Unit 4 Questions MEGATHREAD!

[david10d]

i don't really understand the solutions for this question:
Half equations for the cells:

CH4 + 8OH- -> CO2+ 6H2O + 8e-

2O2 + 4H2O + 8e- -> 8OH-

When set up, the electrolyte contained 0.5mol of dissolved OH-. The cell operates for a period of time during which 0.1mol of CH4 gas is consumed. What amount of OH-, in mol, is now dissolved in the electrolyte? Explain how you arrived at your answer.

need answer please!

another question.

A block of nickel and a block of copper are joined together and then the combined block is placed in a solution of 1 M HCl solution at 25C. It was observered that ubbles of gas are only evolved from the surface of the copper. However if the two blocks are separated and placed in the same acid at 25C, the bubbles only appear on the nickel surface. Explain these observations and including half equations to support your explanations.

[nacho]

hey guys
any tips on summing the whole unit up in 1 day?

[Panicmode]

hey guys
any tips on summing the whole unit up in 1 day?

Study:

- Your chemical (briefly, 1/2 hour sohuld do it)
   . Remember waste management procedures and safety risk
   . Remember rate/yield considerations
   . Remember on major use of your chemical and an equation to go with it.
 
- Equilibrium (Important. Could possibly be done in 1/2 hour depending on how well you know it.)
  . Know how to use / determine K/Ka values
  . Understand Le Chaterlier's Principle
  . Effect of temperature/pressure/addition or removal of reactants on yield and K value
  . Activation energy and catalysts
 
- Fuels (skim this section. Not extremely important)
  . Very basic understanding of nuclear fission and fusion (10 minutes max)
  . Study one biofuel and understand how it's carbon neutral yaddayadda
  . Understand limitations/advantages of renewable/fossil fuels

-  Thermochemical equations (spend majority of time here and on electrolysis and equilibrium)
  . Understand delta H
  . Understand energy profiles
  . Learn how to determine delta H using calorimetry
  . Learn heat of combustion.

- Galvanic cells (spend a more time on this than fuels)
  . Understand principles behind galvanic cell reactions.
  . Know how to use the electrochemical series (and the conditions it's valid under)
  . Know purpose of salt bridge, anode cathode bladiblabla
  . Potential difference and working out voltages
  . Predicting spontaneous redox reactions

- Batteries (spend equal time here as with galvanic cells)
  . Understand how batteries work
  . Know how rechargable batteries work
  . Fuel cell basics (these have come up on every VCAA exam so far!)

- Electrolysis (spend same time here as on equilibrium)
  . Understand and know how to apply Faraday's laws
  . Understand competition at electrodes (will water be preferentrially oxidised?)
  . Know some aplications of electrolysis (electrorefining of copper, extraction of chlorine and sodium, electropating)


Try and fit in at least the last 3 VCAA past exams. There's your cram sesh in a nutshell.


Good luck Nacho...

[DisaFear]

^Pro tier advice, thank you

[nacho]

Massive thanks
I'm hoping I can get a C+ with that
been averaging 30/70 (did one paper two months ago -insight 2008)

[Panicmode]

Massive thanks
I'm hoping I can get a C+ with that
been averaging 30/70 (did one paper two months ago -insight 2008)

If you know (and can apply) everything I've listed, you should be able to easily get 1 58-62/72.

[Mao]

i don't really understand the solutions for this question:
Half equations for the cells:

CH4 + 8OH- -> CO2+ 6H2O + 8e-

2O2 + 4H2O + 8e- -> 8OH-

When set up, the electrolyte contained 0.5mol of dissolved OH-. The cell operates for a period of time during which 0.1mol of CH4 gas is consumed. What amount of OH-, in mol, is now dissolved in the electrolyte? Explain how you arrived at your answer.

I am your saviour.

Overall reaction: CH4 + 2O2 --> CO2 + 2H2O
No net change in OH-.
:. Amount of dissolved OH- remains constant. (Look at the two equations, they are being produced and consumed at the same rate. You can react any amount of CH4, and the electrolyte concentration will not change. This is another advantage of fuel cells.)

[Mao]

A block of nickel and a block of copper are joined together and then the combined block is placed in a solution of 1 M HCl solution at 25C. It was observered that ubbles of gas are only evolved from the surface of the copper. However if the two blocks are separated and placed in the same acid at 25C, the bubbles only appear on the nickel surface. Explain these observations and including half equations to support your explanations.

When Cu/Ni are joined: Ni --> Ni2+ + 2e, 2H+ + 2e --> H2.
So, near the nickel block, ample amounts of Ni2+ are formed. This repels H+ away from the Ni.
Since Cu/Ni are electrically connected, any part of Cu/Ni can act as the cathode. In this case, since the reactants (H+) finds it difficult to approach the Ni electrode due to repulsion, it makes sense that reduction would occur at the Cu surface (where Ni2+ is absent, thus no repulsion). :. Gas bubbles evolve at Cu surface.

When they are separated: Not in electrical contact. Cu does not react with H+, thus no bubble formation at Cu. Only Ni is reacting.
As Ni is no longer in electrical contact with Cu, H+ will have no choice but to approach the Ni electrode (despite the charge-charge repulsion). Bubbles evolve at Ni.
Note that due to this charge-charge repulsion, we will observe a smaller rate of reaction. (i.e. activation energy increases, which captures the repulsive barrier which H+ must overcome to reach the surface of Ni)