AQA CHEM2 ~ May 23rd 2012 ~ AS Chemistry

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Reply 160

berryripple

Getting close :afraid:

Reply 161

MedicalMayhem

I've got a question about energetics and wondered if someone could clarify something. For a calorimetry experiment, the displacement ones, what is the value of m?

So say you have 5g of x added to 50cm^3 of y. Would m be 50, or 55?

Reply 162

clownfish

Original post by MedicalMayhem

m is 50. It is the mass of the water, not the mass of the reactant.

Reply 163

Luppy021

pretty confident for this exam for some reason :s-smilie:

Reply 164

MedicalMayhem

Original post by clownfish

m is 50. It is the mass of the water, not the mass of the reactant.

Thanks a lot!

I've got another question, which I'm not sure whether you could help with :/ When you have reactions of group 2 metals in water, why do some form hydroxides whereas some form oxides? What's the reasoning behind that? :redface:

Reply 165

clownfish

Original post by MedicalMayhem

It's to do with reactivity of the metal and stability of the compound. Although theoretically they all form hydroxides, magnesium hydroxide is very unstable at high temperatures (the exact conditions required for magnesium to react with water), therefore it decomposes into magnesium oxide. The more reactive metals Ca, Sr, Ba all easily react with water to form soluble hydroxides.

Take a look at this link: http://www.google.co.uk/url?sa=t&rct=j&q=group%202%20water%20reactions&source=web&cd=10&ved=0CGUQFjAJ&url=http%3A%2F%2Fwww.a-levelchemistry.co.uk%2FAQA%2520AS%2520Chemistry%2FUnit%25202%2F2.6%2520Group%25202%2C%2520the%2520Alkaline%2520Earth%2520Metals%2F2.6%2520notes.doc&ei=NUpvT7vsL4z68QPXobG_DQ&usg=AFQjCNETgjVZQaJuJhPtB9u9L6nDEsujjw&cad=rja

Reply 166

MedicalMayhem

Original post by clownfish

Ah okay, it doesn't say on the spec, but do you think that we are required to know that?

Reply 167

clownfish

Original post by MedicalMayhem

Ah okay, it doesn't say on the spec, but do you think that we are required to know that?

No i think you just need to know they get more reactive as you go down the group, and so the reaction is more vigorous, and to explain why this trend occurs (increase in shielding, so atomic radius increases, so outer electrons are easier to lose).

58 days till CHEM2:P

Original post by Sorro10

58 days till CHEM2:P

How many topics have you completed? We have so much still to do! :frown:

Reply 170

MedicalMayhem

Erm today we did some stuff on solubility of halides, and learned about the different precipitates. But what's the different to say a white precipitate in a solution and a white solution? As you generally say it's a colourless solution, but would you say it changes to a white ppt? (I'm a bit confused between the two)

Reply 171

Silverland

A white precipitate is separate from the solution.
The solution itself will not be white, but the solid formed separate from it will be white.

Reply 172

MedicalMayhem

Original post by Silverland

A white precipitate is separate from the solution.
The solution itself will not be white, but the solid formed separate from it will be white.

So if something is white - opaque, then it's a white solution?

But cloudy with white floating things, is a white precipitate? Do you mention that it's cloudy, or is it a given? Or would you say that it's a cloudy solution with a white precipitate?

Reply 173

Solza

Original post by MedicalMayhem

You would always say it is a cloudy solution with a white precipitate for anything you are describing with 'white floating things'. Lost 20 marks in an assignment for just failing to put cloudy solution :/

Reply 174

MedicalMayhem

Original post by Solza

Wow, 20 marks?! Hm thanks :smile:

Reply 175

Solza

Original post by MedicalMayhem

Wow, 20 marks?! Hm thanks :smile:

Yea I know, very tough marking criteria and no worries :smile:

Reply 176

Sorro10

Original post by 3.14Pie

How many topics have you completed? We have so much still to do! :frown:

We've finished, but we got told to learn group 2 and extraction of metals at home ourselves which I've done so yea done the whole syllabus now lol.

Reply 177

Sorro10

NEED KINETICS HELP:

Explain why the flour dust in a flour milling factory may explode, whereas it is difficult to ignite ordinary flour in the home. (3 marks)

The question is in the Kinetics topic, I think it's got to do with Concentration of Flour in the air of the factory is greater than at home but idk where to go after that if that is right, work 3 marks..

Reply 178

Sorro10

Another question on Kinetics:

1) Consider the following reaction: Zn(s) + 2HCl(aq) --> ZnCl₂(aq) + H₂(g)

Explain the effect, if any, on the rate of reaction of:

- (i) Increasing the pressure (2 marks)
- (ii) Adding water (2 marks)
- (iii) Using hydrobromic acid (HBr), on the same concentration, instead of hydrochloric acid.

For part

- (i) No effect as it is not in a gaseous state?
- (ii) Not sure
- (iii) Not sure ;p

Help pl0x

Reply 179

3.14Pie

Original post by Sorro10

We've finished, but we got told to learn group 2 and extraction of metals at home ourselves which I've done so yea done the whole syllabus now lol.