VCE Chemistry Question Thread

[Swagadaktal]

Here's a few:

The "fizzing" is a result of a dynamic equilibrium between carbon dioxide and the water in the bottle, described as follows:
CO2 + H2O <-----> H2CO3
Explain why soft drinks can go flat.

A particular reaction has the following equilibrium:

A+2B <------> C, K=7
2 mole of A and 1 mole of B are allowed to equilibriate. How much mole of C are present at the end of this reaction? If 3 mole of A is added to the solution, how much mole of B and C are left after the solution is allowed to equilibriate?

Ok for the first one I think I've got it (im fairly new to equilibrium so excuse my lack of knowedge)
- The fizz is caused from an excess of the reactants which causes carbonic acid to form, giving the fizzing of the soft drink - when H2CO3 is created the concentration increases therefore the rate of reaction backwards also increases - This is when the fizzing starts to die down and the drink slowly becomes less fizzy. At the point where the drink becomes flat, the drink has reached equilibrium, that is the forward and back reactions are occuring at the same rate. At this point the drink is flat and any new carbonic acid formed is negated by the back reaction of carbonic acid becoming H2O and CO2  - how was the wording of this and is the ans correct?

Can you give us a hint for the second question? Not getting anywhere - does it involve any mathematics (beyond simple quadratics algebra?)

[keltingmeith]

Ok for the first one I think I've got it (im fairly new to equilibrium so excuse my lack of knowedge)
- The fizz is caused from an excess of the reactants which causes carbonic acid to form, giving the fizzing of the soft drink - when H2CO3 is created the concentration increases therefore the rate of reaction backwards also increases - This is when the fizzing starts to die down and the drink slowly becomes less fizzy. At the point where the drink becomes flat, the drink has reached equilibrium, that is the forward and back reactions are occuring at the same rate. At this point the drink is flat and any new carbonic acid formed is negated by the back reaction of carbonic acid becoming H2O and CO2  - how was the wording of this and is the ans correct?

Nope, sorry, all wrong - the answer has nothing to do with concentration. Things to think about:

Is the carbonic acid in equilibrium before opening the bottle?
What happens when you open the bottle? (think about the last time you opened a bottle - any bottle, could be bottled water. Something very important happens to the bottle when you open one up for the first time that doesn't happen any other time after it's been opened once)
What can affect an equilibrium? (hint: le Chetelier's principle)

Can you give us a hint for the second question? Not getting anywhere - does it involve any mathematics (beyond simple quadratics algebra?)

EDIT: Ignore what I originally wrote here - I read my own question wrong. But yeah, only requires algebra and such. Ever heard of the ICE method? Because you may find it useful here.

[Jakeybaby]

Does the length of a salt bridge in a galvanic cell affect the voltage produced?

[keltingmeith]

Does the length of a salt bridge in a galvanic cell affect the voltage produced?

Good question - do you think it would? Why or why not? How is the voltage of a galvanic cell produced?

[Jakeybaby]

Good question - do you think it would? Why or why not? How is the voltage of a galvanic cell produced?

I don't think it would.
Due to the fact that a galvanic cell uses the electrical energy available from the transfer of electrons in a redox reaction. The salt bridge just 'completes' the circuit so to speak in my eyes.

[keltingmeith]

I don't think it would.
Due to the fact that a galvanic cell uses the electrical energy available from the transfer of electrons in a redox reaction. The salt bridge just 'completes' the circuit so to speak in my eyes.

I'd say that you've hit the nail on the head! Certainly, a longer salt bridge could affect things such as how long your battery lasts, or how effective the battery is, it's certainly not going to affect the voltage of a cell - this is determined solely by the chemicals used in the cell.

Another thought experiment for anyone else who doesn't believe what I've said - if the size of the salt bridge affects the voltage of the cell, wouldn't its composition also affect it? In which case, why don't we define a KCl salt bridge? Why instead can the salt bridge be whatever you have on hand at the time? Things to think about.

[Elizawei]

Is the answer to part b 1.0 ?
_{sorry for huge img}

[Sine]

(Image removed from quote.)

Is the answer to part b 1.0 ?
_{sorry for huge img}

pH = 0.98

[Jakeybaby]

In my latest Chemical Practical Report, the relationship between the concentration of the electrolytes and the voltage produced was discovered, as the concentration increases, so does the voltage. However, a question that is incorporated into the report is in regards to the future predictions that could be made. Would the voltage eventually plateau? What would happen in regards to future events with the galvanic cell with Cu & Mg?

[Elizawei]

pH = 0.98

Ah ok so 2 sig figs? I thought pH only have 1 decimal place
Thanks for clarification though!

[Sine]

Ah ok so 2 sig figs? I thought pH only have 1 decimal place
Thanks for clarification though!

Yeah I'm pretty sure normal sig fig rules still apply. 

[kimmie]

oh thanks!

Hi, kimmie

Your definition of oxidising strength is spot-on 

[Swagadaktal]

Nope, sorry, all wrong - the answer has nothing to do with concentration. Things to think about:

Is the carbonic acid in equilibrium before opening the bottle?
What happens when you open the bottle? (think about the last time you opened a bottle - any bottle, could be bottled water. Something very important happens to the bottle when you open one up for the first time that doesn't happen any other time after it's been opened once)
What can affect an equilibrium? (hint: le Chetelier's principle)

EDIT: Ignore what I originally wrote here - I read my own question wrong. But yeah, only requires algebra and such. Ever heard of the ICE method? Because you may find it useful here.

OK so for Q1, redo.
For a soft drink, upon initial opening the carbon dioxide reacts with the H2O for the first time, creating the carbonic acid in a forward reaction. As this is an equilibrium reaction, the increase of carbonic acid will lead to a net backwards reaction of H2O and CO2 - i think im somewhere along the right track but im stumped >:|

for Q2,
Is the first C value 1.23 mol? and the 2nd C value 2.47 mol, and B = 2.35 mol?

[jyce]

OK so for Q1, redo.
For a soft drink, upon initial opening the carbon dioxide reacts with the H2O for the first time, creating the carbonic acid in a forward reaction. As this is an equilibrium reaction, the increase of carbonic acid will lead to a net backwards reaction of H2O and CO2 - i think im somewhere along the right track but im stumped >:|

Pretty sure it relates to the fact that, when the drink is opened, CO₂(g) escapes. 

[Swagadaktal]

Pretty sure it relates to the fact that, when the drink is opened, CO₂(g) escapes. 

oooh that would make much more sense - but isn't there CO2 in the atmosphere as well?