HSC 2012-2015 Chemistry Marathon (archive) (1 Viewer)

[Youi_]

re: HSC Chemistry Marathon Archive

What is needed to prepare a buffer solution?

A weak acid/base and its conjugate acid/base

Feel free to expand on my answer

 

[someth1ng]

re: HSC Chemistry Marathon Archive

HCL and H2SO4 are stronger acids and are most likely to fully ionise in water however Acetic Acid is a weak acid as it only partially ionises in water. Distinguishing the Ph levels between Hydrogen Chloride and Sulfuric Acid H2So4 will be higher in Ph as it is diprotic whilst Hcl is monoprotic in which it will have have as many Hydrogen Ions and the calculations would be Ph= -log(2x0.1) and for HCl would be the regular calculation resulting in 1 ph for hcl and ph of ~.7 for H2so4. Therefore h2so4 is stronger then hcl


What is needed to prepare a buffer solution?

Same error as usual - lower pH doesn't mean that it's stronger because you're comparing a monoprotic acid against a polyprotic acid.

When you compare two strong acids, they are effectively equal in strength when in water. To compare the strengths, you need a different solvent and you find out that HCl is actually more likely to protonate than H2SO4 and hence, HCl is the stronger acid.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

A weak acid/base and its conjugate acid/base

Feel free to expand on my answer

In roughly equal amounts (and it's weak base).

 

[Youi_]

re: HSC Chemistry Marathon Archive

Compare a galvanic cell to an electrolytic cell

 

[Menomaths]

re: HSC Chemistry Marathon Archive

Isn't this Industrial Chem?

 

[Youi_]

re: HSC Chemistry Marathon Archive

Isn't this Industrial Chem?

Woops nevermind

 

[Youi_]

re: HSC Chemistry Marathon Archive

Assess the effectiveness of methods used to purify and sanitise water supplies.

 

[leesh95]

re: HSC Chemistry Marathon Archive

-Water treatment and sanitation is balanced between cost and quality. Filtration removes many particles however not extremely small ones. Its speed is considerably appropriate for a big city
-Chlorination is quite effective and economic way of killing bacteria. However the 1998 Giarda scare in Sydney showed that these methods may not be completely effective in removing all parasites thus we could use membrane filters which are very expensive or ozone sterlisation which has an odor.

 

[Youi_]

re: HSC Chemistry Marathon Archive

-Water treatment and sanitation is balanced between cost and quality. Filtration removes many particles however not extremely small ones. Its speed is considerably appropriate for a big city
-Chlorination is quite effective and economic way of killing bacteria. However the 1998 Giarda scare in Sydney showed that these methods may not be completely effective in removing all parasites thus we could use membrane filters which are very expensive or ozone sterlisation which has an odor.

Where is your assessment of the effectiveness i.e very effective, limited in effectiveness

 

[leesh95]

re: HSC Chemistry Marathon Archive

Question: Big bob loves sodium hydrogen carbonate because he used this as his primary standard (in which he used to make a standard solution) to calculate the concentration of hydrochloric and sulfuric acid. Explain why you should not conduct titration with sodium hydrogen carbonate to find the concentration of an unknown acetic acid solution?

I am going to say that this is a weak acid and weak base titration. Thus pH does not change sharply at end point and colour change is hard to observe as we can't find a suitable indicator.

 

[leesh95]

re: HSC Chemistry Marathon Archive

Mine said something like this: "Discuss the basis of the claim that ethanol can be a 'carbon neutral' fuel" (3 marks)
A lot of people got 1-2 marks

I got 2 marks in this one as well. What was the right answer?

 

[Menomaths]

re: HSC Chemistry Marathon Archive

I got 2 marks in this one as well. What was the right answer?

You can say ethanol's been proposed to be greenhouse neutral since the CO2 produced when ethanol is combusted is balanced by the CO2 used by plants during photosynthesis to produce glucose which can be converted into ethanol. However significant energy input is required for the production of fertilizers and distillation process which comes from the combustion of fossil fuels which leads to production of carbon dioxide.

 

[ocatal]

re: HSC Chemistry Marathon Archive

A student claims that the change in enthalpy of all neutralisation reactions is close to -57 kJ/mol. Evaluate the extent to which this is true.

 

[leesh95]

re: HSC Chemistry Marathon Archive

A student claims that the change in enthalpy of all neutralisation reactions is close to -57 kJ/mol. Evaluate the extent to which this is true.

I am not sure about the answer...
I would say for strong acids and alkali the value is fairly close to -57 kj/mol since a strong acid and base are fully ionized in solution and so the reaction is between the hydroxide ions and hydrogen ions to produce water. But with a weak acid or base the value is not close to -57 kj/mol since they are not fully ionized in solution. So this statement is not accurate.

Am I remotely correct?

 

[ocatal]

re: HSC Chemistry Marathon Archive

I am not sure about the answer...
I would say for strong acids and alkali the value is fairly close to -57 kj/mol since a strong acid and base are fully ionized in solution and so the reaction is between the hydroxide ions and hydrogen ions to produce water. But with a weak acid or base the value is not close to -57 kj/mol since they are not fully ionized in solution. So this statement is not accurate.

Am I remotely correct?

Yep that's basically it. Good job! The reason why the value is lower for weak acids is because heat is required to ionise them, meaning less heat is released overall.

 

[Frie]

re: HSC Chemistry Marathon Archive

Yep that's basically it. Good job! The reason why the value is lower for weak acids is because heat is required to ionise them, meaning less heat is released overall.

What happens if the neutralisation is with a base that doesnt have hydroxide, like ammonia, is the enthalpy still the same as a weak base.

 

[ocatal]

re: HSC Chemistry Marathon Archive

What happens if the neutralisation is with a base that doesnt have hydroxide, like ammonia, is the enthalpy still the same as a weak base.

Hmm good question. I would assume it was different to strong bases since the reaction wouldn't involve the standard hydronium and hydroxide ion reaction but I'm not too sure about weak bases.

 

[super47]

What happens if the neutralisation is with a base that doesnt have hydroxide, like ammonia, is the enthalpy still the same as a weak base.

I dont know i'm right but when dealing with weak bases ionising in water, delta H is smaller due to the dynamic equilibrium that weak base create. Due to forward and backwards reaction some heat is also used as reactant, which does not happen when complete ionisation happens(strong acid). Am i right?

 

[leesh95]

re: HSC Chemistry Marathon Archive

Doesn't reaction of ammonia with water produce OH- ions?

 

[kostadalianas]

re: HSC Chemistry Marathon Archive

NH3 + H20 --> NH4+ + OH-

yeah it does