2010 VCAA Chemistry U4 Suggested solutions

[Mao]

would that question be 2 for the equation and 1 for why it exploded, or the other way around

Yeah, that sounds right. I dunno how they could give two for explosion..

I said both of hydrogen gas is highly flammable/explosive and high pressure due to build up of hydrogen gas could lead to an explosion.

That is a VERY good point. As soon as the seal is ruptured it will combust. Didn't think of this at the time.

[Mao]

With question 1.b.iii where methane and hydrogen are "burned", is that the same as combustion?

Because I assumed it was so I wrote down that

CH4 + 2 O2 → CO2 + 2 H2O and
2 H2 + O2 → 2 H2O

Therefore, the answer for the volume of hydrogen needed would be double the volume that Mao has given (12.4 L)

Yes, it's just combustion and your equation is correct.

But the value from the data booklet is for this equation:  H2 + 0.5O2 --> H2O
The value from the data booklet (or any standard tables) is always per 1 mol of the reactant in question.

[d-ea-6]

With question 1.b.iii where methane and hydrogen are "burned", is that the same as combustion?

Because I assumed it was so I wrote down that

CH4 + 2 O2 → CO2 + 2 H2O and
2 H2 + O2 → 2 H2O

Therefore, the answer for the volume of hydrogen needed would be double the volume that Mao has given (12.4 L)

Yes, it's just combustion and your equation is correct.

But the value from the data booklet is for this equation:  H2 + 0.5O2 --> H2O
The value from the data booklet (or any standard tables) is always per 1 mol of the reactant in question.

After I raised the query, I was afraid this was the case

This is what happens when you try a different study approach. I did the bulk of my chem study weeks ago, so when it came to the actual exam I think I forgot some things... (like this, for instance). Should've stuck with cramming!

[D27RII]

surely 19 is not C, since when did stupid electrons decide to go to the anode??? and Mao, the anode and the reluctant are the same, its not like a dissolved reluctant is donating electrons to the non reactive anode, the anode is the reluctant, this makes no sense??

and D is wierd too, the question says "fuel cells and rechargeable cells" <----, doesnt that say taht fuel cells are not rechargeable??

still i think D is most right tho.

BTW, ONE important thing

for my lactic acid assumption i said the we assumed that H+ ion concentration was the same as the lactate ion concentration. is that a fair assumption?

and Hydrogen gas is not flammable, if u put down flammable u will get it wrong, it is combustible, DEFINITELY not flammable lol.

[D27RII]

And Mao is definitely right about question 1b or whatever about volume about H2, the data book shows energy released per mole of candidate fuel

[D27RII]

also, its crucial to point out the basics...lithium is a very reactive element and the strongest reductant there is. Hence in its reaction with water, irrepsective of any Hydrogen gas is pretty darn violent. the hydrogen gas just adds the exploxive factor, but lithium and water otherwise relsesae a lot of heat anyway.

so u would need to mention that the reaction evolves IMMENSE heat
and that hydrogen gas is COMBUSTIBLE

[Mao]

also, its crucial to point out the basics...lithium is a very reactive element and the strongest reductant there is. Hence in its reaction with water, irrepsective of any Hydrogen gas is pretty darn violent. the hydrogen gas just adds the exploxive factor, but lithium and water otherwise relsesae a lot of heat anyway.

so u would need to mention that the reaction evolves IMMENSE heat
and that hydrogen gas is COMBUSTIBLE

Yes and no. The electrochemical series tell us nothing about the rate of reaction. We can only get this rate through experimentation. The reaction between lithium and water is not THAT energetic, I would contribute most of the explosive factor into the pressure build up. If you've done the experiment of Li in water vs Na in water, you can probably remember that Li was the boring one, just fizzing about and never caught on fire. The heat of reaction will predominantly go into increasing the pressure of H2 gas inside the battery by PV=nRT.

[Mao]

and Hydrogen gas is not flammable, if u put down flammable u will get it wrong, it is combustible, DEFINITELY not flammable lol.

I disagree. Just about every MSDS on the planet will say H2 gas is an inorganic highly-flammable gas.

[stonecold]

and Hydrogen gas is not flammable, if u put down flammable u will get it wrong, it is combustible, DEFINITELY not flammable lol.

I disagree. Just about every MSDS on the planet will say H2 gas is an inorganic highly-flammable gas.

I think the examiner will get the jist of what I meant, and I doubt they are that tight.

[8039]

FFS I forgot to balance my ammonia reaction... even tho I wrote it about 20 effing times for all the practice exams I've done.

[D27RII]

PLLLSSS CHECK THIS ANSWER>

for my assumption question i said.

We assumed that the concentration of hydrogen ions was the same as the concentration of lactate ions as the two reactants are shown by the equation to exit in equimolar stoichiometric amounts.

is that a fair assumption considering that there are addition H+ ions in the neutral water itself?

[m@tty]

Okay then, by definitions:

Reductant = A reducing agent

Anode = The positively charged electrode by which the electrons leave a device.

The reductant is the thing that releases electrons. At an atomic level this is the Fe atoms participating in the reaction (is this what you dispute? Dude, it's simple logic xD ). Then, by definition, the electrons produced must leave via the anode - ie. the electrons must "pass from the reductant to the anode as electrons are produced".

Therefore C.

QED.


EDIT:

besdies, electrons flowing from reductant to anode does not produce electricity, option C states "As electricity is being produced" - hence implies that electrons need to flow from reductant to CATHODE, not from reductant to anode.

Yes, that is true. But with intermediate and final steps shown it goes: reductant -> anode -> cathode -> oxidant.

PLLLSSS CHECK THIS ANSWER>

for my assumption question i said.

We assumed that the concentration of hydrogen ions was the same as the concentration of lactate ions as the two reactants are shown by the equation to exit in equimolar stoichiometric amounts.

is that a fair assumption considering that there are addition H+ ions in the neutral water itself?

I think that should be fine

[DarkVisor]

For the last question (The kid with the phosphoric acid fuel cell), I put "Non standard temp" for one point, but I also said that the NSC could cause the "Hydrogen peroxide reaction" to take place... Does anyone think they'll accept is? Wouldn't it technically be correct?

[D27RII]

For the last question (The kid with the phosphoric acid fuel cell), I put "Non standard temp" for one point, but I also said that the NSC could cause the "Hydrogen peroxide reaction" to take place... Does anyone think they'll accept is? Wouldn't it technically be correct?

nope, H2O2 needs enzymes to be formed and enzymes to be decomposed. moreover in the different world of biology, two different catalysts are required for the forward and backward reactions respectively since the catalysts are organic and complex.

it shouldnt really be in the electrochemical series, because the reaction which produced H2O2 will never happen without enzymes and biological organelles, and the decomposing reaction will happen at such a slow temperature that it is completely negligible.

[Shark 774]

also pretty sure 8 is A, dilution decreases ocncentration of all ions - just because hydronium ion conc decresases doesnt necessarily mean OH- increases...

The concentration oh is greater in the water, thus when it is added to the acidic solution the overall concentration of increases.

Also, Vanadium Oxide is fine for catalyst instead of Vanadium Pentoxide.

I am not sure what the right answer is, but I don't agree with your reasoning. The OH- is greater in the water but as it is added to the HCl there is now a greater total volume and not to mention some of the OH- from the water will react with H+.