VCE Chemistry Question Thread

[Gogo14]

1. Is it important to include the units for equilibrium constant? I remember stephen said it doesnt matter, but my textbook always includes the units in the answers. Asking cuz the units are kinda a pain to figure out
2. Why is question 4a decrease? Shouldnt it be increase?

[deStudent]

H2O2 is being oxidised (check the 9th row down on your electrochemical series). O2 cannot be oxidised, so H2O2 must be oxidised.
There aren't any H2O being oxidised below H2O2. So B is wrong.

A and C are obviously wrong. Hence D is the correct answer.
The one with only one half cell with two electrodes is the electrolytic cell.

Weaker reductant = stronger oxidant
Weaker oxidant = stronger reductant

So you can still use the same method.

Thanks Syndicate. But why can't O2 be oxidised? Couldn't O2 be oxidised and H2O2 be reduced or are you using the student's predicted overall as reference?

[zhen]

1. Is it important to include the units for equilibrium constant? I remember stephen said it doesnt matter, but my textbook always includes the units in the answers. Asking cuz the units are kinda a pain to figure out
2. Why is question 4a decrease? Shouldnt it be increase?

Le Chatelier's principle states that if a system in equilibrium is subject to change, the system will adjust itself to partially oppose the effect of the change. So, by increasing the volume, the concentration decreases. But a forward reaction will occur to make more NO2, but this adjustment will only partially oppose the change in concentration, so the overall concentration will be still lower.

[Syndicate]

Thanks Syndicate. But why can't O2 be oxidised? Couldn't O2 be oxidised and H2O2 be reduced or are you using the student's predicted overall as reference?

O2 is really unlikely to oxidise (due to its high electronegative features, which doesn't allow electrons to be lost). O2 is also not available on the right side of the equations (O2 reactions are only going forward not backwards, so they are reducing not oxidising) in the data book. So H2O2 must be oxidised. H2O is not a stronger reductant that H2O2 (in some cases, it might be, however, in this case it isn't), hence, B must be wrong.

[-273.15]

Helloo

Could someone please explain to me:

1. Why the anode is negative and cathode is positive in galvanic cells

2. Why the anode is positive and cathode is negative in electrolytic cells

3. Why the anode and cathode have different polarities (positive or negative) in galvanic and electrolytic cells when oxidation still occurs at the anode and reduction still occurs at the cathode  and the electron flow is still in the same direction?
- does it have something to do with the power source and if so could you explain this clearly?
I assume in electrolytic cells the cathode is negative as it is gaining electrons which have a negative charge hence it becomes negative but this doesnt make sense with galvanic cells..

thank you!

[Gogo14]

I cant seem to get the correct ans for this q.

In the ice table in the answers, the number of moles of H2O at equilibrium is 0.119. But I got 0.14
Besides that everything else is the same.

[Syedali_]

I cant seem to get the correct ans for this q.

In the ice table in the answers, the number of moles of H2O at equilibrium is 0.119. But I got 0.14
Besides that everything else is the same.

books working out is wrong and so is the answer.

[johnhalo]

Hi guys,
I'm having trouble with this question (or 3 questions) and would love some help:

How much energy (in kJ) is released by combustion of one mole of methane if the percentage converted to carbon monoxide (with the balance converted to carbon dioxide) is 0%, 20% and 100%?

Thanks

[vcestressed]

Hey,
May i please know how the answer  for ii is hydrogen?
Thanx! :')

[rpapa]

Hi! I was wondering whether I could get some help with this question? I can't seem to get the molecular formula of the biodiesel molecule.

A biodiesel molecule can be formed from the reaction between glycerol and stearic acid
a) What is the molecular formula of the biodiesel molecule?
b) Write a balanced equation for the complete combustion of this biodiesel molecule.

[cookiedream]

Helloo

Could someone please explain to me:

1. Why the anode is negative and cathode is positive in galvanic cells

2. Why the anode is positive and cathode is negative in electrolytic cells

3. Why the anode and cathode have different polarities (positive or negative) in galvanic and electrolytic cells when oxidation still occurs at the anode and reduction still occurs at the cathode  and the electron flow is still in the same direction?
- does it have something to do with the power source and if so could you explain this clearly?
I assume in electrolytic cells the cathode is negative as it is gaining electrons which have a negative charge hence it becomes negative but this doesnt make sense with galvanic cells..

thank you!

Denoting an electrode as negative or positive depends on whether electrons appear there first or not.
The anode always involves oxidation and the cathode always reduction.

1. So in galvanic cells, because the metal at the anode is oxidising hence releasing electrons, that's where electrons are first appearing so it's negative.
Therefore the cathode in galvanic is positive.

2. However, in electrolytic cells, the negative terminal pumps electrons to the cathode, so in this case the cathode is negative (electrons are appearing here first) and the anode is positive (the power source draws electrons from it)

3. Remember that in a cell, regardless of whether it is discharging (galvanic) or recharging (electrolytic) the polarities remain the same. It's just the reactions that are swapped. Here's a diagram which may help. Notice where the electrons first appear.

Just remember that the following things swap when a cell is switching from discharging to recharging:
- The direction of electron flow
- Location of cathode (therefore, where the reduction reaction occurs)
- Location of anode (therefore, where the oxidation reaction occurs)
- The reactants and products of
          - The oxidation reaction
          - The reduction reaction
          - The overall reaction

[cookiedream]

Hey,
May i please know how the answer  for ii is hydrogen?
Thanx! :')

When you balance the equation (where you add a 2 in front of the water to balance out oxygen), you end up with 2 hydrogen atoms left over. These then produce hydrogen gas

[asdfdsa]

Quick question about galvanic cells - why is the salt bridge a bridge? why not have the anions and cations is separate containers, supplying the anode and cathode respectively? Wouldn't say, a salt bridge containing K2SO4, with SO4 anions near the cathode have trouble getting to the other side?

[sweetiepi]

Quick question about galvanic cells - why is the salt bridge a bridge? why not have the anions and cations is separate containers, supplying the anode and cathode respectively? Wouldn't say, a salt bridge containing K2SO4, with SO4 anions near the cathode have trouble getting to the other side?

Think of it as a circuit, if you have a circuit that isn't open, it doesn't work. However, if you have a closed circuit, the electricity (or this case, electrons) can flow through the "circuit" as it is completed.
Thus the salt bridge containing K2SO4 instead of SO4 2- anions and K+ cations allows the electrons to flow through, but SO4 2- and K+ on its own does not do anything, the two half cells remain as half cells.
(This may be a really haphazard answer, I'll fix it up when I get home, haha)

[Nicko912]

Hi quick question ( Daniell cell ),

If I were to mix Cu(NO₃)₂ solution in a single beaker with ZnSO₄ solution with a copper and zinc electrode in the single beaker, what are two predictions why the cell won't work? ( wire is still present )