VCE Chemistry Question Thread

[whys]

I love how my chem teachers never tell me stuff, so your saying that since 0.25 is derived, that I should discount is and use the 889 which is three sig figs so it would be 222 instead of 2.2 x 10 ^2?
This is so confusing...

Yes, that's basically it. For example, if you had a question with the values 0.0034, 5673 and 10.0, then your answer should be to 2 significant figures as 0.0034 has the least number of sig figs out of all numerical values given. In your working out, if you get an answer that has 1 sig fig, such as 8, this would not change the number of sig figs your answer should be as 8 was an answer from your calculations.

tldr, the number of sig figs of an answer is determined by the least number of sig figs in the question itself

[Snow Leopard]

Here are some questions that I'd really appreciate getting some help with:
1. Why is Caesium more reactive than Francium (since reactivity increases down a group)?
2. Can first ionisation energy by justified/explained by electronegativity? (This is because I noticed that both ionisation energy and electronegativity increase across periods and increase up groups)?
3. For Unit 1/2 Chem do we need to know about Oxidising and Reducing Strengths? (if not , is this important for 3/4 chem)
4. Are domains the same as covalently bonded electrons?
5. Why do you have to put in the location of a double bond for example in but - 1 - ene?
6. Do the exceptions in subshell notation for copper and chromium apply for all the d-block elements? As in do all d-block elements have full or half full orbitals since that is more stable?

More about how pedantic VCAA would be:
7. When writing ions, would I lose marks for writing the subscript as +2, rather than 2+?
Thanks in advance!

[angrybiscuit]

Here are some questions that I'd really appreciate getting some help with:
1. Why is Caesium more reactive than Francium (since reactivity increases down a group)?
2. Can first ionisation energy by justified/explained by electronegativity? (This is because I noticed that both ionisation energy and electronegativity increase across periods and increase up groups)?
3. For Unit 1/2 Chem do we need to know about Oxidising and Reducing Strengths? (if not , is this important for 3/4 chem)
4. Are domains the same as covalently bonded electrons?
5. Why do you have to put in the location of a double bond for example in but - 1 - ene?
6. Do the exceptions in subshell notation for copper and chromium apply for all the d-block elements? As in do all d-block elements have full or half full orbitals since that is more stable?

More about how pedantic VCAA would be:
7. When writing ions, would I lose marks for writing the subscript as +2, rather than 2+?
Thanks in advance!

Hi! I've forgotten most of 1/2 so I apologise if I skip out any questions

2. I guess the more electronegative an atom is the greater the first ionisation energy. First ionisation energy decreases as you go down a group because the atomic radius gets larger and the electrons are further out from the nucleus and less energy is needed to ionise it. The same can be said for electronegativity because as you go down the group electronegativity decreases as atoms are more likely to lose electrons so ionisation energy decreases.
3. Yes oxidizing and reducing strengths is important for U3/4 as they come into play when learning about galvanic cells. I can't remember if you do galvanic cells but you do learn about redox reactions which require you to hone in on your knowledge of oxidation and reduction.
5. You can't just say butene because the double bond can be anywhere. But-1-ene and but-2-ene are very different molecules so you need to know where the double bond is to draw it. For example, with pentene the double bond can either be in carbon 1/2/3 so you need to specify where it is. It's okay to write 1-butene or but-1-ene but you need to say where the double bond is.
7. By convention, stick to writing the charge after the number (2+/3+). Though I haven't heard of being penalised for doing the reverse.

[Snow Leopard]

Hi! I've forgotten most of 1/2 so I apologise if I skip out any questions

2. I guess the more electronegative an atom is the greater the first ionisation energy. First ionisation energy decreases as you go down a group because the atomic radius gets larger and the electrons are further out from the nucleus and less energy is needed to ionise it. The same can be said for electronegativity because as you go down the group electronegativity decreases as atoms are more likely to lose electrons so ionisation energy decreases.
3. Yes oxidizing and reducing strengths is important for U3/4 as they come into play when learning about galvanic cells. I can't remember if you do galvanic cells but you do learn about redox reactions which require you to hone in on your knowledge of oxidation and reduction.
5. You can't just say butene because the double bond can be anywhere. But-1-ene and but-2-ene are very different molecules so you need to know where the double bond is to draw it. For example, with pentene the double bond can either be in carbon 1/2/3 so you need to specify where it is. It's okay to write 1-butene or but-1-ene but you need to say where the double bond is.
7. By convention, stick to writing the charge after the number (2+/3+). Though I haven't heard of being penalised for doing the reverse.

Thank you so much for your help angrybiscuit!
Would anyone else be able to help me out with these questions:
1. Why is Caesium more reactive than Francium (since reactivity increases down a group)?
4. Are domains the same as covalently bonded electrons?
6. Do the exceptions in subshell notation for copper and chromium apply for all the d-block elements? As in do all d-block elements have full or half full orbitals since that is more stable?
Thank you so much in advance!

[Geoo]

Another dumb question.
I have heard conflicting advice with these significant figures.

Take 4000. My teacher says that this has 4 significant figures, and the textbook which has the example 20, also has 2 sig figs. So this means the zeros that are right of integers are significant.

However, other websites my teacher has linked me to, and the youtube videos made be Khan academy and the tyler guy say that 4000 has only one sig fig the same with 20.

So i'm confused as to which is correct with all of the conflicting resources.

[whys]

Another dumb question.
I have heard conflicting advice with these significant figures.

Take 4000. My teacher says that this has 4 significant figures, and the textbook which has the example 20, also has 2 sig figs. So this means the zeros that are right of integers are significant.

However, other websites my teacher has linked me to, and the youtube videos made be Khan academy and the tyler guy say that 4000 has only one sig fig the same with 20.

So i'm confused as to which is correct with all of the conflicting resources.

Not a dumb question! Your teacher is right. VCAA will consider 4000 as 4 and 20 as 2 significant figures.
Non-zero digits are always considered significant. Leading zeros are never significant but following zeros and zeros between non-zero digits are always significant.

For example:
750.002      6 significant figures
123000       6 significant figures
0.00012      2 significant figures
125.000      6 significant figures

[Evolio]

Hey Geoo!
No, there's no such thing as a dumb question!

It is 4 sig figs. For the purpose of VCE Chemistry, trailing zeros are ALWAYS significant which is what VCAA has specifically specified. However, trailing zeroes are traditionally significant (outside VCE Chemistry's specifications) when the number contains a decimal place which explains why with Khan Academy they said 4000 only has one sig fig.

Hope this clears it up.

EDIT: whys beat me to it

[Geoo]

Thanks for clearing it up guys!!

Legends!

[Erutepa]

Thank you so much for your help angrybiscuit!
Would anyone else be able to help me out with these questions:
1. Why is Caesium more reactive than Francium (since reactivity increases down a group)?
4. Are domains the same as covalently bonded electrons?
6. Do the exceptions in subshell notation for copper and chromium apply for all the d-block elements? As in do all d-block elements have full or half full orbitals since that is more stable?
Thank you so much in advance!

1) the increase in reactivity down a group is a trend with exceptions (not a universal rule). for reasons outside of VCE chem, really heavy elements (like francium) actually has a greater first ionisation energy. For these group 1 elements, the reactivity is determined by how easily the element will lose its outer electron, thus francium having a greater first ionisation energy than caesium means it is less reactive (since it hold on to its valence electron more strongly).

4) electron domains are bond locations or lone pairs. Its important to note that bond locations and bonded electrons are not the same - for example the carbon in CO2 and CH4 both have the same number of electrons participating in bonding, however while the carbon in CO2 posesses 2 double bonds and thus has 2 bonding locations, the carbon in CH4 posesses 4 single bonds and thus has 4 bonding locations. Since both carbons do not have any lone pairs, CO2 has 2 electrons domains, and CH4 has 4 electron domains.

6) The exception does not occur for all d-block elements, although it does happen for other ones. In addition to this there are a bunch of other exception. You really don't need to know any of them except for chromium and copper, but if you are curious I found a list of some exceptions on this site here (about a 3rd of the way down the page)

Hope this helps 

[Snow Leopard]

4) electron domains are bond locations or lone pairs. Its important to note that bond locations and bonded electrons are not the same - for example the carbon in CO2 and CH4 both have the same number of electrons participating in bonding, however while the carbon in CO2 posesses 2 double bonds and thus has 2 bonding locations, the carbon in CH4 posesses 4 single bonds and thus has 4 bonding locations. Since both carbons do not have any lone pairs, CO2 has 2 electrons domains, and CH4 has 4 electron domains.
Hope this helps 

Thanks so much for your help Erutepa!
So can electron domains be either bond locations where electrons can be covalently bonded or lone pairs according to the molecule or something? Or have I got it wrong?

Also, this was in my textbook: "Metals – Ionic compounds (Base oxides)" and I was wondering if it meant/would make more sense if it said: "metals and non-metals - ionic compounds (base oxides)"?

[Owlbird83]

Thanks so much for your help Erutepa!
So can electron domains be either bond locations where electrons can be covalently bonded or lone pairs according to the molecule or something? Or have I got it wrong?

Also, this was in my textbook: "Metals – Ionic compounds (Base oxides)" and I was wondering if it meant/would make more sense if it said: "metals and non-metals - ionic compounds (base oxides)"?

Yeah, electron domains are bond locations or lone pairs.

I found this online, but it said that metals and oxygen can form basic oxides, which are ionic compounds that react with water to form basic compounds. And it said nonmetals and oxygen can form acidic oxides, which react with water to form acidic compounds.
I am pretty sure you don't need to know this because I never heard of this last year (unless the study design changed?)

You are right in saying that metal + nonmetal forms ionic compound! (But the textbook also makes sense because I think it's trying to say that if it's a metal oxide it'll be a basic oxide.)

[Snow Leopard]

Yeah, electron domains are bond locations or lone pairs.

That makes sense! Thanks for your help Owlbird38!!!
Sorry to be a pain, but I was also wondering, are bond locations places in the molecule that have been covalently bonded? Whereas lone pairs are electrons in the molecule that haven't been covalently bonded?

[Owlbird83]

That makes sense! Thanks for your help Owlbird38!!!
Sorry to be a pain, but I was also wondering, are bond locations places in the molecule that have been covalently bonded? Whereas lone pairs are electrons in the molecule that haven't been covalently bonded?

Yeah, that's right. A bond location refers to either a single, double or triple covelant bond, which will equal one bond location. And the lone pairs haven't been bonded. 

[whys]

I am confused regarding the following dot point in the study design:

the comparison of fossil fuels (coal, crude oil, petroleum gas, coal seam gas) and biofuels (biogas, bioethanol, biodiesel) with reference to energy content, renewability and environmental impacts related to sourcing and combustion

Are we required to memorise the energy released by each fossil fuel/biofuel or are we just required to know which ones produce more/less energy than others?

[Evolio]

Hey whys. I found this response by Vaike where it said we don't need to know the exact energy content of each fuel but it would be useful to have a general idea of which has more or less.

Would it be necessary or useful to memorise the specific order of different bio/fossil fuels in order of highest to lowest energy content?

Probably not. It's a good idea to have some understanding of general trends in energy content, but I don't believe you specifically need to know the energy value for each fuel, or exactly how they compare. It won't hurt to do so, but just a having some idea of where abouts they lie in relation to each other should suffice.