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OCR Chemistry A Exam Thread (Breadth - May 27 2016 and Depth - June 10 2016) Watch

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    (Original post by Cherx)
    Oh gosh not that.. Hoping they don't ask of calorimetry I mean they did have quite a hefty calculation question on it.

    Ah ok. I need to go over it too. Lmao I'm going over so many stuff rn. The only green chemistry we have is the polymer stuff isn't it? So waste polymers you could make into other new polymer, sort and recycle, organic feedstock.
    yh I think so, also combustion for energy.
    Good luck
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    (Original post by JamieHarris1998)
    Test for sulfates is adding dilute HCl then BaCl and if a white precipitate forms a sulfate is present.
    I can't remember the calorimetry test
    Oxidation might ask about why we reflux (prevents the loss of volatile liquids by preventing the evapourated liquids from escaping by condensing them back).
    Also about why we left the top open, which was to prevent explosions from pressure created by heating gas which would be danagerous if sealed by a stopper when condensing
    Ah so we do reflux so the vapours are condensed and returned to the vessel to ensure it undergoes full oxidation. As for the top open on what the test tube? Thats a really good point! Didn't think about that! Also the anti bumbing granules are to prevent against violent boiling.Which can then be easily filtered out.
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    Revising these practicals just gave me a flashback to when I tried picking up the boiling tripod after finishing reflux and scolded my hand. Was so stupid
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    (Original post by 4nonymous)
    yh I think so, also combustion for energy.
    Good luck
    Thank you! I still have another good 30 mins ahahah. Hmm maybe go over the synthetic routes. Is this likely to come up?
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    (Original post by Cherx)
    Thank you! I still have another good 30 mins ahahah. Hmm maybe go over the synthetic routes. Is this likely to come up?
    there was a longish question on that in the breadth, so i doubt it
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    (Original post by Cherx)
    Ah so we do reflux so the vapours are condensed and returned to the vessel to ensure it undergoes full oxidation. As for the top open on what the test tube? Thats a really good point! Didn't think about that! Also the anti bumbing granules are to prevent against violent boiling.Which can then be easily filtered out.
    For reflux, you use a pear shaped flask and a condenser which prevents the vapours from escaping from the top.
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    (Original post by Cherx)
    Thank you! I still have another good 30 mins ahahah. Hmm maybe go over the synthetic routes. Is this likely to come up?
    I'm not sure, I don't think I'm gonna go over it
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    (Original post by amysed)
    there was a longish question on that in the breadth, so i doubt it
    I didn't really know the synthetic routes that well so left that out..

    (Original post by 4nonymous)
    I'm not sure, I don't think I'm gonna go over it
    Might go over disproportionation quickly now instead.
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    I think the main practical based questions that might come up is either qualitative analysis, titrations or purifying an organic liquid.
    Titrations:
    Preparing a standard solution-
    1. Weigh out a particular mass of the solid.
    2. Add it to a beaker.
    3. Dissolve all of it in distilled water.
    4. Add the solution to a 250cm3 volumetric flask.
    5. Make up the solution to the graduation line (the bottom of the meniscus must be touching the graduation line)
    6. Invert the flask several times to mix thoroughly.
    Actual Titration:
    1. Take 25cm3 of the standard solution using a pipette and add it to the conical flask (this is because the concentration and volume of the standard solution is known).
    2. Add the solution with the unknown concentration to the burette. Make sure the jet space of the burette is filled with the solution.
    3. Add an indicator (eg.methyl orange which turns orange at end point or phenolphthalein which turns pink at endpoint)
    4. Titrations the solutions approximately once until endpoint is reached to get an idea of the titre.
    5. Repeat the titration several times to get several titres.
    6. Calculate the mean titres by taking an average of concordant results (within 0.1 cm3 of each other)

    I think that's all. Did I miss anything out?



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    (Original post by snickercell)
    So if they give us a table of formation values to work out combustion, would you do products- reactants?! Or the other way around?
    Products - reactants as formation values given
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    (Original post by Cherx)
    I didn't really know the synthetic routes that well so left that out..



    Might go over disproportionation quickly now instead.
    Thats a good idea. I think maybe the reaction with chlorine and water or chlorine and NaOH may come up
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    KiwiFruit What's purifying an organic liquid?
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    (Original post by kiwifruit99)
    i think the main practical based questions that might come up is either qualitative analysis, titrations or purifying an organic liquid.
    Titrations:
    Preparing a standard solution-
    1. Weigh out a particular mass of the solid.
    2. Add it to a beaker.
    3. Dissolve all of it in distilled water.
    4. Add the solution to a 250cm3 volumetric flask.
    5. Make up the solution to the graduation line (the bottom of the meniscus must be touching the graduation line)
    6. Invert the flask several times to mix thoroughly.
    Actual titration:
    1. Take 25cm3 of the standard solution using a pipette and add it to the conical flask (this is because the concentration and volume of the standard solution is known).
    2. Add the solution with the unknown concentration to the burette. Make sure the jet space of the burette is filled with the solution.
    3. Add an indicator (eg.methyl orange which turns orange at end point or phenolphthalein which turns pink at endpoint)
    4. Titrations the solutions approximately once until endpoint is reached to get an idea of the titre.
    5. Repeat the titration several times to get several titres.
    6. Calculate the mean titres by taking an average of concordant results (within 0.1 cm3 of each other)

    i think that's all. Did i miss anything out?

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    thank you so much
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    (Original post by 4nonymous)
    Thats a good idea. I think maybe the reaction with chlorine and water or chlorine and NaOH may come up
    Yep so Cl2 and H20 -> HClO and HCL

    Then there's the reaction with dilute NaOH (aq)

    You make NaClO and NaCl and H20.
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    (Original post by 4nonymous)
    thank you so much
    Yes thank you kiwi! I coudnt think of anything else to add. I mean they could ask extra questions like why use a white tile. Or why your answer may be less. Something about losing moles when transferring to the beaker? Erm add above the meniscus lowers the conectration? These are off the top of my head so could be wrong.
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    Qualitative analysis:
    For anions-
    The sequence of tests is carbonate then sulphate then halide.
    Carbonates:
    1. Add dilute acid (preferable HNO3)
    2. A salt, water and water and carbon dioxide is produced. The carbon dioxide can be tested by using limewater which forms a cloudy precipitate.
    Sulphates-
    1. Add a barium compound (such as barium nitrate) to the mixture.
    2. If sulphates are present, an barium sulphate (an insoluble white precipitate is formed).
    Halides:
    1. Silver nitrate is added to mixture.
    2. AgCl is a white precipitate, AgBr is a cream precipitate and AgI is a yellow precipitate.
    4. AgCl dissolves in dilute ammonia, AgBr dissolves in concentrated ammonia and AgI is insoluble in ammonia.

    For cations-
    Ammonium ion-
    1. Add sodium hydroxide solution to the solution.
    2. Ammonia gas should be produced. This is pungent and turns litmus paper blue.

    Does anyone know the equation for the ammonium ion test?


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    (Original post by asinghj)
    Products - reactants as formation values given
    Oh ok, thank you and good luck!
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    Purifying an organic liquid:
    We normally do this after a distillation.
    1. Put the distillate of organic liquid into a separating funnel.
    2. Add sodium chloride to separate the aqueous layer from the organic layer.
    3. add sodium hydrogen carbonate to react an remaining acid.
    4. Run organic layer into a conical flask
    5. Add a drying agent (such as magnesium sulphate) to remove waters of crystallization.
    6. Distilled the remaining liquid to obtain pure product.




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    (Original post by Cherx)
    Yes thank you kiwi! I coudnt think of anything else to add. I mean they could ask extra questions like why use a white tile. Or why your answer may be less. Something about losing moles when transferring to the beaker? Erm add above the meniscus lowers the conectration? These are off the top of my head so could be wrong.
    You're welcome guys 😊👍


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    (Original post by kiwifruit99)
    You're welcome guys 😊👍


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    Oh yes when you transfer the solution add the washings of the beaker to the volumetric flask!
 
 
 
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