A-level Chemistry Revision Squad!

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Reply 100

FemaleBo55

Original post by Dinasaurus

Anyone else here doing A2?

Yepp

& it's 'chemically' killing me & I feel like crying :lol:

(What deep poetic rhyme LOL :u:

)

Reply 101

TheAlphaParticle

can someone explain how to find limiting reagents and what use is it?

Reply 102

samb1234

Original post by TheAlphaParticle

can someone explain how to find limiting reagents and what use is it?

Right I'm going to go for a bit of a different approach, I'm going to do a fully worked example explaining each stage as I go to hopefully allow you to answer anything they ask. Note that I'm just making this question up it doesn't come from anywhere.

Question: Sodium metal reacts with chlorine gas to form a white crystalline soild of sodium chloride. John reacts 24.2g of sodium metal with 1 mole of chlorine gas, and he produces 26.4g of sodium chloride. Calculate his % yield.

Step 1: Write out the full balanced symbol equation, so firstly you need to work out the formula of the product. Sodium is a group one metal so will form a 1+ ion, and chlorine is in group 7 so will form a 1- ion. Therefore the formula of the product is NaCl. We know that chlorine is diatomic so the initial, unbalanced equation is:

Na +Cl2 -> NaCl

To balance we need there to be the same amount of each element on both sides of the equation . At the moment there are 2 chlorines on the left and only one on the right, so we need 2 NaCl:

Na +Cl2 -> 2NaCl

We now have the right number of chlorines on both sides, but we now have 2 sodiums on the right and only one on the left, so we need 2 Na so the final, balanced equation would be:

2Na +Cl2 -> 2NaCl

Step 2

Now that we have the equation, we first want to work out the number of moles we have of every reagent. In this example, we are told that we have one mole of chlorine. We have 24.2g of sodium metal, and one mole of sodium would weigh 23g. Therefore we have:

24.2/23 = 1.05... moles of sodium, and 1 mole of chlorine (given in question in this example, but if it wasn't you just work out the number of moles of all the reagents makes no difference).

Step 3

We now need to look at the balanced equation and the number of moles we have to work out the limiting reagent. What does this mean? Well imagine you were making a cake, and each cake took 3eggs and a kg of flour. If you only had 3kg of flour, even if you had 100 eggs you would still only be able to make 3 cakes, so the flour would be the limiting reagent.

So we know from the balanced equation that for every 2 moles of sodium reacting we need 1 mole of chlorine. However, you should clearly be able to see that we do not have twice as much sodium as there is chlorine. Even though we have 1 mole of chlorine, since we do not have 2 moles of sodium the chlorine is in excess.

Step 4

Now that we have identified the sodium as the limiting reagent, we are able to establish the expected yield. Looking back at the original equation, the number of moles of Na is the same as the number of moles of NaCl. Therefore if we have 1.05... moles of Na, we would theoretically expect to get 1.05... moles of NaCl.

Therefore the expected yield in grams would be (mass of one mole) x (number of moles we have). The expected yield is therefore 1.05... x (23+35.5) = 61.6g

Step 5

Now we have the expected yield we can work out the % yield.

% yield = mass we got/ expected mass x 100

= 26.4/61.6 *100 = 42.9%

Written for something else, but if you read through should hopefully answer your question

Reply 103

Dinasaurus

Original post by FemaleBo55

Yepp

& it's 'chemically' killing me & I feel like crying :lol:

(What deep poetic rhyme LOL :u:

)

Does it rhyme? I guess if you put me on the next line.

Yeah it's pretty annoying, I've barely started F325 D:

Reply 104

FemaleBo55

Original post by Dinasaurus

Does it rhyme? I guess if you put me on the next line.

Yeah it's pretty annoying, I've barely started F325 D:

This is how you know I don't read over what I've written 😂 In my head I thought I erased because I wrote something else instead of the "poem" but I guess my brain worked faster than my hands 🙄

I'm hoping you mean revision for it lol and not lesson 🙈 I know exactly how you feel! I'm in the same position as you!!! I haven't even touched or opened my books nor made any notes in F335 😭 Because I have the long annoying coursework for chemistry!!! It's been a month and I am no where near finished with it!

Reply 105

Dinasaurus

Original post by FemaleBo55

I did Chemistry A2 last year and failed, so I'm resitting but I've only done like 1 'lesson' for F325. I have another tomorrow, luckily for me I seem to remember quite a bit from last year so it's not as bad as having to learn it from scratch but still a bit worrying.

Reply 106

olivia7001

Does anyone have a document of all equations you need to know for AQA A2?

Reply 107

Dr Cobbold

I was wondering since our upcoming chemistry exams are coming up if people wanted to be part of a chemistry whatsapp revision group. I guessing most of us use whatsapp so I guess it will be more suitable for everyone then. If you are interested either pm me or responded to this message

Posted from TSR Mobile

Reply 108

FemaleBo55

Can someone please explain how I do 2b, it's worth 5 marks -.-
The answer is in the spoiler LOL :tongue:

Spoiler

Reply 109

samb1234

Original post by FemaleBo55

Can someone please explain how I do 2b, it's worth 5 marks -.-
The answer is in the spoiler LOL :tongue:

Spoiler

You can work out how many moles of NaOH react, which will be the same as the number of moles of acid. There is four times that number of moles in whole sachet, as you use one quarter of the solution in the titration. You now have a number of moles and can just multiply by the mass of the acid then divide by 25

Reply 110

Ladymusiclover

Anyone know the difference between the chemical and physical properties of transition metals? I'm getting confused,

Reply 111

olivia7001

Does anyone know when it's electrophillic/nucleophillic substitution and when it's addition-elimination?
(For aqa a2)

Reply 112

samb1234

Original post by Ladymusiclover

Anyone know the difference between the chemical and physical properties of transition metals? I'm getting confused,

Physical properties are things like appearance, hardness, solids etc etc, whereas chemical properties are things about how they react eg form complexes with ligands

Reply 113

Dohaeris

Hmm... is most of this thread AQA-focused?

I'm CIE, and our questions seem massively different to those being asked here. Is there actually a big difference between CIE and AQA Chemistry paper/syllabus-wise?

Reply 114

thatcooldude2.0

Original post by olivia7001

Does anyone know when it's electrophillic/nucleophillic substitution and when it's addition-elimination?
(For aqa a2)

Electrophile - positive species
Nucleophile - negative species
Substitution - replacing an atom/ group with another
So Nucleophilic substitution when a negative species (eg OH-) replaces a group/atom. Electophilic sub when a positive species replaces a group/atom.
Addition - a group/ molecule is added to compound (Eg electophilic addition of HBr to an alkene -> bromoalkane)
Elimination - a group/ molecule is taken away (/eliminated) from the compound (usually H2O in dehydration reactions such as dehydration of alcohols -> alkenes)
Hope this helps :smile:

Original post by thatcooldude2.0

Not sure if you do aqa or not but I see on the markscheme sometimes it will say 'addition-elimination' as the name of the mechanism and sometimes says 'substitution' and idk if there's a difference?
Thanks for ur help btw

Reply 117

P____P

Original post by olivia7001

Addition elimination is a 2-step reaction mechanism (the elimination part and then the addition of the nucleophile that is attacking) (AQA), but sub. has one step, and could be an electrophile or a nucleophile.

you probably know that they are 2 different mechanisms, any idea what paper you found that on? it was probably only in that year they accepted that.

hope the mini-mindmap helps. (not everything you need though)

MECH.pdf314.1KB

Reply 118

Ladymusiclover

Original post by samb1234

Physical properties are things like appearance, hardness, solids etc etc, whereas chemical properties are things about how they react eg form complexes with ligands

Thanks!

Reply 119

flynnmoore 1597

Hi guys. Having some serious trouble with chemistry :frown:

I've been getting Ds in tests & assessments but I need a B for university! I understand things in class but exam questions make no sense to me :frown:

Help?