty trick but use the a-levelchemistry website and try out some questions there :biggrin:

tell me how it goes

Reply 830

Kallisto

22

Original post by Super199

Can someone explain the elimination reaction and what each arrow is showing in the image I attached. Thanks :smile:

Well, by this elimination reaction, the Br-atom and H-atom are splitted. As there are no another atoms which are added or substituded, the reaction has this name.

This reaction is caused by breaking up the bonds. In the case of H-atom, the C-C-bond is changed to C=C-bond by adding H-atom's bond to it. So, the H-Atom lost the bond to the molecule and becomes a positive H-ion which is attracted by the negative OH-ion and reacts to water (H2O) at the end.

In the case of Br-Atom, the bond is shifted to the named atom, so it lost the bond to the molecule and becomes an negative Br-ion. And so you got two by-products, namely Br- and H2O. The purpose of such reactions is to get new products which can be processed again. In your case, its C3H6.

In general, the arrows show you up that bonds are shifted to split atoms of a molecule.

Reply 831

Entertainment Forum Helper

Original post by Kallisto

Well, by this elimination reaction, the Br-atom and H-atom are splitted. As there are no another atoms which are added or substituded, the reaction has this name.

This reaction is caused by breaking up the bonds. In the case of H-atom, the C-C-bond is changed to C=C-bond by adding H-atom's bond to it. So, the H-Atom lost the bond to the molecule and becomes a positive H-ion which is attracted by the negative OH-ion and reacts to water (H2O) at the end.

In the case of Br-Atom, the bond is shifted to the named atom, so it lost the bond to the molecule and becomes an negative Br-ion. And so you got two by-products, namely Br- and H2O. The purpose of such reactions is to get new products which can be processed again. In your case, its C3H6.

In general, the arrows show you up that bonds are shifted to split atoms of a molecule.

Kind of sorted this but I was just about to post another query, care to help me? It's to do with oxidising and reducing agents. I understand the redox thing but how do I know which one. Can you possibly explain with an example?

Reply 832

Kallisto

22

Original post by Super199

Kind of sorted this but I was just about to post another query, care to help me? It's to do with oxidising and reducing agents. I understand the redox thing but how do I know which one. Can you possibly explain with an example?

Maybe its helpful to understand in the view of electrons? Look at this pattern of oxidation and reduction.

Whenever an agent loses electrons, its an oxidation. The agent gets a positive charge and can be attracted by agents with negative ones.

Whenever an agent gains electrons, its a reduction. The agent gets a negative charge and can be attracted by agents with positive ones.

Reply 833

Original post by Kallisto

Maybe its helpful to understand in the view of electrons? Look at this pattern of oxidation and reduction.

Whenever an agent loses electrons, its an oxidation. The agent gets a positive charge and can be attracted by agents with negative ones.

Whenever an agent gains electrons, its a reduction. The agent gets a negative charge and can be attracted by agents with positive ones.

Ah I was thinking something complicated for no reason. So oxidising agent is the one that is oxidised and reducing agent is the other one?

Reply 834

cupsoftea

1

Original post by Super199

Ah I was thinking something complicated for no reason. So oxidising agent is the one that is oxidised and reducing agent is the other one?

an oxidising agent oxidises (removes electrons from another species), thus is reduced itself (gains these electrons)

reducing agent reduces, (donates electrons) and thus is oxidised itself (loses electrons)

Reply 835

Entertainment Forum Helper

Original post by cupsoftea

an oxidising agent oxidises (removes electrons from another species), thus is reduced itself (gains these electrons)

reducing agent reduces, (donates electrons) and thus is oxidised itself (loses electrons)

Gosh I'm confused now :frown:

Care to give an example?

Reply 836

Kallisto

22

Original post by Super199

Ah I was thinking something complicated for no reason. So oxidising agent is the one that is oxidised and reducing agent is the other one?

Here is a concrete reaction with oxidation numbers.

As you can see copper has a twice positive charge, after gaining two electrons, the charge was neutralized. By three gaining electrons, the charge would be negative (-1)!

Iron has a neutral charge before losing electrons. After losing two electrons, the charge has got a twice positive charge. So note:

an Oxidation decreases the oxidation number (getting more positive), but a reduction increases it (getting more negative).

(edited 9 years ago)

Reply 837

cupsoftea

1

Original post by Super199

Gosh I'm confused now :frown:

Care to give an example?

I'll explain from scratch

A species which is called an oxidising agent is a species which oxidises
another species. The oxidising process is one where a species loses electrons.

A species which is called a reducing agent is a species which reduces another species. This other species being reduced, is gaining electrons.

Emphasis is on the fact whatever the agent is, the process it is named after is happening to another species, not itself. so a reducing species reduces another species, and thus itself is oxidised .

an oxidating reagent oxidises another species, and thus itself must be reduced.

for example

Fe + Cu2+ --> Fe2+ + Cu

In this process, two electrons have been lost from Fe, these two electrons are donated to the Cu2+, in order for that to form Cu. The Fe becomes Fe2+ (as it has lost two electrons)

Therefore, Fe is the reducing species (it has donated electrons, electron gain has occured in the Cu). As a result of this donation, Fe has been oxidised (lost electrons).

The Cu is the oxidising species (it has taken electrons from the Fe). In doing so, it has gained these electrons and therefore Cu itself has been reduced.

The oxidising and reducing names come from the process the reagent is carrying out, thus the reverse process is actually happening to itself (think about the electrons being gained and lost in the processes)

Hope this helps, you may need to read it a few times for it to click, but once it does you'll realise its quite an easy concept, it's just initially confusing.

Reply 838

username1426305

Original post by Super199

Gosh I'm confused now :frown:

Care to give an example?

use OILRIG where oxidation is loss and reduction is gain, so if a substance is oxidised, it's lost electrons and if it's reduced its gained electrons.

An oxidising agent Oxidises something else

A reducing agent Reduces something else

If an oxidising agent oxidises something, it's taking electrons from it so it itself has gained those electrons, or is reduced. It's the other way around for the reducing agent, that reduces something else, or gives electrons to it and so is oxidised itself. Look at this equation

Mg + Cu2+ ----> Mg2+ + Cu

The magnesium goes from neutral to becoming 2+ so it's lost electrons, meaning it's been oxidised by Cu2+ so that's the oxidising agent. Those electrons that have been removed go to copper to form a deposit of copper, which has been reduced. Therefore Mg is the reducing agent because it gives electrons to Cu2+

Hope that helps, best way to get your head around these is to practice. :smile:

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Reply 839

Georgiam247