Mod 6 extended response (1 Viewer)

[NexusRich]

What should I mention in my response to get those full marks ? Any tips appreciated, thanks !

 

[Qeru]

What should I mention in my response to get those full marks ? Any tips appreciated, thanks !

Basically everything in the second dot point of module 6.
Off the top of my head:
-History of Acids and Bases particuarily arrhenius+Bronsted Lowry Define, Describe and explain,Include conjugate acid/base pairs and general equation for acid and base disassociation.
-Strength of Acids and bases vs concentration Define and Explain (you can link it to bronsted lowry definition)
-pH Define and explain
-Explain all the 'protic,' i.e. amphiprotic, diprotic etc.
-You could also talk about acidic and basic salts although I dont know if this is relevant.

Now if this was an assess question you would usually say how important water is in defining acids and bases, I think I would still include this here (even though it says explain).

Note everything must be linked to the role of water.

 

[thush@decode]

What should I mention in my response to get those full marks ? Any tips appreciated, thanks !

If you also want to push the boat, you can also talk about how the pH 7 being the cutoff between an acidic and a basic solution (with pH 7 being neutral pH) is predicated on Kw =[H+][OH-] being 10^(-14).

Which is only true when the temperature is 298 K.

Meaning the stimulus material is actually only accurate for the temperature 298 K.

Being an endothermic reaction, the Kw value for the equilibrium 2H2O <--> H3O+ + OH- increases with rising temperature. For example, at body temperature (37 degrees = 310 K), Kw = 10^(-13.6) approximately, with neutral pH being 6.8, and a pH of 6.9 will be considered basic.
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As far as the question goes, I'd probably have gone:

• describe water being the solvent, and defining the terms "acidic solution", "basic solution" and pH, and explaining that these terms are only relevant when acids are dissolved in solution including aqueous solutions
• explain the self-ionisation of water equation, and the value of Kw and its significance - you could include the caveat of pH 7 being the neutral pH being only at a temperature of 298 K
• explain how Lowry-Bronsted acids, when dissolved in water, protonate water to form H3O+ ions, increasing [H3O+] to make it greater than [OH-] making the solution more acidic by definition, decreasing the pH too
  • you could even talk about Lewis acids if you really wanted to push the boat
• doing an identical explanation for Lowry-Bronsted bases

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These questions are marked "holistically", just like English essays are. You just need to show off that you know your stuff and blow the examiner's mind with your knowledge and linguistic flair in scientific communication.

 

[CM_Tutor]

If you also want to push the boat, you can also talk about how the pH 7 being the cutoff between an acidic and a basic solution (with pH 7 being neutral pH) is predicated on Kw =[H+][OH-] being 10^(-14).

Which is only true when the temperature is 298 K.

Meaning the stimulus material is actually only accurate for the temperature 298 K.

I would go further and say that the question is simply wrong. Even at 298 K, I would not accept the assertion that acids are defined as being solutions with pH < 7.

Aqueous systems are acidic if they contain greater concentration of hydronium ions than hydroxide ions. They are neutral if [H₃O⁺] = [OH^-]. They are basic if [OH^-] > [H₃O⁺].

As @thush@decode has noted, the pH value which is corresponds to a neutral solution is temperature dependent, and it can be calculated from the value of K_w.

It is never accurate to say that neutral is defined by pH = 7. Neutral is defined by [H₃O⁺] = [OH^-] and it just so happens that, at 298 K, this occurs when pH = 7. It is reasonable to say that acids can be recognised at 298 K by the fact that they have a pH below 7, but that is not the definition. I could equally recognise an acidic solution by its ability to turn blue litmus red, but if I tried to argue that the ability to turn blue litmus red is the defining characteristic of an acid, I would run into difficulties.

 

[thush@decode]

I would go further and say that the question is simply wrong. Even at 298 K, I would not accept the assertion that acids are defined as being solutions with pH < 7.

Fully agree here. And also fully agree with the definitions of acidic and basic solutions.

The logical sequence here is:

• A neutral solution is defined as one in which [H3O+] = [OH-].
• If and only if the temperature is 298 K, this implies that Kw = 10^(-14), which in turn implies that a neutral solution would have a pH 7.

In other words, if temperature is 298 K, this means that if a solution is neutral, its pH would have to be 7 (by implication, not by definition).

This is me speculating now, and @CM_Tutor could clarify this better than I can, but surely NESA would not slip in question stems that are deliberately inaccurate? Surely this is an oversight by whomever wrote this question.

 

[CM_Tutor]

Fully agree here. And also fully agree with the definitions of acidic and basic solutions.

The logical sequence here is:

• A neutral solution is defined as one in which [H3O+] = [OH-].
• If and only if the temperature is 298 K, this implies that Kw = 10^(-14), which in turn implies that a neutral solution would have a pH 7.

In other words, if temperature is 298 K, this means that if a solution is neutral, its pH would have to be 7 (by implication, not by definition).

This is me speculating now, and @CM_Tutor could clarify this better than I can, but surely NESA would not slip in question stems that are deliberately inaccurate? Surely this is an oversight by whomever wrote this question.

I'd probably modify this slightly, to the sequence:

1. Neutral aqueous solution defined by [H₃O⁺] = [OH^-].
2. Since the auto-ionisation of water involves 2 H₂O(l) <----> H₃O⁺(aq) + OH^-(aq) and thus K_w = [H₃O⁺][OH^-], we can infer that a neutral solution has the property that K_w = [H₃O⁺]².
3. K_w, being an equilibrium constant, is temperature dependent. At 298 K, since K_w = 1.00 x 10^-14, it follows that [H₃O⁺] = 1.00 x 10^-7 mol L^-1 and thus the neutral pH at this temperature is 7.00. That the neutral pH at 298 K is 7.00 is a property of the system that is consequent on the definition of neutrality and the value of the K_w.

However, the differences between these sequences are minor.

As for the question "surely NESA would not slip in question stems that are deliberately inaccurate?"...

• I doubt that this question is intentionally misleading, it is just badly written.
• Badly written questions are more common following a syllabus change.
• The procedure for writing the HSC is exhausting and meticulous (I know someone who helped write the Chemistry HSC a few years ago). However, it is also ultimately answerable to NESA and mistakes do happen. There have been serious mistakes made in some past examinations.
• Badly written questions are more common on school exams or trial papers written by outside organisations and sold to schools.

The bigger issue, in my opinion, is to prepare students to recognise problems in questions and to respond to them in appropriate ways.