Chemistry 3/4 2013 Thread

[brightsky]

yeah so the LHS electrode is positive (having electrons pulled forcibly away from it) and the RHS electrode is negative. so the anions will migrate towards the positive LHS electrode, the anode. the cations will migrate towards the negative RHS electrode, the cathode. as a general rule, anions migrate towards anodes ALWAYS and cations migrate towards cathodes ALWAYS. that is how anions and cations are defined.

[Scooby]

yeah so the LHS electrode is positive (having electrons pulled forcibly away from it) and the RHS electrode is negative. so the anions will migrate towards the positive LHS electrode, the anode. the cations will migrate towards the negative RHS electrode, the cathode. as a general rule, anions migrate towards anodes ALWAYS and cations migrate towards cathodes ALWAYS. that is how anions and cations are defined.

shouldn't the anode be negative, since it's a fuel cell?

[brightsky]

shouldn't the anode be negative, since it's a fuel cell?

oh woops soz you're right didn't read the question properly. but the point still holds. the RHS electrode is the positive cathode and the LHS electrode is the negative anode. cations migrate towards cathodes, so H+ goes right. anions migrate towards anodes so H2PO4- goes left. you can also think about it this way. electrons are migrating away from the LHS electrode. but we can't have a build up of positive charge there so we need some negative charge in that region to balance things up.

[Scooby]

oh woops soz you're right didn't read the question properly. but the point still holds. the RHS electrode is the positive cathode and the LHS electrode is the negative anode. cations migrate towards cathodes, so H+ goes right. anions migrate towards anodes so H2PO4- goes left. you can also think about it this way. electrons are migrating away from the LHS electrode. but we can't have a build up of positive charge there so we need some negative charge in that region to balance things up.

yeah, that makes sense. thanks brightsky

[Jeggz]

How would you describe the main difference between direct and back titration? I'm struggling to be concise in my response

[jono88]

write a balanced overall equation that shows hydrogen peroxide, H202,reacting as a reductant. Don't really know how to write a full equation for this. oxidant has to be higher than the one acting on +0.68 is all i know for it. why does h202 need to be on the left side of the equation?

[ahat]

How would you describe the main difference between direct and back titration? I'm struggling to be concise in my response

For a direct titration, solutions are slowly added together until an endpoint is reached, using a precisely measured amount of primary standard. In back titration, the solution is neutralised instantaneously by adding as excess of the standard. This is done so accuracy can be preserved when trying to analyse solutions like ammonia, i.e. volatile compounds. If you a direct titration was used instead, the process would be inaccurate because as the aliquot is slowly delivered, the ammonia evaporates. Instead, using a back titration, all of the ammonia is reacted instantly and another titration is then used to determine the amount of excess standard solution.
*I'm sure someone could be more detailed *

I myself have a few questions.

1. In pure water when heated, the pH of the resulting solution will decrease as hydronium concentration increases. Nevertheless, the solution itself is neutral because hydroxide and hydronium are at equal concentrations, which seems somewhat contradictory because the pH indicates an acidic solution. So my question is, are there any other cases where the pH decreases/increases but the solution is still neutral? Thanks.   

2. When doing questions using acidity constants, in what cases do we make the assumption (let the acid be HA) that [H⁺] = [A^-], or more simply, the concentration of products on the left hand side are equal? In what cases can we make this assumption and in what cases should we not (I came across a VCAA question where people did but it was incorrect - the specific question has slipped my mind)?

Just to make sure you understand my question:



Let [H⁺] = [A^-]



When do and don't we make this assumption?

Thank you!

[Jaswinder]

have no clue what this question means and what its asking for :/

[ahat]

Describe how this relationship between absorbance and concentration can be experimentally determined:

Absorbance (A) = 4.15[X]
The first thing that we should notice is that this curve should follow a linear relationship. This is the assumption that we have to experimentally prove. Thus, all we must do is observe the absorbance as the concentration increases. Foremost, it would be logical to run an experiment with several standards of increasingly high concentrations. Run them through a UV-Vis spectrophotometer at 290nm and record their absorbance. If the data follows a linear trend, the the relationship that A = 4.15[X] can be deemed to be a reasonable assumption.

[clıppy]

Just a quick question from the Insight 2013 paper.
We're asked to write the oxidation and reduction reactions from a methane fuel cell after only being told it is in acidic conditions.
Is the question assuming we know the principles of a methane fuel cell, or should we be able to find all the information we need from it? I believe the study design said it was not necessary to know exact details about particular fuel cells so I'm not sure what this question believes a student would know.

[Dismounted]

write a balanced overall equation that shows hydrogen peroxide, H202,reacting as a reductant. Don't really know how to write a full equation for this. oxidant has to be higher than the one acting on +0.68 is all i know for it. why does h202 need to be on the left side of the equation?

Have a look in your data book - you'll see a half equation for hydrogen peroxide acting as a reductant. To get the overall balanced equation, it depends on what else you have in the environment...

In pure water when heated, the pH of the resulting solution will decrease as hydronium concentration increases. Nevertheless, the solution itself is neutral because hydroxide and hydronium are at equal concentrations, which seems somewhat contradictory because the pH indicates an acidic solution. So my question is, are there any other cases where the pH decreases/increases but the solution is still neutral?

What defines if something is neutral? A neutral solution is something that has equal amounts of H⁺ and OH^-.

pH indicates the concentration of H⁺ only. By itself, it tells us nothing about the concentration of OH^-. So therefore it can't tell us anything about acidity/alkalinity without further information.

So why does the pH decrease for a neutral solution at a higher temperature? Because of equilibrium. Water ionisation is endothermic, so as temperature rises, the value of K increases, thus increasing the concentration of the products: H⁺ and OH^-.



The fact that "pH 7 = neutral" is only valid at 25 degrees C, where the value of K is your well-known 10^-14. Working out/being given a value for K is the "further information" we need to determine the concentration of OH^- from pH.

That should also answer your actual question - when does the pH of a neutral solution change? Any time the value of K is changed! (i.e. temperature)

When doing questions using acidity constants, in what cases do we make the assumption (let the acid be HA) that [H⁺] = [A^-], or more simply, the concentration of products on the left hand side are equal?

You make the assumption when they are being produced in equal amounts (i.e. 1:1 mole ratio) and when nothing is reacting with either product (which would reduce the concentration of one or the other).

[ECheong]

Just a quick question from the Insight 2013 paper.
We're asked to write the oxidation and reduction reactions from a methane fuel cell after only being told it is in acidic conditions.
Is the question assuming we know the principles of a methane fuel cell, or should we be able to find all the information we need from it? I believe the study design said it was not necessary to know exact details about particular fuel cells so I'm not sure what this question believes a student would know.

hi! I'm working off a very cloudy memory in that I vaguely remember a question (it might have been insight) that stated that the products of the oxidation reaction of methane are the same as its combustion. This would make sense, since when you burn something, it's essentially very quick oxidation

[Dismounted]

Just a quick question from the Insight 2013 paper.
We're asked to write the oxidation and reduction reactions from a methane fuel cell after only being told it is in acidic conditions.
Is the question assuming we know the principles of a methane fuel cell, or should we be able to find all the information we need from it? I believe the study design said it was not necessary to know exact details about particular fuel cells so I'm not sure what this question believes a student would know.

They've told you enough information - you have methane, and it is in acidic conditions.

A fuel cell basically "splits" a combustion into two parts - so what is the combustion reaction for methane? What is oxidised? What is reduced? (One of the half equations will involve methane, the other, oxygen. This is in an acidic environment, so which half equation from your data book involves oxygen in an acidic environment? There's one down.)

[barydos]

For the equilibrium reaction of ammonium ions with water, the product NH3 is given in (aq) in the assessment report.
Is it wrong to put it as (g)?

Might as well also ask, do we get consequential marks in chem? As in we don't lose answer marks for consequent parts of the question?

[Sentar]

For the equilibrium reaction of ammonium ions with water, the product NH3 is given in (aq) in the assessment report.
Is it wrong to put it as (g)?

Might as well also ask, do we get consequential marks in chem? As in we don't lose answer marks for consequent parts of the question?

It should be (aq) as aqueous NH3 is produced, if you put ammonium ions in water it doesn't evaporate. Some may evaporate but that's another equilibrium reaction (NH3 (aq) <-> NH3 (g)). You might get away with (g), but it's not worth the risk.

On the 2nd point I don't know, I hope so.