VCE Chemistry Question Thread

[RazzMeTazz]

Can someone please explain how to do the following question (attached below).

Thankyou 

The answers are:
Aluminium: 74 kJ
Copper: 32 kJ

[jessss0407]

Hi!

Can someone please explain why, out of the following, H2O2 has the highest oxidation number for oxygen?
A. O2
B. H2O2
C. Na2O
D. H2SO4
Thanks!

[keltingmeith]

Hi!

Can someone please explain why, out of the following, H2O2 has the highest oxidation number for oxygen?
A. O2
B. H2O2
C. Na2O
D. H2SO4
Thanks!

Because they're silly. The answer is O2.

[jyodesh.com]

Hi!

Can someone please explain why, out of the following, H2O2 has the highest oxidation number for oxygen?
A. O2
B. H2O2
C. Na2O
D. H2SO4
Thanks!

Sorry but I can't because O₂ has the highest oxidation number
For C and D, oxygen is more electronegative than the atoms it is bonded to so if we think of the bonds having a completely ionic character, oxygen will have a charge of -2.
For B, each oxygen has bonds to one oxygen and one hydrogen. This means it has an oxidation number of -1
For A, each oxygen only has bonds to another oxygen so it has an oxidation number of 0.

[jyodesh.com]

Can someone please explain how to do the following question (attached below).

Thankyou 

The answers are:
Aluminium: 74 kJ
Copper: 32 kJ

In this question they're not asking for the amount of heat absorbed by water, they're asking for the amount of heat absorbed by the pots to bring them to the boiling point of water. It's also saying that the temperature of the pot is equal to the temperature of the water. So to heat the pot from 18 to 100 degrees celsius you'll need E=cx(1x10³)x(100-18) joules to heat up the pot. (Note this is only referring to the energy required to heat the pot and not the water inside it. If we had say 1.0kg of water inside each pot we would need a total of 343 (water)+74 (aluminium)=417kJ to heat the aluminium pot)

[knightrider]

How would you do this question?

Write full and ionic chemical equations for the reactions between:

 zinc and ethanoic acid

[@#035;3]

chemical equation: 2CH3COOH(aq) + Zn(s) -> Zn(CH3COO)2(aq) +H2(g)

ionic equation: Zn(s) + 2H+(aq) -> Zn2+(aq) + H2(g)

hope this helped

[lzxnl]

chemical equation: 2CH3COOH(aq) + Zn(s) -> Zn(CH3COO)2(aq) +H2(g)

ionic equation: Zn(s) + 2H+(aq) -> Zn2+(aq) + H2(g)

hope this helped

Actually, the chemical and ionic equations are the same here. Ethanoic acid mostly exists in its non-dissociated form because it is a weak acid.

[jyodesh.com]

Actually, the chemical and ionic equations are the same here. Ethanoic acid mostly exists in its non-dissociated form because it is a weak acid.

But its the protons that are getting reduced not the ethanoic acid so i think @#035;3's answer is fine

[knightrider]

chemical equation: 2CH3COOH(aq) + Zn(s) -> Zn(CH3COO)2(aq) +H2(g)

ionic equation: Zn(s) + 2H+(aq) -> Zn2+(aq) + H2(g)

hope this helped

Thanks @#035;3   

where did the 2H+(aq) come from ?

[knightrider]

How would you do this question ?

Write full and ionic chemical equations for the reactions between

magnesium hydrogen carbonate and hydrochloric acid

[warya]

How would you do this question ?

Write full and ionic chemical equations for the reactions between

magnesium hydrogen carbonate and hydrochloric acid

Write the full equation with states, take out anything that has not changed states. These are the spectator ions. You are left with a full ionic equation.

[warya]

I'm having trouble understanding standard electrode potentials, particularly what voltage/potential difference is?

[jyodesh.com]

I'm having trouble understanding standard electrode potentials, particularly what voltage/potential difference is?

The negative charge on electrons makes them want to get away from eachother. So if you cramp a whole lot of electrons in one place, they're going to want to spread out i.e. they will have a high electrical potential energy. This is analogous to water being held in a dam.
The electrons are going to want to move to a place where there are less electrons i.e. a place that has a lower potential energy. This difference in electrical potential energy is known as voltage.

(For the purposes of the explanation I switched around the positive-negative convention of measuring potential energy)

[Sine]

To be honest, I'm not too happy with that response. It's much simpler, I feel, to measure the pH of, say, a 1 M NaOH solution and add NaHCO3 and watch the pH drop due to NaHCO3's acidity. Similarly, add some NaHCO3 to 1 M HCl and you'll see the pH rise due to its basicity. That is a sufficient demonstration. Otherwise, you have to somehow justify the existence of CO3 2- ions and H2CO3 molecules, which isn't trivial.

Generally a good train of thought to use, but not always. The hydrogen sulfate ion is still a much stronger acid than it is a base, despite having a negative charge.

What is the reason for amphiprotic substances to be either stronger acids/bases?