Unit 4 Questions MEGATHREAD!

[Greatness]

Im not  the best at chem but i think the reactinos occuing are O2 + 4H+ + 4e- -> 2H2O
and H2 -> 2H+ + 2e-     to get the overall equation multpliy the 2nd one by 2. The H+s will cancel out but i dont get the part about basic and stuff lol
im gonna say B..... wait for someone else to give their answer

not too sure abot the electrode question.
i think equilibrium means that the reactions have reached their limits? lol dunno
when we recharge a battery, the cathode doesnt become the anode and vice versa. the charges of the electrodes change so thee positive cathode becomes the negative cathode. reduction still occurs at the cathode and oxidation at the anode.
yeah im not too sure what we need to know about batteries but i reckon know the structures, how they work and adv/disadv...
i started doing exams this week i hope this helped a little bit :s lol

[HenryP]

I believe the reaction occurring at the cathode would be O2 +2H2O +4e- --> 4OH- and the anode reaction would be 2OH- + H2 --> 2H2O +2e-
In response to swarley, I think your answer may be incorrect as there is no H+ ions for the oxygen to react with.
I believe the answer will be C as the build up of OH- ions occurs in the cathode half cell.
If I'm wrong though please ignore everything lol. Not really that great at chem to be honest.

[nbhindi]

Hey guys, when I'm deriving the reduction half equation of the alkaline cell, I always get the following:

MnO₂ + H⁺ + e^- = MnO(OH) + OH^-

However, my textbook has the following half equation:

2MnO₂ + 2H₂O + 2e^- = 2MnO(OH) + 2OH^-

I can see that they have added OH^- to both sides, however, is my answer incorrect???
Thanks 

[b^3]

Remember it is an alkaline cell, i.e. OH^- and basic conditions. If it were acidic conditions then your equation works, but since it is alkaline conditions, the textbok equation works.

[nbhindi]

Remember it is an alkaline cell, i.e. OH^- and basic conditions. If it were acidic conditions then your equation works, but since it is alkaline conditions, the textbok equation works.

I understand that the electrolyte is KOH and the reaction occurs in an alkaline environment, but then how do I derive that half equation using the "KOHES" method???

[Mao]

Remember it is an alkaline cell, i.e. OH^- and basic conditions. If it were acidic conditions then your equation works, but since it is alkaline conditions, the textbok equation works.

I understand that the electrolyte is KOH and the reaction occurs in an alkaline environment, but then how do I derive that half equation using the "KOHES" method???

Do KOHES as usual. Then add base to neutralize the acid.

[nbhindi]

Do KOHES as usual. Then add base to neutralize the acid.

Ahhh cheers Mao now I get it 

[Mr. Study]

Would anyone happen to have a website explaining concentration versus time graphs?

Hopefully that's not a noob question. xD

[Vincezor]

but then how do I derive that half equation using the "KOHES" method???

Sorry what is this "KOHES" method? I've never heard of it before :S

[tony3272]

K - Balance your key elements first
O - Balance your oxygen by adding H₂0
H - Balance hydrogens by adding H⁺
E - Balance the charge by adding e^-
S - Add the states

[luken93]

Question 3
Which of the following changes will always ensure an increase in the rate of a chemical reaction?

I Adding a catalyst
II Increasing temperature
III Increasing concentration
IV Increasing the surface area of the reactant

A. I only
B. I, II and III only
C. I, II and IV only
D. I, II, III and IV

Can someone explain how increasing surface area DOESN'T increase the rate of reaction?

[HenryP]

I remember doing that question. I picked D since I thought that increasing the surface area increased the rate. According to the answers however I think option IV should've been "Adding more reactants". In that case the answer would be B.

[luken93]

I remember doing that question. I picked D since I thought that increasing the surface area increased the rate. According to the answers however I think option IV should've been "Adding more reactants". In that case the answer would be B.

Yeah, that's what I thought as well, I was tripping out when it said surface area DOESN'T increase the rate of reaction, like that's the old trick in the book!

[HarveyD]

Just wondering what you guys would put for this question

[Milkshake]

2H20(l) => O2(g) + 4H+(aq) + 4e-, since H20(l) is the strongest reductant present in the solution, so we expect to see the solution at the anode become acidic and produce oxygen gas.

EDIT: The concentration of Cl2 I took was wrong