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March 29, 2024, 10:17:41 pm

Author Topic: VCE Chemistry Question Thread  (Read 2313757 times)  Share 

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eagles

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Re: VCE Chemistry Question Thread
« Reply #270 on: February 03, 2014, 10:30:35 am »
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If 3.0 g of NaOH is added to 500mL of 0.10 M HCl, what's the resulting pH?
Thanks!

jgoudie

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Re: VCE Chemistry Question Thread
« Reply #271 on: February 03, 2014, 05:11:54 pm »
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Here is a hint:

Step 1) Write an equation.
Step 2) Find mole of each reactant.
Step 3) This is limiting reagent, so work out which one is left over. (we know its limiting regent as we have knowledge of both reactants).
Step 4) Work out how much is left over.
Step 5) Work out pH based on what is left over. 


If 3.0 g of NaOH is added to 500mL of 0.10 M HCl, what's the resulting pH?
Thanks!
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #272 on: February 03, 2014, 06:56:50 pm »
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Is the mole of the original solution = to the mole of the diluted solution?
The q. I just did is implying that it is
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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #273 on: February 03, 2014, 07:14:03 pm »
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Is the mole of the original solution = to the mole of the diluted solution?
The q. I just did is implying that it is
Absolutely right. When you dilute something, you are adding water to it, hence, the amount of solute will not change, the only thing that is changing is the concentration.
You should recall the equation from unit 1/2 that:
c1 x V1 = c2 x V2
( this is because n1 = n2)
Hope this helps!
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eagles

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Re: VCE Chemistry Question Thread
« Reply #274 on: February 03, 2014, 07:17:41 pm »
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Thank you!



Here is a hint:

Step 1) Write an equation.
Step 2) Find mole of each reactant.
Step 3) This is limiting reagent, so work out which one is left over. (we know its limiting regent as we have knowledge of both reactants).
Step 4) Work out how much is left over.
Step 5) Work out pH based on what is left over.

fadzsta1

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Re: VCE Chemistry Question Thread
« Reply #275 on: February 03, 2014, 08:17:49 pm »
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Hey guys,
From Chem Heinemann 3/4 book - pg 59 Q22a:

The thermite process can be used to weld lengths of railway track together. A mould
is placed over the two ends of rails to be joined and it is filled with a ‘charge’ of
aluminium powder and iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron which joins the rails together.
a) Write a half equation for the conversion of iron(III) oxide to metallic iron.

My answer:        Fe2O3(s) + 6H+(aq) + 6e- ----> 2Fe(s) + 3H2O(l)
Actual answer:   Fe2O3(s) + 6e- ----> 2Fe(l) + 3O2-(s)

Where the heck did that oxygen ion come from? Isn't it supposed to be water to balance the hydrogen ions? How is that ion solid? So many questions... any help would be appreciated! Thanks!!
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #276 on: February 03, 2014, 08:21:34 pm »
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Hey guys,
From Chem Heinemann 3/4 book - pg 59 Q22a:

The thermite process can be used to weld lengths of railway track together. A mould
is placed over the two ends of rails to be joined and it is filled with a ‘charge’ of
aluminium powder and iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron which joins the rails together.
a) Write a half equation for the conversion of iron(III) oxide to metallic iron.

My answer:        Fe2O3(s) + 6H+(aq) + 6e- ----> 2Fe(s) + 3H2O(l)
Actual answer:   Fe2O3(s) + 6e- ----> 2Fe(l) + 3O2-(s)

Where the heck did that oxygen ion come from? Isn't it supposed to be water to balance the hydrogen ions? How is that ion solid? So many questions... any help would be appreciated! Thanks!!

The end of q22 a) says 'Write a half equation for the conversion of iron(III) oxide to,metallic iron and oxide ions
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fadzsta1

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Re: VCE Chemistry Question Thread
« Reply #277 on: February 03, 2014, 08:29:10 pm »
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.... Perhaps the older version of the book has a typo there. Thanks for your help :)
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #278 on: February 03, 2014, 08:38:08 pm »
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.... Perhaps the older version of the book has a typo there. Thanks for your help :)

Ohhh I guess it does!  Do you have the heinemann chemistry student workbook? There's a similar Q. to this (Page 22, c))




Have I figured the half equation correctly?

Fe2O3(s) --> 2Fe(l) + 3H2O(l)
Fe2O3(s) +6H+ + 6e --> 2Fe(l) + 3H2O
Fe2O3(s) +6H+ + 6e --> 2Fe(l) + 6H+ + 3O2-
(now the 6H+ cancel out)
Fe2O3(s) + 6e --> 2Fe(l) + 3O2-

would 3O2- be in an (aq) state since it is now an ion? In the book it says that it is (s)?
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fadzsta1

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Re: VCE Chemistry Question Thread
« Reply #279 on: February 03, 2014, 08:46:50 pm »
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I don't understand why the H2O dissociates, then cancels out the hydrogen ions... I also don't understand why the O2- ions are in the solid state....
I'm just gonna wait here till one of the experts on here delivers...
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #280 on: February 03, 2014, 08:53:54 pm »
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I don't understand why the H2O dissociates, then cancels out the hydrogen ions...

I did that so that that it is an ionic equation and then cancelled out the hydrogen ions since they are spectator ions *NOT SURE IF CORRECT* (probably isn't)
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #281 on: February 03, 2014, 09:08:56 pm »
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Ohhh I guess it does!  Do you have the heinemann chemistry student workbook? There's a similar Q. to this (Page 22, c))




Have I figured the half equation correctly?

Fe2O3(s) --> 2Fe(l) + 3H2O(l)
Fe2O3(s) +6H+ + 6e --> 2Fe(l) + 3H2O
Fe2O3(s) +6H+ + 6e --> 2Fe(l) + 6H+ + 3O2-
(now the 6H+ cancel out)
Fe2O3(s) + 6e --> 2Fe(l) + 3O2-

would 3O2- be in an (aq) state since it is now an ion? In the book it says that it is (s)?

I think there is a serious misunderstanding here. The thermite reaction does not involve water here. It is just aluminium powder and iron (III) oxide. The overall reaction is Fe2O3(s) + 2Al(s) => 2 Fe(l) + Al2O3(s)
Redox reactions don't always have to proceed in aqueous solution!

Also, even if this reaction did occur in water, aluminium oxide is insoluble in water.
EVEN IF aluminium oxide dissolved in water, the oxide ion cannot exist in water in aqueous form. Think about it: the oxide ion's conjugate acid is the hydroxide ion, which is a strong base. Therefore, the oxide ion must be a ridiculously powerful base capable of immediately deprotonating most things around it (like water). Aqueous oxide ions quickly become hydroxide ions.

I don't understand why the H2O dissociates, then cancels out the hydrogen ions... I also don't understand why the O2- ions are in the solid state....
I'm just gonna wait here till one of the experts on here delivers...

The water doesn't dissociate. Oxide ions react with water by O2-(aq) + H2O(l) => 2OH-(aq)
Now if this hydroxide encounters hydrogen ions...you know what happens.
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fadzsta1

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Re: VCE Chemistry Question Thread
« Reply #282 on: February 03, 2014, 09:29:45 pm »
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You are a god i love you
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #283 on: February 03, 2014, 09:43:57 pm »
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Ohhh I understand. Thanks so much :-)

What would the oxidation half equation be?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #284 on: February 03, 2014, 10:16:35 pm »
+1
Ohhh I understand. Thanks so much :-)

What would the oxidation half equation be?

Well...given the question...I'd say it would be Fe2O3(s)+ 6e- => 2Fe(l) + 3O2- (s)

Even though I dislike the practice of writing an ion as a solid. And yes the metallic iron is molten as per the question.

Edit: I've read the question incorrectly; this is the reduction half equation.
Edit again: how did I put the electrons on the wrong side of the equation.
« Last Edit: February 06, 2014, 10:38:59 am by lzxnl »
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