Hey there! Sign in to join this conversationNew here? Join for free
    • Political Ambassador
    • Thread Starter
    Offline

    15
    ReputationRep:
    Political Ambassador
    (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)
    Offline

    14
    ReputationRep:
    can someone explain how to find limiting reagents and what use is it?
    Offline

    10
    ReputationRep:
    (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
    Offline

    22
    ReputationRep:
    (Original post by FemaleBo55)
    Yepp & it's 'chemically' killing me & I feel like crying :lol:

    (What deep poetic rhyme LOL)
    Does it rhyme? I guess if you put me on the next line.

    Yeah it's pretty annoying, I've barely started F325 D:
    • Political Ambassador
    • Thread Starter
    Offline

    15
    ReputationRep:
    Political Ambassador
    (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!
    Offline

    22
    ReputationRep:
    (Original post by FemaleBo55)
    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!
    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.
    Offline

    2
    ReputationRep:
    Does anyone have a document of all equations you need to know for AQA A2?
    Offline

    3
    ReputationRep:
    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
    • Political Ambassador
    • Thread Starter
    Offline

    15
    ReputationRep:
    Political Ambassador
    Name:  Screen Shot 2016-03-08 at 20.40.23.png
Views: 175
Size:  51.6 KB

    Can someone please explain how I do 2b, it's worth 5 marks -.-
    The answer is in the spoiler LOL
    Spoiler:
    Show
    48.4%
    Offline

    10
    ReputationRep:
    (Original post by FemaleBo55)
    Name:  Screen Shot 2016-03-08 at 20.40.23.png
Views: 175
Size:  51.6 KB

    Can someone please explain how I do 2b, it's worth 5 marks -.-
    The answer is in the spoiler LOL
    Spoiler:
    Show
    48.4%
    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
    Offline

    19
    ReputationRep:
    Anyone know the difference between the chemical and physical properties of transition metals? I'm getting confused,
    Offline

    2
    ReputationRep:
    Does anyone know when it's electrophillic/nucleophillic substitution and when it's addition-elimination?
    (For aqa a2)
    Offline

    10
    ReputationRep:
    (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
    Offline

    3
    ReputationRep:
    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?
    Offline

    9
    ReputationRep:
    (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
    Offline

    3
    ReputationRep:
    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


    Posted from TSR Mobile
    Offline

    2
    ReputationRep:
    (Original post by thatcooldude2.0)
    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
    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
    Offline

    2
    ReputationRep:
    (Original post by olivia7001)
    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
    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)
    Attached Images
  1. File Type: pdf MECH.pdf (306.7 KB, 101 views)
    Offline

    19
    ReputationRep:
    (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!
    Offline

    0
    ReputationRep:
    Hi guys. Having some serious trouble with chemistry 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 Help?
 
 
 
Poll
Do you agree with the PM's proposal to cut tuition fees for some courses?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.