VCE Chemistry Question Thread

[keltingmeith]

Hello

I have a question about redox reactions: According to my teacher's notes, "A gain of oxygen is oxidation and loss of oxygen in reduction". WHY? How does oxygen cause oxidation (loss of electrons)?

This is a really interesting point, actually.

You see, redox used to only be concerned with oxygen. In fact, "oxidation" used to identify as the addition of oxygen - you've learned the definitions in the opposite order they were defined.

What's been found over the years that there are reactions very similar to redox reactions, but don't involve oxygen. Thus, we eventually found that more generally, it wasn't gaining oxygen that was really important, it was the losing of the electrons to the oxygen (which then bonded to the molecule).

However, particularly in organic chemistry, it is still very useful to still think of oxidation as a gaining in oxygen. (In fact, I teach all my students this definition before we go into redox in terms of electrons)

A bit long winded, but I hope that helped you out.

[thushan]

Hello

I have a question about redox reactions: According to my teacher's notes, "A gain of oxygen is oxidation and loss of oxygen in reduction". WHY? How does oxygen cause oxidation (loss of electrons)?

Oxygen is an extremely electronegative element. Therefore, when it bonds with most atoms (except itself, and Fluorine the Emperor), it will pull electron density away from the other atom. This is interpreted in redox chemistry as this other atom losing electrons, and therefore being oxidised.

[scarletmoon]

I have two questions:
What are two major parts of a risk assesment?
(colourimetry) If standard solutions were heated for 10min but the sample was heated for 5 min would the concentration of the sample be higher or lower than true value? The element analysed was phospohrous which was a cle ar solution so it had to be converted to phosphomolybdate

[izzywantsa97]

I have two questions:
What are two major parts of a risk assesment?
(colourimetry) If standard solutions were heated for 10min but the sample was heated for 5 min would the concentration of the sample be higher or lower than true value? The element analysed was phospohrous which was a cle ar solution so it had to be converted to phosphomolybdate

For our risk assessments, major parts were the first aid/treatment of one of the chemicals used, and then disposal of all products and reactants

For your second question, I'm assuming (could be wrong) that the reaction won't have completed as a result of the decreased time, so the colour would be fainter (therefore less light absorbed) and the concentration determined to be lower?
Feel free to correct me!

[yang_dong]

hello,
What volume of the 0.15 HCl solution would, if added to 100 mL of pure water, make the pH decrease by 0.05 pH units?

thank u ~~~~~

[thushan]

Hey - where did you get the question from? The issue is that there are a lot of technicalities with this question that makes it rather difficult to solve (due to stuff you'll learn in Unit 4).

[yang_dong]

teacher gave it to us

i heard i have to use some henderson-haddlebach law or something right?

[EspoirTron]

teacher gave it to us

i heard i have to use some henderson-haddlebach law or something right?

Henderson-Hasslebach is beyond the scope of VCE Chemistry last I checked. Generally, it is used for buffer systems in the human blood or body. Or really anything you want. It just helps accentuate the effects of adding an acid or base to a buffer system.
pH= pKa +log([A-]/[HA]) where the base is 10 and A- represents the conjugate base of the acid.

[chocolate.cake.1]

Hello 

In UV-Vis, light is passed through the sample and some of it is absorbed by the atoms as electrons jump to higher energy levels. The absorbance of the sample is then calculated based on the amount of light that reaches the detector.

BUT don't the electrons in the sample emit energy as they return to ground state? Wouldn't this energy emitted also reach the detector and therefore there will be no absorbance reading (because all the energy to begin with would reach the detector as it is just being absorbed and then emitted again)?

Sorry if I don't make sense

[lzxnl]

Henderson-Hasslebach is beyond the scope of VCE Chemistry last I checked. Generally, it is used for buffer systems in the human blood or body. Or really anything you want. It just helps accentuate the effects of adding an acid or base to a buffer system.
pH= pKa +log([A-]/[HA]) where the base is 10 and A- represents the conjugate base of the acid.

Also, it's not ideal for use in pure water or adding a strong acid

hello,
What volume of the 0.15 HCl solution would, if added to 100 mL of pure water, make the pH decrease by 0.05 pH units?

thank u ~~~~~

This question is VERY fiddly. As you're adding HCl to pure water, you have to take into account the auto-ionisation of water. In addition, you need to take into account the volume change too that you get from adding the HCl solution. VCAA wouldn't ask something like this.

[Champ101]

what is the name of this organic?

[Maths Forever]

what is the name of this organic?

First, you need to identify the longest carbon chain. In this case the longest chain consists of 7 carbon atoms, all of which contain single C-C bonds.

Next, number the carbon atoms in the main chain from the end which is closest to the first branch point (this will be from the left).

Name substituents in alphabetical order (di, tri and tetra do not count in this order). There is a chlorine atom coming off carbon atom 2, and three methyl groups coming off carbon atoms 2, 4 and 5.

So the IUPAC name is 2-chloro-2,4,5-trimethylheptane.

I hope this helps! 

[Champ101]

First, you need to identify the longest carbon chain. In this case the longest chain consists of 7 carbon atoms, all of which contain single C-C bonds.

Next, number the carbon atoms in the main chain from the end which is closest to the first branch point (this will be from the left).

Name substituents in alphabetical order (di, tri and tetra do not count in this order). There is a chlorine atom coming off carbon atom 2, and three methyl groups coming off carbon atoms 2, 4 and 5.

So the IUPAC name is 2-chloro-2,4,5-trimethylheptane.

I hope this helps! 

aren't there only 2 methyl group and 1 ethyl group?

[Mieow]

aren't there only 2 methyl group and 1 ethyl group?

if you count it like that then you did not identify the longest possible carbon chain in the molecule. I've used my absolutely breathtaking graphic design skills(/sarcasm) to try to show you where the substituents are. As you can see the "ethyl group" was actually a part of the 7-carbon chain, with your side groups being the chlorine and 3 methyl groups

[cosine]

Probably a really stupid question, but can two different precipitates form when you mix two different solutions?