VCE Chemistry Question Thread

[Jakeybaby]

Cool, no carbonate, just water and the salt. Got it

Yep, that's correct:

Acid + Base -> Water + Salt

[FutureDoctor2k16]

Been around a while as a guest. Thought Id finally make an account cos I have a qsn. Hopefully I'm posting this in the right place...

Just wondering: What are some common errors for equilibrium pracs? Especially for equilibrium pracs where it's like all qualitative. Like what if we get a prac where the equilibrium reaction is coloured (like this one: http://3.bp.blogspot.com/-NnKFxXywSJA/VGgtgTWCsZI/AAAAAAAAAhs/ohF0LZZgMjI/s1600/colourchangeHI.JPG) and basically we're just recording the colours of the solution/colour changes after we add more reactant/product/whatever. What sort of errors could we even have?

[vox nihili]

Been around a while as a guest. Thought Id finally make an account cos I have a qsn. Hopefully I'm posting this in the right place...

Just wondering: What are some common errors for equilibrium pracs? Especially for equilibrium pracs where it's like all qualitative. Like what if we get a prac where the equilibrium reaction is coloured (like this one: http://3.bp.blogspot.com/-NnKFxXywSJA/VGgtgTWCsZI/AAAAAAAAAhs/ohF0LZZgMjI/s1600/colourchangeHI.JPG) and basically we're just recording the colours of the solution/colour changes after we add more reactant/product/whatever. What sort of errors could we even have?

What do you think?

Not being snarky or anything, but the more of these errors you can work out for yourself first the more will stick

[Sine]

Been around a while as a guest. Thought Id finally make an account cos I have a qsn. Hopefully I'm posting this in the right place...

Just wondering: What are some common errors for equilibrium pracs? Especially for equilibrium pracs where it's like all qualitative. Like what if we get a prac where the equilibrium reaction is coloured (like this one: http://3.bp.blogspot.com/-NnKFxXywSJA/VGgtgTWCsZI/AAAAAAAAAhs/ohF0LZZgMjI/s1600/colourchangeHI.JPG) and basically we're just recording the colours of the solution/colour changes after we add more reactant/product/whatever. What sort of errors could we even have?

With errors for equilibrium think about what variables impact the position of equilibrium and then think about what we have done to control those variables.
Variables would be
Temperature, above or below 298K exothermic vs endothermic
Volume (pressure): note volume doesn't impact equilibrium if it's the same concentration.
Concentration

[FutureDoctor2k16]

What do you think?

Not being snarky or anything, but the more of these errors you can work out for yourself first the more will stick

With errors for equilibrium think about what variables impact the position of equilibrium and then think about what we have done to control those variables.
Variables would be
Temperature, above or below 298K exothermic vs endothermic
Volume (pressure): note volume doesn't impact equilibrium if it's the same concentration.
Concentration

But what if all of those were kept constant? (sorry... not trying to be difficult or anything)

I hate errors...

Like the only thing I could think of was that maybe the test tubes could have been rinsed with water (not distilled - so like contaminants and stuff?) or maybe the test tube hadn't been rinsed and there'd been other chemicals in it? And I don't even know if that would count because the question doesn't mention rinsing the test tubes or anything. I can't think of anything and I've tried. I've been stuck on this question all day and it's driving me up the wall just a little...

Apparently I should be able to think of "at least 3 errors". Can't even think of one. 

[HasibA]

hey guys , have a chem pac sac and need helps with some questions -><- is reversible arrow
basically, it's this reaction: Fe3+(aq) + SCN-(aq) -><- Fe(SCN)2+(aq)
the product of the reaction (Fe(SCN)2+) is deep red in colour. Fe3+ solution is pale yellow, SCN- is colourless.
basically, a mixture of chemicals have been added to test tubes containing this Fe(SCN)2+ solution. in one of them, a specficed amount of water was added to it; in another, the test tube was with the Fe(SCN)2+ solution's test tube was placed in a beaker with iced water. The mixture with the water added became a bit lighter than the original red solution. The mixture placed in the iced water became darker.
can anyone help me with these? struggling to find the reason why/how iced water causes a different colour change- thank YOU



edit: another q that's similar to the one up above.

CoCL2(s) forms complex ions, [Co(H2O)6]2+ (pink) and (CoCL4)2- (blue) when dissolved in water. when added to dilute HCL or saturated NaCl the following equilimbrium is established.
(CoCl4)2-(aq) + 6H2O(l) -><- [Co(H2O)6]2+(aq) + 4Cl-(aq)
at relatively low concentrations of chloride ions, the solution is pink. The Equilibrium is sensitive to temp as well as to concentrations of solutes.  why does the addition of water causes the colour to be lighter pink, the addition of 12M HCL cause the colour to become blue, the addition of 0.1M AgNO3 cause the colour to become cloudy pink, the test tube contain the solution placed in iced water cause pink, and when the test tube is heated in a water bath, why does the colour become blue.


i know this is a lot, but i tried solving it , and have no answers, so i have no idea :p thanks guys means a lot!

[zsteve]

hey guys , have a chem pac sac and need helps with some questions -><- is reversible arrow
basically, it's this reaction: Fe3+(aq) + SCN-(aq) -><- Fe(SCN)2+(aq)
the product of the reaction (Fe(SCN)2+) is deep red in colour. Fe3+ solution is pale yellow, SCN- is colourless.
basically, a mixture of chemicals have been added to test tubes containing this Fe(SCN)2+ solution. in one of them, a specficed amount of water was added to it; in another, the test tube was with the Fe(SCN)2+ solution's test tube was placed in a beaker with iced water. The mixture with the water added became a bit lighter than the original red solution. The mixture placed in the iced water became darker.
can anyone help me with these? struggling to find the reason why/how iced water causes a different colour change- thank YOU

Iced water reduces the temperature of the mixture, and changes the value of K. This results in an shift in the equilibrium position and a change in colour. Judging from the results, the reaction should be exothermic as cooling it causes a right shift and thus a darker solution. You might want to google to confirm that the reaction is indeed exothermic.

[sweetiepi]

hey guys , have a chem pac sac and need helps with some questions -><- is reversible arrow
basically, it's this reaction: Fe3+(aq) + SCN-(aq) -><- Fe(SCN)2+(aq)
the product of the reaction (Fe(SCN)2+) is deep red in colour. Fe3+ solution is pale yellow, SCN- is colourless.
basically, a mixture of chemicals have been added to test tubes containing this Fe(SCN)2+ solution. in one of them, a specficed amount of water was added to it; in another, the test tube was with the Fe(SCN)2+ solution's test tube was placed in a beaker with iced water. The mixture with the water added became a bit lighter than the original red solution. The mixture placed in the iced water became darker.
can anyone help me with these? struggling to find the reason why/how iced water causes a different colour change- thank YOU

(While I was typing this, zsteve responded and I actually like his help better )
Hey! From my understanding, the deepening of the red from the iced water experiment is due to the temperature of the mixture. The colder temperature allows the Fe ³⁺ and the SCN^- to react more efficiently, providing a net forward reaction. That is, the reaction favours the exothermic reaction in colder temperatures, as it produces more heat. If I'm not /that/ clear, I'm sure the true blue chem squad will clarify this.

Also, this may help a little: http://www.ausetute.com.au/lechatsp.html

[HasibA]

thanks guys- i edited my og post, can you guys help with those? thanks so much- no answers provided so needed to confirm!

[jyce]

edit: another q that's similar to the one up above.

CoCL2(s) forms complex ions, [Co(H2O)6]2+ (pink) and (CoCL4)2- (blue) when dissolved in water. when added to dilute HCL or saturated NaCl the following equilimbrium is established.
(CoCl4)2-(aq) + 6H2O(l) -><- [Co(H2O)6]2+(aq) + 4Cl-(aq)
at relatively low concentrations of chloride ions, the solution is pink. The Equilibrium is sensitive to temp as well as to concentrations of solutes.  why does the addition of water causes the colour to be lighter pink, the addition of 12M HCL cause the colour to become blue, the addition of 0.1M AgNO3 cause the colour to become cloudy pink, the test tube contain the solution placed in iced water cause pink, and when the test tube is heated in a water bath, why does the colour become blue.


i know this is a lot, but i tried solving it , and have no answers, so i have no idea :p thanks guys means a lot!

- The addition of water dilutes the solution, and the system will only be able to partially oppose this change. Therefore, overall the solution will be more dilute at the new equilibrium position, and more dilute = less colour intensity, i.e. less pink.

- Addition of HCl increases the concentration of chloride ions, thereby causing a net backwards reaction towards the blue ion.

- Addition of AgNO₃ precipitates the chloride ions, thereby causing a net forwards reaction towards the pink ion. The cloudiness is due to the formation of AgCl precipitate.

- If ice water (i.e., a decrease in temperature) causes a net forwards reaction (hence the colour of the solution becoming pink) and an increase in temperature causes a net backwards reaction (hence the colour of the solution becoming blue), then the reaction must be exothermic in the forwards direction, and endothermic in the reverse.

Not sure if you were wanting explanations, or confirmation in whether the colour changes you stated were correct?

[HasibA]

snip

needed explanations, but isnt it exothermic in the forward reaction bc in ice water, temp decreases , and it wants to inc that temp back to eq, hence forwards is exothermic? this right .. thanks tho
edit: misread your post, you're right, ty

[jyce]

needed explanations, but isnt it exothermic in the forward reaction bc in ice water, temp decreases , and it wants to inc that temp back to eq, hence forwards is exothermic? this right .. thanks tho
edit: misread your post, you're right, ty

Haha, I realised my mistake and changed it already, so yeah you're right

[HasibA]

Haha, I realised my mistake and changed it already, so yeah you're right

still- thanks heaps for the help- you're a gun!

[blacksanta62]

Jyce for mod

[blacksanta62]

Hey guys, with the attached question(s):
1) The first attachment - The mol ratios won't always be 1:1 will it? And why can we get the conc. of the two from the mol? i.e. [H3O^+] and [CH3COO^-]
2) Second attachment - Do we need to know this for the end of year exam?
Thank you
Edit: I worded my question weirdly, I mean why can we say that because we have 1 mol of H3O... etc. we can say their conc. are equal? I think I may have just answers my question while typing this but I would still like confirmation