October 19, 2019, 05:41:03 pm

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#### -_-zzz

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##### Re: VCE Chemistry Question Thread
« Reply #7980 on: April 15, 2019, 04:33:49 pm »
+2
Hi!
Why does the shielding effect remain constant across the periods of the periodic table?

Because the number of occupied electron shells which 'shield' the valence electrons from the attractive force of the positive nucleus remains constant.
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#### Rameen

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##### Re: VCE Chemistry Question Thread
« Reply #7981 on: April 15, 2019, 04:38:07 pm »
0
Because the number of occupied electron shells which 'shield' the valence electrons from the attractive force of the positive nucleus remains constant.

ok thank you!!
Am I required to memorise the polyatomic ions given in the textbook, for unit 1 chemistry AOS1?
I have a test on the topics: atomic structure, periodic table, metallic bonding, ionic bonding and quantifying chemistry when school starts.

#### -_-zzz

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##### Re: VCE Chemistry Question Thread
« Reply #7982 on: April 15, 2019, 06:45:48 pm »
+1
ok thank you!!
Am I required to memorise the polyatomic ions given in the textbook, for unit 1 chemistry AOS1?
I have a test on the topics: atomic structure, periodic table, metallic bonding, ionic bonding and quantifying chemistry when school starts.

Yes you will need to know basic polyatomic ions throughout chemistry as their formulae won't always be given to you in SACs/exams.
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#### catpacksnapback

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##### Re: VCE Chemistry Question Thread
« Reply #7983 on: April 16, 2019, 01:02:29 pm »
0
Hi there! I have a precipitation reaction that I noticed I had forgotten to balance when I wrote it out. However, upon closer inspection, I'm actually not sure about what the best way to go about balancing this would be.
$\ce{Ba(NO3)2(aq) + Na2SO4(aq) -> BaSO4(s) + Na(NO3)2(aq)}$
You may have already picked where the imbalance is - we started with two sodium (Na) atoms, but for some bizarre reason, my instinct wrote what you see above. I was about to simply put a subscript 2 after the sodium in the precipitate (BaSO4(s)) but we were just told that the golden rule is to put any balancing number before the compound. I think I've confused myself. Anyone mind un-confusing me?
Thank you
« Last Edit: April 16, 2019, 01:08:48 pm by catpacksnapback »
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#### zuijinde

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##### Re: VCE Chemistry Question Thread
« Reply #7984 on: April 16, 2019, 01:44:09 pm »
+4
Hi there! I have a precipitation reaction that I noticed I had forgotten to balance when I wrote it out. However, upon closer inspection, I'm actually not sure about what the best way to go about balancing this would be.
$\ce{Ba(NO3)2(aq) + Na2SO4(aq) -> BaSO4(s) + Na(NO3)2(aq)}$
You may have already picked where the imbalance is - we started with two sodium (Na) atoms, but for some bizarre reason, my instinct wrote what you see above. I was about to simply put a subscript 2 after the sodium in the precipitate (BaSO4(s)) but we were just told that the golden rule is to put any balancing number before the compound. I think I've confused myself. Anyone mind un-confusing me?
Thank you

You got the Barium sulfate correct, however the other compound is wrong

The Na has a +1 charge, and since it's Na2 (or 2 mols of Na), the overall charge is +2. Same deal with NO3-

Therefore, the overall equation should be Ba(No3)2(aq) + Na2So4(aq) = BaSo4(s) + 2 NaNo3(aq)

#### catpacksnapback

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##### Re: VCE Chemistry Question Thread
« Reply #7985 on: April 16, 2019, 02:43:52 pm »
0
You got the Barium sulfate correct, however the other compound is wrong

The Na has a +1 charge, and since it's Na2 (or 2 mols of Na), the overall charge is +2. Same deal with NO3-

Therefore, the overall equation should be Ba(No3)2(aq) + Na2So4(aq) = BaSo4(s) + 2 NaNo3(aq)

Thank you for the answer! I'm really, really stuck on this for some reason.
Are there rules for balancing these kinds of equations? I just can't seem to get it.
Panicking.

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#### zuijinde

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##### Re: VCE Chemistry Question Thread
« Reply #7986 on: April 16, 2019, 05:09:43 pm »
+1
Thank you for the answer! I'm really, really stuck on this for some reason.
Are there rules for balancing these kinds of equations? I just can't seem to get it.
Not that I know of.

Write out the compounds into its constituent ions, that way it's easier to see the charges and whether they need balancing or not.

#### -_-zzz

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##### Re: VCE Chemistry Question Thread
« Reply #7987 on: April 16, 2019, 10:46:39 pm »
+1
Thank you for the answer! I'm really, really stuck on this for some reason.
Are there rules for balancing these kinds of equations? I just can't seem to get it.

There is really no shortcut to balancing these types of equations. After a decent amount of practice it should just become second nature to you. That being said I wouldn't worry too much about balancing complex equations (such as those for precipitation) as they don't really come up in 3/4.
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#### yplee0926

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##### Re: VCE Chemistry Question Thread
« Reply #7988 on: April 17, 2019, 03:35:06 am »
0
hey guys!
In galvanic cells, do the half cells have to contain redox conjugate pairs?
I was thinkinf about a cell (that I made up) where one half cell has Pt electrode with Cu2+ solution, and another half cell with Mg electrode and Mg2+ solution. Would the reaction still proceed to have Cu deposit on Pt electrode? I’m just confused whether half cells with conjugate redox pairs are the only ones in the textbook because they’re conventional and common or because they’re the only possible combo.
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#### -_-zzz

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##### Re: VCE Chemistry Question Thread
« Reply #7989 on: April 17, 2019, 02:04:27 pm »
+2
hey guys!
In galvanic cells, do the half cells have to contain redox conjugate pairs?
I was thinkinf about a cell (that I made up) where one half cell has Pt electrode with Cu2+ solution, and another half cell with Mg electrode and Mg2+ solution. Would the reaction still proceed to have Cu deposit on Pt electrode? I’m just confused whether half cells with conjugate redox pairs are the only ones in the textbook because they’re conventional and common or because they’re the only possible combo.

They most certainly don't need to. Perhaps the reason why we often use conjugate redox pairs is because that's what the ECS is based off and thus they allow us to make predicitions about particular cell reactions. Hope that helps.
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#### yplee0926

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##### Re: VCE Chemistry Question Thread
« Reply #7990 on: April 17, 2019, 04:35:50 pm »
0
They most certainly don't need to. Perhaps the reason why we often use conjugate redox pairs is because that's what the ECS is based off and thus they allow us to make predicitions about particular cell reactions. Hope that helps.

Thanks for the response! Would it matter though in the cell that I proposed? The reaction would still be quite predictable usinf ECS (from my knowledge)
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#### colline

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##### Re: VCE Chemistry Question Thread
« Reply #7991 on: April 17, 2019, 09:58:47 pm »
0
Hey everyone! Got two questions regarding significant figures:

1. If I'm using data from the data book, and the sig figs are lower than the ones in the question, which one should I follow?
(The question I'm on has 3 sig figs as the lowest number, but I'm also required to use the molar mass of hydrogen which is given in the data book as 1.0 which is 2 sig figs)

2. Can someone confirm if this is correct: (this is part of the working out for a larger question)
There's been a temperature change from 18.5 to 13.3 degrees Celsius. So delta T is 5.2 (one decimal place). The smallest number of significant figures given in the question is 3, but 5.2 has two significant figures. I'm now required to leave my answers to two significant figures instead of three because that's the lowest number of s.f in the working out, even if the question had 3 s.f.
Is this correct? This was a question on a practice SAC and I left my answer to 3 s.f., and lost a mark.

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#### DBA-144

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##### Re: VCE Chemistry Question Thread
« Reply #7992 on: April 17, 2019, 10:13:00 pm »
0
Hey everyone! Got two questions regarding significant figures:

1. If I'm using data from the data book, and the sig figs are lower than the ones in the question, which one should I follow?
(The question I'm on has 3 sig figs as the lowest number, but I'm also required to use the molar mass of hydrogen which is given in the data book as 1.0 which is 2 sig figs)

2. Can someone confirm if this is correct: (this is part of the working out for a larger question)
There's been a temperature change from 18.5 to 13.3 degrees Celsius. So delta T is 5.2 (one decimal place). The smallest number of significant figures given in the question is 3, but 5.2 has two significant figures. I'm now required to leave my answers to two significant figures instead of three because that's the lowest number of s.f in the working out, even if the question had 3 s.f.
Is this correct? This was a question on a practice SAC and I left my answer to 3 s.f., and lost a mark.

Pretty sure that 2 is correct. You don't know for sure that it's 13.30 or 18.50, so you only know the accuracy of your calculation to 2 sf. Here, you are 'certain' it is that value, but don't know it is, say, 5.20 or 5.2001 or 5.2000001. Hence, you also carry over this uncertainty to any other calculation you make using this value. Therefore, you should be using the fewest number of sf given to you in your working. I am NOT certain about how VCAA or your school does it, this is how I understand it only.

For 1, I believe that values in the data book are correct to an unlimited degree and hence we would use 3 sf in example 1.

Pretty sure that significant figures are only a mark all up in the exam, so you don't need to stress too much about this, but it's a great idea to be getting this clarified. I have struggled with this stuff for too long haha

#### -_-zzz

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##### Re: VCE Chemistry Question Thread
« Reply #7993 on: April 17, 2019, 10:31:36 pm »
+2

Pretty sure that 2 is correct. You don't know for sure that it's 13.30 or 18.50, so you only know the accuracy of your calculation to 2 sf. Here, you are 'certain' it is that value, but don't know it is, say, 5.20 or 5.2001 or 5.2000001. Hence, you also carry over this uncertainty to any other calculation you make using this value. Therefore, you should be using the fewest number of sf given to you in your working. I am NOT certain about how VCAA or your school does it, this is how I understand it only.

For 1, I believe that values in the data book are correct to an unlimited degree and hence we would use 3 sf in example 1.

Pretty sure that significant figures are only a mark all up in the exam, so you don't need to stress too much about this, but it's a great idea to be getting this clarified. I have struggled with this stuff for too long haha

DBA is correct. The values from the data book are assumed to have an 'unlimited' number of sig figs so if you're using a value from the data book that has 2 numbers in it (e.g. 2.5) but the question gives you values with 3 sig figs, then you would round your final answer to 3 sig figs. In regards to your first question, if for example you have 2 numbers which are correct to 3 sig figs such as 24.3 and 19.3 and you are required to subtract 19.3 from 24.3, the value in which you yield can only be rounded accurately to 2 sig figs (5.3). Thus if you were to go and derive an answer by multiplying/dividing with '5.3', then your final answer should be to 2 sig figs despite the fact that the values given to you in the question were rounded to 3 sig figs. Hope that makes sense
« Last Edit: April 17, 2019, 10:38:17 pm by -_-zzz »
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#### persistent_insomniac

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