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# AQA GCSE Chemistry - C2 & C3 (14th May 2015) watch

1. Please can someone describe the 3 salts methods (i.e bases + acid, alkali + acid, insoluble salts, soluble salts)... It would really help
2. (Original post by amber.991)
Step 1: work out the number of moles in the burette (this will be the substance which you know both the volume AND concentration of) moles = vol x conc (remember, you will be given a volume value in cm3 but you need to use it in dm3, so just divide by 1000)

Step 2: work out the number of moles in the flask - this is done by working out the ratio of each substance for example CH3COOH + NaOH you have 1:1 ratio, but if it was CH3COOH + 2NaOH the ratio would be 1:2. Depending on the ratio, use the moles from the burette to work it out. So, if the ratio is 1:1 the moles will be exactly the same. If it's 1:2 the moles in flask will be double the moles in the burette.

Step 3: work out the concentration of the substance: conc = vol / moles

Sorry if this is confusing, tell me if you need more help.
(Original post by zc555)
You make it sound so straight forward! Thank you so much! I'll have a go practising this now.
an example from my-GCSEScience:
http://imgur.com/jLveoS8
3. (Original post by harryleavey)
Please can someone describe the 3 salts methods (i.e bases + acid, alkali + acid, insoluble salts, soluble salts)... It would really help
I've been revising these all day so hopefully this is right!

Making salts from acid + metal or insoluble base
1. Add the metal/metal oxide/metal hydroxide to the acid
2. The reaction will have finished when the excess solid sinks to the bottom of the conical flask.
3. Filter out the excess metal/metal oxide/metal hydroxide and you will have the salt solution left.
4. Evaporate some of the water to concentrate the solution.
5. Leave the rest to evaporate slowly. This is known as crystallisation.

I've got to go but if no one else does the rest, I'll explain later
4. (Original post by XxRAEXx)
C3-
Could be-
describing a caliormetry/titration experiment
Testing for ions
Hard water/soft water and purification
Haber process
Explaining difference in reactivity of g1 &g7
calorimetry came up as a 6 marker last year.
5. (Original post by spnlove)
Newlands' Law of Octaves:
- noticed every 8th element had similar properties so arranged elements in rows of seven according to atomic mass
- BUT pattern broke down in 3rd row due to transition metals
- Problems: groups contained elements which didn't have similar properties; he mixed up metals and non-metals; didn't leave any gaps for undiscovered elements

Mendeleev:
- arranged the known elements in order of atomic mass
- left gaps to keep elements with similar proplerties in same columns (the groups)
- he could predict properties of undiscovered elements from this

Hope that helps, if you have any questions feel free to ask
Thank you. Can you say why scientists didn't accept Mendeleev and Newlands ideas

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6. (Original post by Nai18)
Thank you. Can you say why scientists didn't accept Mendeleev and Newlands ideas

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Newland's:
Didn't leave gaps
mixed metals and non-metals (e.g. O and Fe)
elements that didn't have similar properties in same group (e.g. C and Ti)

Mendeleev:
Wasn't much evidence (UNTIL new elements were discovered and worked/fitted)
Wasn't much use for a periodic table at the time
7. (Original post by fabsafxo)
hi is that about C2 OR C3???
Haber process is C3!
8. (Original post by Nai18)
Thank you. Can you say why scientists didn't accept Mendeleev and Newlands ideas

Posted from TSR Mobile
Yeah, so for Newland it was mainly because he didn't leave any gaps and also he mixed up properties.

For Mendeleev, scientists just thought the table was a bit of fun and there wasn't much evidence to suggest that the elements fit together in that way. It was only once they found new elements which fit into the gaps that scientists realised it could be a useful tool; especially once the atomic structure (protons,neutrons e.t.c.) had been discovered as that matched up to the table.
9. (Original post by PawanAviator)
A* answer right there!
yeah atoms/particles on the surface of a material are often more reactive than those in the centre, so a larger surface area means the material is more reactive.

c:
10. (Original post by pollyy)
Titration: Is used to find how much acid is needed to neutralise an alkali (or vice versa) which is then used to calculate an unknown concentration.
Did you want help with the calculation or the titration method?

As for the ion tests; to test for halides, add dilute nitric acid (not HCL as that contains chlorine which is a halide!) then add silver nitrate solution, and use the colour of the precipitate to distinguish between halides:
Chlorides- white precipitate
Bromides- cream precipitate
Iodides- yellow precipitates

Sulfate test: add dilute HCL then barium chloride solution, if a WHITE precipitate is formed, then the substance is a sulfate!

Sodium Hydroxide test (for metal ions): Add NaOH, then use colour of precipitate to distinguish-
Iron (II)- Green precipitate
Iron (III)- Reddish brown precipitate
Copper (II)- Light blue precipitate
Magnesium, Calcium and Aluminium ALL form white precipitates, but aluminium ions dissolve in excess NaOH into a colourless solution, and calcium ions can be distinguished through the flame test (red flame).

Hope this helps and I can try and help with titration too!
OMG your a life saver! Thank you so much!!!!! I think I know how to do calculations I just struggle on the actual writing up a Titration Calculation (in a 6 marker)
11. A tip for titration calculations:

An easy formula for titration which incolves no conversions and just one calculation is:

Unknown concentration = Known concentration x (Volume of substance you know conc. of)/(Volume of other substance)

This does work but if marks are awarded for method, this won't necessarily get them! U just use it for checking my answers Hope it helps!
12. What's the difference between intermolecular and electrostatic forces?
13. (Original post by zc555)
OMG your a life saver! Thank you so much!!!!!
haha it's okay! I just posted a quick way to do titration calculations is you need help with that too!
14. (Original post by maxjackson5)
1) Find the Ar's for the elements involved
2) Divide each mass/percentage by the Ar of its element
3)Divide each answer by the smallest answer that you just got
4)Put them into the correct formula (i.e if you had 2 iron and 3 oxygen, the formula would be Fe203).

Hope this helped! If you're struggling just tell me which part you don't understand and I'll message you back
What's the "Ar"
15. (Original post by Kinnon)
What's the "Ar"
Relative atomic mass IIRC
16. (Original post by pollyy)
haha it's okay! I just posted a quick way to do titration calculations is you need help with that too!
Thanks
17. (Original post by metrize)
What's the difference between intermolecular and electrostatic forces?
An electrostatic force is the force of attraction in all directions between oppositely charged ions in an ionic compound like NaCl. (This is what holds the ions in a regular structure, hence ionic substances have high mp and bp, and tend to be of regular shape.)

Intermolecular forces are forces of attraction between molecules in a substance (not found in ionic compounds). These are usually found in substances which have covalent bonds holding atoms together in molecules, but the intermolecular forces hold the MOLECULES together.
18. (Original post by thepurplerain)
I've been revising these all day so hopefully this is right!

Making salts from acid + metal or insoluble base
1. Add the metal/metal oxide/metal hydroxide to the acid
2. The reaction will have finished when the excess solid sinks to the bottom of the conical flask.
3. Filter out the excess metal/metal oxide/metal hydroxide and you will have the salt solution left.
4. Evaporate some of the water to concentrate the solution.
5. Leave the rest to evaporate slowly. This is known as crystallisation.

I've got to go but if no one else does the rest, I'll explain later
For the other two:

Making soluble salt from acid + alkali

1. Add indicator to acid
2. Add alkali until indicator changes colour and record how much alkali was used
3. Repeat this using exactly same volumes of alkali and acid but without the indicator so that the salt isn't contaminated
4. Then evaporate the water to concetrate the solution
5. Leave the rest to evaporate slowly (crystallisation)

Making insoluble salt - Precipitation reaction

1. Mix two solutions containing ions you need (i.e. for lead chloride you could mix lead nitrate solution and sodium chloride solution)
2. Once precipitation has occurred (it will leave a solid), you just filter the solution
3. Wash and dry precipitate on filter paper

If anyone has any corrections for those please tell me cause I find this stuff really hard to remember!
19. main topics to revise for c2 anyone?
20. (Original post by spnlove)
For the other two:

Making soluble salt from acid + alkali

1. Add indicator to acid
2. Add alkali until indicator changes colour and record how much alkali was used
3. Repeat this using exactly same volumes of alkali and acid but without the indicator so that the salt isn't contaminated
4. Then evaporate the water to concetrate the solution
5. Leave the rest to evaporate slowly (crystallisation)

Making insoluble salt - Precipitation reaction

1. Mix two solutions containing ions you need (i.e. for lead chloride you could mix lead nitrate solution and sodium chloride solution)
2. Once precipitation has occurred (it will leave a solid), you just filter the solution
3. Wash and dry precipitate on filter paper

If anyone has any corrections for those please tell me cause I find this stuff really hard to remember!
Thanks. But do you not heat the first one you posted to start the crystallisation?

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