HSC 2012-2015 Chemistry Marathon (archive) (11 Viewers)

[Ekman]

re: HSC Chemistry Marathon Archive

So basically that meaning of amphiprotic also applies for amphoteric and vice versa? Because if a substance reacts with an acid/base it's basically donating/accept a proton?

All amphiprotic substances are amphoteric, but not all amphoteric substances are amphiprotic. For example, Al2O3 is amphoteric but it doesn't contain a hydrogen atom to be classified as amphiprotic

Proton transfer reactions are known as neutralisation reactions. However just because a substance reacts with an acid or base, doesn't necessarily mean its a neutralisation reaction. For example, sulfuric acid can react with iron to form iron sulfate and hydrogen gas. This isn't a neutralisation reaction, its a redox reaction. So not all substances that react with acids or bases are neutralisation reactions.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

All amphiprotic substances are amphoteric, but not all amphoteric substances are amphiprotic. For example, Al2O3 is amphoteric but it doesn't contain a hydrogen atom to be classified as amphiprotic

Proton transfer reactions are known as neutralisation reactions. However just because a substance reacts with an acid or base, doesn't necessarily mean its a neutralisation reaction. For example, sulfuric acid can react with iron to form iron sulfate and hydrogen gas. This isn't a neutralisation reaction, its a redox reaction. So not all substances that react with acids or bases are neutralisation reactions.

This is a good example, thanks!

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Anyone know a simple way to know conjugate acid/base? I know that a conjugate base is the acid with a proton removed and the conjugate acid is the base with a proton added, but I kind of forget this, is there another simple easy to remember way?

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Anyone know a simple way to know conjugate acid/base? I know that a conjugate base is the acid with a proton removed and the conjugate acid is the base with a proton added, but I kind of forget this, is there another simple easy to remember way?

It doesnt get more simpler than that

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Guys, this was taken form 2010 HSC sample answers, basically it describes a titration experiment. But it only says clean, rinse, fill with distilled water for the equipment, in my notes I've got rinse 3 times and lastly with the solution it's going to contain, something like that. What should I write?

 

[psyc1011]

re: HSC Chemistry Marathon Archive

Write your one. If you don't rinse with the solution, then there will be left some water (on the edges) which will dilute the solution that is going into the thing. This decrease precision of titration

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Write your one. If you don't rinse with the solution, then there will be left some water (on the edges) which will dilute the solution that is going into the thing. This decrease precision of titration

Actually i just realised after rereading the sample answer that it's already right, because it already says for the burette and the pipette to clean AND rise the the solution it's going to contain

 

[BlueGas]

re: HSC Chemistry Marathon Archive

How do I answer this question?

 

[Fizzy_Cyst]

re: HSC Chemistry Marathon Archive

Guys, this was taken form 2010 HSC sample answers, basically it describes a titration experiment. But it only says clean, rinse, fill with distilled water for the equipment, in my notes I've got rinse 3 times and lastly with the solution it's going to contain, something like that. What should I write?

Pipette / Burette must be rinsed with water then solution.

Conical flask just water.

 

[Fizzy_Cyst]

re: HSC Chemistry Marathon Archive

How do I answer this question?

CaCl2 --> Ca2+ + 2Cl-

[Cl- ions] = 2x [CaCl2]

[Cl-] = 3x10^-3 mol L^-1
Mass Cl- = 3x10^-3 x 35.45 = 0.106g L^-1 = 106mg L^-1 = 106ppm

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

CaCl2 --> Ca2+ + 2Cl-

[Cl- ions] = 2x [CaCl2]

[Cl-] = 3x10^-3 mol L^-1
Mass Cl- = 3x10^-3 x 35.45 = 0.106g L^-1 = 106mg L^-1 = 106ppm

What did you do in the bold step?

 

[Fizzy_Cyst]

re: HSC Chemistry Marathon Archive

What did you do in the bold step?

Convert from mol L^-1 to g L^-1 by multiplying by the mass of the chloride ion

 

[leehuan]

re: HSC Chemistry Marathon Archive

Q: Previous theories of acids and bases could not explain why certain substances could be classified as them.

(i) Despite this, Arrhenius' theory was arguably a big step in understanding how acids and bases work. What did Arrhenius refer to as acids and bases, and what were some of the consequences of his theory? Include relevant equations where appropriate. (4)
(ii) Currently, Bronsted-Lowry theory is commonly accepted to explain acids and bases. Explain the principles of this theory and why it can be considered more valid than Arrhenius' theory.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Q: Previous theories of acids and bases could not explain why certain substances could be classified as them.

(i) Despite this, Arrhenius' theory was arguably a big step in understanding how acids and bases work. What did Arrhenius refer to as acids and bases, and what were some of the consequences of his theory? Include relevant equations where appropriate. (4)
(ii) Currently, Bronsted-Lowry theory is commonly accepted to explain acids and bases. Explain the principles of this theory and why it can be considered more valid than Arrhenius' theory.

i) Arrhenius' theory defined acids as substances that produce hydrogen/hydronium ions in water, and bases as substances that produce hydroxide ions in water. However, Arrhenius theory was limited in that it only applies to aqueous solutions, and only accounts for substances that have hydrogen or hydroxide in their structure.
Acid Reaction: HCl + H2O ----> H30+ + Cl-
Base Reaction: Don't really have a basic reaction, it would be great to know one though

ii) Bronsted-Lowry define acids as proton donors and bases as proton gainers. This is different from Arrhenius' theory as acids and bases no longer have to be in aqueous solutions. For example, the reaction: HCl(g) + NH3(g) ----> NH4Cl (s), has no water present, so by Arrhenius' theory this is not an acid or a base reaction.

 

[leehuan]

re: HSC Chemistry Marathon Archive

i) Arrhenius' theory defined acids as substances that produce hydrogen/hydronium ions in water, and bases as substances that produce hydroxide ions in water. However, Arrhenius theory was limited in that it only applies to aqueous solutions, and only accounts for substances that have hydrogen or hydroxide in their structure.
Acid Reaction: HCl + H2O ----> H30+ + Cl-
Base Reaction: Don't really have a basic reaction, it would be great to know one though

ii) Bronsted-Lowry define acids as proton donors and bases as proton gainers. This is different from Arrhenius' theory as acids and bases no longer have to be in aqueous solutions. For example, the reaction: HCl(g) + NH3(g) ----> NH4Cl (s), has no water present, so by Arrhenius' theory this is not an acid or a base reaction.

Forgot to put a mark allocation of 3 on the second part so I'll just let that be.

Base reaction is really just how substances can ionise to produce OH- in solution as well, so you can just make one up e.g. NaOH(s) -> Na+ + OH-. But that wasn't the second equation I was looking for. 2/4 maybe 3/4 - I was also looking for some consequences. Some were:
-Neutralisation of acids and bases: Acid + Base -> Water + Salt
-Relative strength of certain acids and bases

Edit: Yeah, probably 3/4 if I think about it: Description, 1 required equation, problems

 

[Mr_Kap]

re: HSC Chemistry Marathon Archive

Convert from mol L^-1 to g L^-1 by multiplying by the mass of the chloride ion

So essentially Mvc = m ?

Where:
v = 1
M = 35.45
c = 3x10^-3 mol L^-1

 

[BlueGas]

re: HSC Chemistry Marathon Archive

So essentially Mvc = m ?

Where:
v = 1
M = 35.45
c = 3x10^-3 mol L^-1

Do you mean the formula n = m/M? Where n is the number of mol, m is the mass, and M is the molecular weight

 

[leehuan]

re: HSC Chemistry Marathon Archive

He tried to put two formulae into one it seems...

n=m/M

C=n/V

Therefore n=CV, so CV = m/M thus m = MCV

Which I do NOT recommend quoting, because the examiners might get confused at where you got that formula from.
Note that Fizzy started a new line of working out for his conversion anyway.

 

[BlueGas]

re: HSC Chemistry Marathon Archive

Quick question, do I have to remember the pH ranges for phenol, methyl orange, bromothyl blue, etc?

 

[rand_althor]

re: HSC Chemistry Marathon Archive

Quick question, do I have to remember the pH ranges for phenol, methyl orange, bromothyl blue, etc?

Yes. I think it is part of this dot point:

"Identify data and choose resources to gather information about the colour changes of a range of indicators"