VCE Chemistry Question Thread

[Individu]

I just work out the half equations and add the half equations together to get the overall equations and cancel out the electrons.

Thanks for the help. However, why do the oxygens have to be included in the half equation; can't we just write Fe3+ + 3e- --> Fe(l)?

[JellyBeanz]

Thanks for the help. However, why do the oxygens have to be included in the half equation; can't we just write Fe3+ + 3e- --> Fe(l)?

You can, although in your question, Fe2O3 is being reduced, hence oxygens have to be balanced on the other side.

When you are reducing Fe3+ to Fe then obviously there will be no oxygens to balance

I think you may have gotten confused in the question where you were required to write a reduction reaction of Fe2O3 and you instead wrote one for Iron(III) ion XD

[The Usual Student]

My school just started IR, would love some help for the following?

1) are the different types of vibrations that occur during IR required knowledge?
2) Can an Ionic substance be analysed by IR? I understand the textbook says no as you can't vibrate an ionic attraction but I just need clarification. Essentially, can is IR STRICTLY for organic compounds or is it just useful for organic compounds but can be employed for many different substances.
3) what is a broad spectrum and what is it for in the IR setup?

[Swagadaktal]

The thermite process can be used to weld lengths of railway
track together. A mould placed over the ends of the two rails
to be joined is filled with a charge of aluminium powder and
iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron, which joins the rails together.

a Write a half equation for the conversion of iron(III) oxide
to metallic iron.

b Is the half equation you wrote for part a an oxidation or a
reduction process?
c Write the overall equation for the thermite process.


My initial answer for a was Fe3+ (s) + 3e- --> Fe(l) but this is wrong apparently. It's meant to be Fe2O3(s) + 6e– —> 2Fe(l) + 3O 2–(s) according to the textbook. My main question is why do we have to include the oxygens in this half-equation? (oxygens don't seem to change oxidation number)

Because Iron(3) oxide - the oxygen is essential to the reaction because it's not dissolved in anything (it's not aq, it's a solid). Whereas say if you have Potassium permanganate (KMnO4) it can become MnO4- because the potassium is a spectator ion. It has no bearing on the reaction. But you cant make MnO4- Mn+7 because the 4 oxygens have bearing on the reaction, as H2O is a product. You feel?

[Individu]

Because Iron(3) oxide - the oxygen is essential to the reaction because it's not dissolved in anything (it's not aq, it's a solid). Whereas say if you have Potassium permanganate (KMnO4) it can become MnO4- because the potassium is a spectator ion. It has no bearing on the reaction. But you cant make MnO4- Mn+7 because the 4 oxygens have bearing on the reaction, as H2O is a product. You feel?

Perfect explanation, thanks man.

[Swagadaktal]

My school just started IR, would love some help for the following?

1) are the different types of vibrations that occur during IR required knowledge?
2) Can an Ionic substance be analysed by IR? I understand the textbook says no as you can't vibrate an ionic attraction but I just need clarification. Essentially, can is IR STRICTLY for organic compounds or is it just useful for organic compounds but can be employed for many different substances.
3) what is a broad spectrum and what is it for in the IR setup?

Um I dont know whether points 1 and 3 are part of the study design, i haven't come across it in the book myself tbh.
for 2) it only works in with functional groups. The IR gives info about whether the functional group is an NH or whether it has a Cl. Maybe the IR goes beyond that but in terms of vce chem das all you need to know: Functional groups.
Also, the types of bonds we dealing with is dipole-dipole and dispersion, which are not as strong as ionic substances (they're significantly weaker) - so you wouldn't expect to measure something so strong with a machine that has been designed to analyse things proportionally weaker.

Maybe I'm wrong im just coming to these conclusions from my own understanding of the stuff. 

[JellyBeanz]

Um I dont know whether points 1 and 3 are part of the study design, i haven't come across it in the book myself tbh.
for 2) it only works in with functional groups. The IR gives info about whether the functional group is an NH or whether it has a Cl. Maybe the IR goes beyond that but in terms of vce chem das all you need to know: Functional groups.
Also, the types of bonds we dealing with is dipole-dipole and dispersion, which are not as strong as ionic substances (they're significantly weaker) - so you wouldn't expect to measure something so strong with a machine that has been designed to analyse things proportionally weaker.

Maybe I'm wrong im just coming to these conclusions from my own understanding of the stuff.

1) Heinemann book shows how there are bending, scissoring, vibrations on the molecule when performing IR. But again, this isn't needed, you don't really need to know in detail how the vibrations affect the molecule or how IR works per se. But you will be asked to interpret and analyse IR spectrums which is basically all you need to know.

[Sine]

Chemicals that absorb IR radiation should have ; covalent bonds, one of these bonds are polar and polarity of molecule changes during bond movement.

[vaish_vj]

Heyyy,
i have my second 3/4 chem sac on Monday and it is about aspirin and oil of wintergreen (Methyl salicylate).
Is the key structural difference between the two that fact that aspirin has no distinct hydroxyl function group or the different positioning of the methyl group?

[Sine]

Heyyy,
i have my second 3/4 chem sac on Monday and it is about aspirin and oil of wintergreen (Methyl salicylate).
Is the key structural difference between the two that fact that aspirin has no distinct hydroxyl function group or the different positioning of the methyl group?

The difference is where the condensation reaction occurs the reactant and salicylic acid.

Yes aspirin as an ester and carboxylic functional groups.
Methyl salicylate has a hydroxyl and ester functional groups.

[William3558]

Hi!
A have a couple of small questions regarding organic compounds and Spectroscopy.

1. In my text, it says that the vibrations of symmetric molecules cannot be detected in IR
spectroscopy. Why?
Is this because Both sides of the molecule will form the same peaks, making it more difficult to determine its structure?

2. Why do molecules absorb less IR radiation the higher the mass is?

3.What is Na2CO3 used to test for? I remember seeing this in a question once. Is it possible that it is used to test if a substance is an acid?

Thanks! 

[Individu]

Sup guys, does anyone know the significance of redox reactions occuring in acidic solution? I know that H+ ions are required for certain redox reactions to occur but must we know why are they needed? I mean, would these redox reactions occur in water for example (neutral solution)?

[Elizawei]

Sup guys, does anyone know the significance of redox reactions occuring in acidic solution? I know that H+ ions are required for certain redox reactions to occur but must we know why are they needed? I mean, would these redox reactions occur in water for example (neutral solution)?

The H+ ions are needed to balance the redox equation because if you think about it - when you write redox equations you balance the equation by adding however many H+ ions required. The acidic solution provides these H+ ions.
Water is amphiprotic (can donate or give away a proton depending on what it's reacting with) and can actually be both an oxidising/reducing agent in redox equations (but it's pretty weak so it's usually just acts as solvent for redox equations)

[Adequace]

Can someone explain to me how to draw molecules with the valence bond structure.

All I know is that 1 bond = linear, 2 bonds = 'V' shaped, 3 bonds = pyramid and 4 bonds = tetrahedron. I'm not sure if this is correct but CO2 doesn't follow this when it has 2 double bonds. Are there any rules I'm missing?

[nerdgasm]

I think you also have to take into account the 'lone pairs' of valence electrons when considering the structure, as well as the total number of atoms bonded to the 'central' atom. Having electrons too close to each other causes unnecessary repulsion, as negative charges repel. Off the top of my head, the rules you have quoted appear to be for atoms where the number of bonded atoms + number of lone pairs is 4. For example, the O atom in H2O would have two bonded atoms and two lone pairs, so the diagram you would draw would be V-shaped. Likewise, the C in CH4 would have four bonded atoms and no lone pairs, so the diagram you would draw should indicate tetrahedral geometry. However, in CO2, note that the C is bonded to two atoms, but has no lone pairs remaining. The best way to minimise electron-electron repulsion in this model would therefore be to have the two O atoms as far away from each other as possible, which is why the resultant structure is linear.

My approach to learning these was to first realise that (at VCE level), the key determinant of structure is the combined number of atoms bonded to the central atom, and central atom lone pairs. From there, I considered the 'parent' geometries of this combined number. For example, the parent geometry when the number of bonded atoms + lone pairs = 4, is tetrahedral. Now, should there be 3 bonded atoms and 1 lone pair, then the lone pair would take up one position on the tetrahedron, so if we only look at the central atom and 3 bonded atoms, then we get something that looks trigonal pyramidal. Likewise, should there be 2 bonded atoms and 2 lone pairs, each lone pair takes up a position on the tetrahedron, but if we just look at the central atom and 2 bonded atoms, we get the 'bent' structure.