VCE Chemistry Question Thread

[Bri MT]

A note on pH questions recently: remember that these are not examinable in the current 3/4 chem study design. Nothing wrong with asking but be aware

[Chocolatepistachio]

For all molecules that are trigonal planar will the bond angle always be 120 and tetrahedral will always be 109.5

[keltingmeith]

For all molecules that are trigonal planar will the bond angle always be 120 and tetrahedral will always be 109.5

Roughly, yes

[ErnieTheBirdi]

I was just wondering is there a way to determine the state of matter of a compound?
For example:
Magnesium Chloride reacts with Potassium Hydroxide to form Magnesium Hydroxide and Potassium Chloride
                  MgCl2                      +                      2KOH                     -->              Mg(OH)2                   +             2KCl

Also, How would I determine what the ionic formula would be?

[Chocolatepistachio]

For this question hydrogen gas combines with chlorine gas to form hydrogen chloride. If 2.00 moles of hydrogen gas reacts with excess chlorine gas, what mass of hydrogen chloride can be obtained?

n(HCl) = 4.00
m=4.00x 36.458 = 145.832
would this be right

The answer says 36.5g

[keltingmeith]

I was just wondering is there a way to determine the state of matter of a compound?
For example:
Magnesium Chloride reacts with Potassium Hydroxide to form Magnesium Hydroxide and Potassium Chloride
                  MgCl2                      +                      2KOH                     -->              Mg(OH)2                   +             2KCl

Also, How would I determine what the ionic formula would be?

Wording is important - if the question tells you something is in a certain state, always note that first. Yes, this time it doesn't - but I'm still going to say, just because too often I've given out rules like these, and had people complain to me when the question makes its own rules. Just because we try to predict chemistry, doesn't mean we can - so often questions will tell you what actually happened in an experiment, because it goes against what you'd expect based on the "rules" you've been taught so far.

So, the question then becomes - how do we know the states if we're not told what they are? Well, next you need to go back to the question and ask - is this a solid state reaction, a gaseous reaction, or a solution/liquid reaction? It's easily to tell if something is a gas - they'll usually use words like, "gas", "vessel", or they'll give you information relating to pressure, temperature, etc. Solid-state reactions, you can basically ignore - they're above VCE level. Tbh, I'll be more impressed if you can find one. So, what makes something a liquid or solution reaction?

So, if we're dealing with metals, it HAS to be a solution reaction, with one caveat. Metals aren't liquid at room temperature (unless you're dealing with mercury or galium). HOWEVER, if they mention that the metals are in a molten state, then that means we've heated everything up the point that the metals ARE a liquid, so it is a liquid reaction. That means the states for everything become liquid or gas - and for these kinds of reactions, typically, it's just liquid, because gases become scary at high temperatures.

So the final step is, if it's a solution reaction, how do we know if something's solid, or aqueous? Well, it'll be aqueous if the compound is soluble in water, and a solid if it is not soluble.

Finally, to write the ionic equation, simply write everything as if it was dissociated - and remember it will only dissociate if it's soluble (in an aqueous state). Eg, MgCl₂ (aq) ---> Mg²⁺ (aq) + 2Cl^- (aq), MgCl₂ (s) ---> MgCl₂ (s).

To write the net-ionic equation, write the ionic equation, then just remove anything that appears on both sides. For example:
NaOH (aq) + HCl (aq) ---> NaCl (aq) + H₂O (l), becomes:
Na⁺ (aq) + OH^- (aq) + H⁺ (aq) + Cl^- (aq) ---> Na⁺ (aq) + Cl^- (aq) + H₂O (l), becomes:
OH^- (aq) + H⁺ (aq) ---> H₂O (l)

For this question hydrogen gas combines with chlorine gas to form hydrogen chloride. If 2.00 moles of hydrogen gas reacts with excess chlorine gas, what mass of hydrogen chloride can be obtained?

n(HCl) = 4.00
m=4.00x 36.458 = 145.832
would this be right

The answer says 36.5g

You're right, they just multiplied by 2 instead of dividing by 2

[Chocolatepistachio]

For this question I got the wrong answer which part did I get wrong
The gas N2O can be prepared by heating a mixture of ammonium chloride and sodium nitrate according to the following equation : NH4Cl + NaNO3 ——> N2O + Nacl + 2H2O

What mass of N2O can be prepared from 100.0g of ammonium chloride


n(N2O) = 1.869 mol
m(N2O) =nM = 1.86943842x 44.02
= 82.29g

[Owlbird83]

For this question I got the wrong answer which part did I get wrong
The gas N2O can be prepared by heating a mixture of ammonium chloride and sodium nitrate according to the following equation : NH4Cl + NaNO3 ——> N2O + Nacl + 2H2O

What mass of N2O can be prepared from 100.0g of ammonium chloride


n(N2O) = 1.869 mol
m(N2O) =nM = 1.86943842x 44.02
= 82.29g

I got the same answer as you. Is there any more to the question? It seems like you are correct to me.

[Chocolatepistachio]

No there is no more to the question

[jnlfs2010]

Hey everyone, I'm not quite sure how to answer this question. I can find the mol of copper lost, I can find the amount of Q with the information that is stated. But I'm not really sure how to go to get the Avogadro constant (STAV 2011 UNIT 4).

Any help would be appreciated thanks =)

[Answer- 6.23x10^23]

[SmartWorker]

For redox reactions, for overall reactions do we show the charges of ions and do we show spectator ions?

For example for this question: If Copper(II) sulfate solution is stored in an aluminium container.

Which of these reactions would be correct or are both correct?
(1) 2Al(s) + 3CuSO₄ (aq) ------> Al₂(SO₄)₃ (aq) + 4Cu (s)
or
(2) 2Al(s) + 3Cu²⁺(aq) ------> 3Cu(s) + 2Al³⁺ (aq)

Thank you!

[coldairballoon]

This might be a trivial question, but does a peptide bond/linkage consist of just the bond between C and N, or does it also include the O and the H? And is there a difference between a peptide bond and a peptide linkage, or did someone lie to me?

[keltingmeith]

For this question I got the wrong answer which part did I get wrong
The gas N2O can be prepared by heating a mixture of ammonium chloride and sodium nitrate according to the following equation : NH4Cl + NaNO3 ——> N2O + Nacl + 2H2O

What mass of N2O can be prepared from 100.0g of ammonium chloride


n(N2O) = 1.869 mol
m(N2O) =nM = 1.86943842x 44.02
= 82.29g

You're right, they're wrong

Hey everyone, I'm not quite sure how to answer this question. I can find the mol of copper lost, I can find the amount of Q with the information that is stated. But I'm not really sure how to go to get the Avogadro constant (STAV 2011 UNIT 4).

Any help would be appreciated thanks =)

[Answer- 6.23x10^23]

You'll need to use the following equations/facts:

The units for Avogadro's constant are mol^(-1)
The units for Faraday's constant are C/mol
The charge on an electron is supplied in your data booklet

I get a slightly different answer, which makes me think they may have used a different method, but this one will work. Let me know if you need further direction.

For redox reactions, for overall reactions do we show the charges of ions and do we show spectator ions?

For example for this question: If Copper(II) sulfate solution is stored in an aluminium container.

Which of these reactions would be correct or are both correct?
(1) 2Al(s) + 3CuSO₄ (aq) ------> Al₂(SO₄)₃ (aq) + 4Cu (s)
or
(2) 2Al(s) + 3Cu²⁺(aq) ------> 3Cu(s) + 2Al³⁺ (aq)

Thank you!

I think the answer to your question is that only the first is correct, but just to make it crystal clear: both are correct chemical equations. The first is just the full equation, and the second is the net ionic equation.

This might be a trivial question, but does a peptide bond/linkage consist of just the bond between C and N, or does it also include the O and the H? And is there a difference between a peptide bond and a peptide linkage, or did someone lie to me?

I'd include the O and H, because the fact that it's an amide is important - without the carbonyl, it's just an amine. There might also be a technicality that the peptide bond is only the N-C bond, while the linkage is the full group, but that's the first I've heard of it, and I don't anybody's find to deduct marks for something that unbelievably minor and petty

[amanaazim]

when a base such as O² and it becomes OH- what happens to the other oxygen ion.

[Chocolatepistachio]

O2- is an oxide ion with a charge of -2

O2- + H+——> OH-