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

[Drsoccerball]

re: HSC Chemistry Marathon Archive

I'm not sure If I've learnt that yet. Would that be when there is a weak acid - hence it couldn't fully ionise?

Pretty much

 

[SuchSmallHands]

Chemistry marathon 2015

Can we maybe just stick to POM questions?It's clear that some people haven't properly started acidic environment yet and so their answers will be way off just because they're not up to that point in the course.

 

[zhertec]

re: HSC Chemistry Marathon Archive

Hydronium Ions, H30+, are positively charged ions that results from the presence of an acid in water, where a hydrogen ion reacts with the water molecule to form Hydronium ions. Due to their charge they increase the acidity of a substance by a lot as they are donating 3 protons, instead of 1 as with an H+ ion.

Err Hydronium ions cannot donate 3 protons....Hydronium ions are formed when a proton is donated into the medium (being water) in which it reacts with H2O, forming a coordinate covalent bond. Later on you learn that H+ ions do not usually exist in aqueous solution by themselves, and due to their instability as an ion, they quickly react with another molecule.

The concentration of Hydronium ions is proportional to pH, which determines the basisity/acidity of the substance. Since the pH follows a graph of -log(base 10) (H+ conc.), an increase in the concentration of Hydronium ions would decrease the pH of the substance, making it more acidic.

 

[Kaido]

re: HSC Chemistry Marathon Archive

My response would be like this, I provided a general equation for clarity:
The conjugate base is what is formed when an acid donates a proton.

HA (acid) --> H+ (proton) + A- (conjugate base)

In your answers, the idea is definitely there but the wording is poor. Be careful when you use words like "compound" and "molecule". The conjugate base may not be a molecule nor a compound. A molecule, by definition, is a structure of atoms that is neutral (that's key!) - when talking about the conjugate base, in many (most) cases, it won't be neutral. As for compounds, it's something that is pure, in this context, it's not really accurate to refer to the conjugate as a compound.

Sweet, thanks for clearing that up

 

[Ekman]

re: HSC Chemistry Marathon Archive

How does the concentration of hydronium ions affect the acidity of a substance? 2 marks

For this question all you have to mention is:
-The higher the concentration of hydronium ions, the more acidic the solution is.
-The absence of hydronium ions result in the solution to not be acidic.
I don't really see why you have to mention strong and weak acids, since the question is only asking for concentration of hydronium ions in solution and how it effects the acidity of that solution. I guess you could mention it, but the question is asking specifically 'concentration' of hydronium ions alone, rather than the 'degree of ionization' of an acid.

EDIT: After all, strength and concentration of an acid or base are two completely different factors that define an acid or base. By definition, strength is the degree of ionization, whereas concentration is the amount of ions per litre of solution.

 

[siggy]

re: HSC Chemistry Marathon Archive

Thanks for clearing that up zhertec. As said before haven't really completely covered everything in the acidic course although I value these questions as I'm still learning a lot of things from the new acidic topic anyway
So the hydronium ion essentially acts as an H+ ion? Or is that too simplified?

I believe the 14ers are meant to ask the questions though, not us Drsoccerball, although correct me if I'm wrong - any question is good

 

[someth1ng]

re: HSC Chemistry Marathon Archive

The concentration of Hydronium ions is proportional to pH, which determines the basisity/acidity of the substance.

Ummm, no.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

By definition, strength is the degree of ionization, whereas concentration is the amount of ions per litre of solution.

You might want to check your definitions.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

So the hydronium ion essentially acts as an H+ ion? Or is that too simplified?

Well, H+ ions don't exist in solutions, strictly speaking. What happens is that an acid donates a proton to a base, in this case, an acid, HA, donates to water, H2O to make H3O+.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Thanks for clearing that up zhertec. As said before haven't really completely covered everything in the acidic course although I value these questions as I'm still learning a lot of things from the new acidic topic anyway
So the hydronium ion essentially acts as an H+ ion? Or is that too simplified?

I believe the 14ers are meant to ask the questions though, not us Drsoccerball, although correct me if I'm wrong - any question is good

Well im the only one posting questions...

 

[Librah]

re: HSC Chemistry Marathon Archive

"In reference to water chemistry, pure water is defined as a neutral solution and is always assigned a pH of 7 as commonly known to be."

Assess the validity of this statement, and justify your response with reference to Bronsted Lowry and equilibrium theory. Include relevant chemical equations. (May want to skip this question unless your already a little more than halfway through acidic environment.)

Haven't assigned marks, so just answer as well as you can.

 

[enigma_1]

re: HSC Chemistry Marathon Archive

"Pure water is defined as a neutral solution and is always assigned a pH of 7 as commonly known to be."

Assess the validity of this statement, and justify your response with reference to Bronsted Lowry and equilibrium theory. Include relevant chemical equations. (May want to skip this question unless your already a little more than halfway through acidic environment.)

Haven't assigned marks, so just answer as well as you can.

They're probably just starting acidic, but I'll post up a question for POM for those of you who aren't too familiar with acidic yet.

Nice and easy first up: Assess the potential of cellulose in being used as a raw material for the production of petrochemicals. (5 marks)

Pay close attention to what the marking criteria might include.

 

[Ekman]

re: HSC Chemistry Marathon Archive

You might want to check your definitions.

Acid strength is an acid's ability or tendency to lose a proton. Different acids should have different strengths. (http://chemistry.tutorvista.com/inorganic-chemistry/acid-strength.html)
In chemistry, "concentration" has its common definition: The amount (grams, mols, pounds, etc.) of a substance dissolved in a specified amount of solvent (in this case, water).(http://www.newton.dep.anl.gov/askasci/chem03/chem03575.htm)

So yes, I am sure about my definitions...

 

[Librah]

re: HSC Chemistry Marathon Archive

Acid strength is an acid's ability or tendency to lose a proton. Different acids should have different strengths. (http://chemistry.tutorvista.com/inorganic-chemistry/acid-strength.html)
In chemistry, "concentration" has its common definition: The amount (grams, mols, pounds, etc.) of a substance dissolved in a specified amount of solvent (in this case, water).(http://www.newton.dep.anl.gov/askasci/chem03/chem03575.htm)

So yes, I am sure about my definitions...

I think he was referring to your use of "litres." Also maybe your definition for acid strength may have been a bit ambiguous. For bolded part, doesn't have to be a solvent, can be a gas.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

They're probably just starting acidic, but I'll post up a question for POM for those of you who aren't too familiar with acidic yet.

Nice and easy first up: Assess the potential of cellulose in being used as a raw material for the production of petrochemicals. (5 marks)

Pay close attention to what the marking criteria might include.

Cellulose is derived from bio mass which is renewable as it is comes from every living and non living organism.
At the moment the material for production of petrochemicals is the cracking of hydrocarbons. Hydrocarbons come from crude oil which is non-renewable showing that one day the source of hydrocarbons is going to eventually run out. Scientists attempt to use other materials to produce substances like petrol, Polymers ect... which is not only environmentally friendly but also renewable. The only alteranative to this is the use of cellulose. Cellulose is converted to glucose by the use of proteins and then glucose is seperated into ethanol and carbon dioxide. It can also be used to produce polymers which are biodegradable. The only barrier to the permanent use is the cost of this. The cellulose around us is not enough to industrially supply to a whole country. Many crops need to be planted in order to grow extra cellulose thus also reducing other produce. In the near future cellulose it is unlikely for cellulose to be used as a raw material for the production of materials.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

"Pure water is defined as a neutral solution and is always assigned a pH of 7 as commonly known to be."

Assess the validity of this statement, and justify your response with reference to Bronsted Lowry and equilibrium theory. Include relevant chemical equations. (May want to skip this question unless your already a little more than halfway through acidic environment.)

Haven't assigned marks, so just answer as well as you can.

Does your answer have to include carbonic acid, polluted water, le chateliers princible and a chemical equation ? not sure who Bronsted lowry is though

 

[Librah]

re: HSC Chemistry Marathon Archive

Does your answer have to include carbonic acid, polluted water, le chateliers princible and a chemical equation ? not sure who Bronsted lowry is though

You'll learn it soon, should leave the question for now though if you haven't gotten up to the self ionization of water.

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

You'll learn it soon, should leave the question for now though if you haven't gotten up to the self ionization of water.

I think we just started that..
?

 

[Librah]

re: HSC Chemistry Marathon Archive

I think we just started that..
?

Need to split it up and add an equilibrium, it's important to see that H20 is both a proton donor and acceptor. H20(l)+H20(l)<---> H30+(aq) +OH-(aq) .

 

[Ekman]

re: HSC Chemistry Marathon Archive

I think he was referring to your use of "litres." Also maybe your definition for acid strength may have been a bit ambiguous.

Even then, concentration is measured in moles per litre, so I still don't understand the problem