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The Informant
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Ask me any problems you might be having with AS Biology or Chemistry Units 1, 2 and 3
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G1998
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which board do you do?
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The Informant
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AQA for both

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honeymoon
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I'm not really sure about the differences between reflux and distillation and the apparatus required. Could you possibly help me please?



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The Informant
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Honeymoon,

This is a great question. This is in the alcohols topic (chapter 16 AQA textbook). In order to answer your question, you need to know some basic things bout alcohols.
On our AS spec, we need to know three types of alcohols:
- 1o alcohols
- 2o alcohols
- 3o alcohols
You should know that they all react differently. The oxidising reagent that is used in the oxidation of these alcohols, is acidified potassium dichromate solution, (H+/K2Cr2O7(aq))
- 1o alcohols can be partially oxidisied to form aldehydes (distillation)
- 1o alcohols can be completely oxidised to form carboxylic acids (reflux)
- 2o alcohols can be oxidised to form ketones
- 3o alcohols cannot be oxidised

REFLUX (forming Carboxylic acid)

Heating under reflux essentially means that you heat the alcohol, with the oxidising agent, without losing any volatile solvents, reactants or products. The apparatus set up (on the right) shows you how it is achieved. As the alcohol is oxidised, aldehydes are formed, but since they have a lower boiling point, they begin to evaporate. The water jacket, or Liebig condenser condenses the aldehyde as it is formed, causing it to drip back into solution, and allow for complete oxidation to occur. This means that the alcohol can be completely oxidised to form the carboxylic acid that we want, without losing anything, and ensuring that minimal aldehyde is formed in the proxess.

DISTILLATION(aldehyde)

The important phrase, for the formation of aldehydes is: "distill as it forms". We cannot use the reflux set up to form aldehydes, as complete oxidation will take place, producing the carboxylic acid. Instead, as mentioned above, as the aldehyde has a lower boiling point, it will evaporate as it is formed, and hence, as a gas, we can condense it and remove it as it forms. A branched arm, the Liebig condenser (again) allows us to distill the product as it forms. Note the angle of the water jacket, it is angled downwards, which means that as the aldehyde condenses, it drips into the receiving flask, and not back into the distilling flask.

If you want to produce a ketone, you can use either set up, provided that you know it is a 2o acohol, as partial or complete oxidation will result in a ketone being produced anyway.

Hope that answers your question
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honeymoon
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(Original post by The Informant)
Honeymoon,

This is a great question. This is in the alcohols topic (chapter 16 AQA textbook). In order to answer your question, you need to know some basic things bout alcohols.
On our AS spec, we need to know three types of alcohols:
- 1o alcohols
- 2o alcohols
- 3o alcohols
You should know that they all react differently. The oxidising reagent that is used in the oxidation of these alcohols, is acidified potassium dichromate solution, (H+/K2Cr2O7(aq))
- 1o alcohols can be partially oxidisied to form aldehydes (distillation)
- 1o alcohols can be completely oxidised to form carboxylic acids (reflux)
- 2o alcohols can be oxidised to form ketones
- 3o alcohols cannot be oxidised

REFLUX (forming Carboxylic acid)

Heating under reflux essentially means that you heat the alcohol, with the oxidising agent, without losing any volatile solvents, reactants or products. The apparatus set up (on the right) shows you how it is achieved. As the alcohol is oxidised, aldehydes are formed, but since they have a loer boiling point, they begin to evaporate. The water jacket, or Liebig condenser condenses the aldehyde as it is formed, causing it to drip back into solution, and allow for complete oxidation to occur. This means that the alcohol can be completely oxidised to form the carboxylic acid that we want, without losing anything, and ensuring that minimal aldehyde is formed in the proxess.

DISTILLATION(aldehyde)

The important phrase, for the formation of aldehydes is: "distill as it forms". We cannot use the reflux set up to form aldehydes, as complete oxidation will take place, producing the carboxylic acid. Instead, as mentioned above, as the aldehyde has a lower boiling point, it will evaporate as it is formed, and hence, as a gas, we can condense it and remove it as it forms. A branched arm, the Liebig condenser (again) allows us to distill the product as it forms. Note the angle of the water jacket, it is angled downwards, which means that as the aldehyde condenses, it drips into the receiving flask, and not back into the distilling flask.

If you want to produce a ketone, you can use either set up, provided that you know it is a 2o acohol, as partial or complete oxidation will result in a ketone being produced anyway.

Hope that answers your question
thank you so much, that really helps i really appreciate it
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The Informant
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(Original post by honeymoon)
thank you so much, that really helps i really appreciate it
You're more than welcome
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mevish98
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How does water effetlct the enthalpy value?
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The Informant
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What do you mean and in what context?
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The Informant
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(Original post by haemo)
Hi.

When I do AQA past papers, and when 'How science works' questions come up to do with vaccines, I hardly ever get them correct.

I would like to know the basics (this is very vague, I apologise) on vaccines (without going into T and B cells/phagocytosis). For example, a question asked 'Why do we populate a large population' or something along those lines, and the mark scheme wanted you to say that non-vaccinated people would contact vaccinated people and that it destroys the virus/virus is not carried in vaccinated people.


Haemo,

I think you're slightly confused about vaccination. What you are talking about is most likely herd immunity. This is the resistance of a group to a pathogen, due to immunity of a large proportion of the group to that population.
In any poopulation, you will have people of all types, the young, the old, the disabled, and everyone else. Not everyone can have a vaccination (religous reasons, weak immune systems etc.), so to protect the vulnerable people, everyone else will try to be vaccinated. This means that the pathogen has no carrier in the population, meaning that the vulnerable people are less likely to become infected.
Instead, if no vaccination was done, potentially everyone in the community could become infected, which would be detremental. Via herd immunity, we can protect people.

Some style of questions are just learning the mark schemes for the points. I've found some good immunity questions of varying marks, and listed the available marking points for you as well:
1. What is a pathogen? [1 mark]
  1. (Micro)organism that causes disease/harm to body/an immune response

2. When a pathogen enters the body it may be destroyed by phagocytosis. Describe how [4 marks]

  1. Phagocyte attracted by a substance/recognises (foreign) antigen
  2. (Pathogen) engulfed/ingested
  3. Enclosed in a vacuole/vesicle/phagosome
  4. (Vacuole) fuses/joins with lysosome
  5. Lysosome contains enzymes
  6. Pathogen digested/molecules hydrolysed

3. When a pathogen causes an infection plasma cells secrete antibodies which destroy this pathogen. Explain why these antibodies are only effective against a specific pathogen [2 marks]

  1. Antigens (on pathogen) are a specific shape/have specific tertiary/3D structure
  2. Antibody fits/binds/is complementary to antigen/antibody-antigen complex forms

OR

  1. Antibodies are a specific shape/have specific tertiary/3D structure
  2. Antigens (on pathogen) fit/bind//are complementary to antibody/antibody-antigen complex forms

4. Scientists use an antibody to detect an antigen on the bacterium that causes stomach ulcers. Explain why the antibody will only detect this antigen [3 marks]

  1. Antibody/variable region has specific amino acid sequence/primary structure
  2. The shape/tertiary structure of the binding site
  3. Complementary to/fits/binds with these antigens
  4. Forms complex between antigen and antibody

5. Vaccines protect people against disease. Explain how [5 marks]

  1. Vaccines contain antigens/antigens are injected
  2. Dead pathogens/weakened pathogens
  3. Memory cells made
  4. On second exposure memory cells produce antibodies/become active/recognise pathogens
  5. Rapidly produce antibodies/produces more antibodies
  6. Antibodies destroy pathogens
  7. Herd effect/fewer people to pass on disease


Remember:
  1. Underlined words are needed for the mark
  2. (Brackets) are not necessary for the mark

Hope this answers your question!
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skittles100
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Why is a water bath used instead of a bunsen burner for an alcohol experiment?

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logicmaryam
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Why is a water bath used instead of a bunsen burner for an alcohol experiment?

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water bath heats it gently which is important in the oxidation of primary alcohols to aldehydes. for primary alcohols to carboxylic acids a reflux is used
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logicmaryam
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did anyone get carbonate ions for chem task 2? What is the test for it besides the limewater one? some people got effervescence but I didnt
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The Informant
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(Original post by logicmaryam)
did anyone get carbonate ions for chem task 2? What is the test for it besides the limewater one? some people got effervescence but I didnt
There was effervescence involved with one of the solution in task 2

You can test the presence of carbonate ions by adding BaCl2, this forms a white precipitate, BaCO3, which dissolves on the addition of HNO3, giving rise to effervescnece
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skittles100
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(Original post by logicmaryam)
water bath heats it gently which is important in the oxidation of primary alcohols to aldehydes. for primary alcohols to carboxylic acids a reflux is used
oh so you would use a waterbath to make sure there is no complete oxidation to aldehydes?
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Gaiaphage
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(Original post by skittles100)
Why is a water bath used instead of a bunsen burner for an alcohol experiment?

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(Original post by logicmaryam)
water bath heats it gently which is important in the oxidation of primary alcohols to aldehydes. for primary alcohols to carboxylic acids a reflux is used
Actually that's not true, it's because alcohols are highly flammable and a bunsen burner may set the sample alight but a water bath will not.
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The Informant
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(Original post by skittles100)
oh so you would use a waterbath to make sure there is no complete oxidation to aldehydes?
You can still use a bunsen burner. In one of my earlier answers, you just "distil as it forms" to obtain the aldehyde
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logicmaryam
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(Original post by Gaiaphage)
Actually that's not true, it's because alcohols are highly flammable and a bunsen burner may set the sample alight but a water bath will not.
Thanks for clearing that up ^^ makes sense.
PS is your name from the Gone series?
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skittles100
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(Original post by The Informant)
You can still use a bunsen burner. In one of my earlier answers, you just "distil as it forms" to obtain the aldehyde
but using a water bath is easier? im trying to form a answer
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Gaiaphage
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(Original post by logicmaryam)
Thanks for clearing that up ^^ makes sense.
PS is your name from the Gone series?
No problem! Yes it is, I'm impressed
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