AS Chemistry Past Year- P1 May/June2011/1 Watch

Sindy12
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So this is a 2011 past year question.

Q16. A student observed the reactions when NaCl and NaI were each reacted with conc. H2SO4 and with conc. H3PO4. The observations are recorded in the table.

NaCl+H2SO4---> colourless acidic gas
NaCl+ H3PO4---> colourless acidic gas
NaI+ H2SO4---> purple fume
NaI+ H3PO4---> colourless acidic gas

which deduction can be made from these observation?

A. Conc. H3PO4 is a stronger oxidising agent than conc. H2SO4
B. Conc. H3PO4 is a stronger oxidising agent than iodine
C. Conc. H2SO4 is a stronger oxidising agent than chlorine
D. Conc. H2SO4 is a stronger oxidising agent than iodine

The answer according to the mark scheme is D. I know H2SO4 oxidised I- to I2 but I2 didn't oxidise anything in the above reactions, how can you compare?
After all, iodine is a weak oxidising agent, H3PO4 could be a stronger oxidising agent than iodine!

Anyone, please advise.
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Sindy12
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Err...I encounter a past year question which I don't understand again.

May June 2012 variant 1, paper 1

Q13. When a mineral was heated with Bunsen flame to constant mass, a colourless gas that turned limewater milky was evolved. The remaining solid was cooled, and then added to HCl(aq). Vigorous effervescence was seen. What was the mineral?

A. CaCO3
B. MgCO3.Mg(OH)2.3H2O
C. BaCO3.CaCO3
D. CaCO3.MgCO3

The answer according to mark scheme is C. How can it be?
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charco
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(Original post by Sindy12)
So this is a 2011 past year question.

Q16. A student observed the reactions when NaCl and NaI were each reacted with conc. H2SO4 and with conc. H3PO4. The observations are recorded in the table.

NaCl+H2SO4---> colourless acidic gas
NaCl+ H3PO4---> colourless acidic gas
NaI+ H2SO4---> purple fume
NaI+ H3PO4---> colourless acidic gas

which deduction can be made from these observation?

A. Conc. H3PO4 is a stronger oxidising agent than conc. H2SO4
B. Conc. H3PO4 is a stronger oxidising agent than iodine
C. Conc. H2SO4 is a stronger oxidising agent than chlorine
D. Conc. H2SO4 is a stronger oxidising agent than iodine

The answer according to the mark scheme is D. I know H2SO4 oxidised I- to I2 but I2 didn't oxidise anything in the above reactions, how can you compare?
After all, iodine is a weak oxidising agent, H3PO4 could be a stronger oxidising agent than iodine!

Anyone, please advise.
Oxidising agents gain electrons by removing electrons from other species. In the reaction between HI and sulfuric acid the acid is removing electrons from the iodide(-1) making iodine(0).

This suggests that sulfuric acid is a stronger oxidising agent than iodine (it can't get the electrons back)

In the reaction the sulfuric acid is reduced to sulfur dioxide (VI --> IV)
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charco
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(Original post by Sindy12)
Err...I encounter a past year question which I don't understand again.

May June 2012 variant 1, paper 1

Q13. When a mineral was heated with Bunsen flame to constant mass, a colourless gas that turned limewater milky was evolved. The remaining solid was cooled, and then added to HCl(aq). Vigorous effervescence was seen. What was the mineral?

A. CaCO3
B. MgCO3.Mg(OH)2.3H2O
C. BaCO3.CaCO3
D. CaCO3.MgCO3

The answer according to mark scheme is C. How can it be?
Presumably barium carbonate does not decompose on heating ...
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Sindy12
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(Original post by charco)
Presumably barium carbonate does not decompose on heating ...
I googled it just now and found out that although MgO and CaO are basic oxides...they dont react with acids to form H2! In fact, BaCO3+HCl=BaCl+H2O+CO2

Oh my...I always thought the effervescence was because of hydrogen gas!

Thank you anyway!
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charco
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(Original post by Sindy12)
I googled it just now and found out that although MgO and CaO are basic oxides...they dont react with acids to form H2! In fact, BaCO3+HCl=BaCl+H2O+CO2

Oh my...I always thought the effervescence was because of hydrogen gas!

Thank you anyway!
Your equation is incorrect: BaCO3 + 2HCl --> BaCl2 + CO2 + H2O

There are no oxides that react with acids to form hydrogen!!!

These are your general reactions of acids (you should learn them)

acid + reactive metal --> salt + hydrogen (this is a redox reaction)

acid + metal oxide --> salt + water (this is neutralisation)

acid + carbonate --> salt + water + carbon dioxide

acid + hydrogencarbonate --> salt + water + carbon dioxide

acid + sulfite --> salt + water + sulfur dioxide
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