HSC Chemistry Question Thread

[itssona]

heeey would someone please be able to tell me if this sounds right:

im comparing the densities of water and ice, but not too sure if this is valid, and our teacher often contradicts sites so idk

   • Structure of Liquid Water
-   Water molecules are randomly orientated
-   Overall structure of water is random
   - Molecules much closer together than in ice (so there is h bonding as h bonding is the strongest)
   -    The more h bonding present, the bigger the density

   • Structure of Ice
     -     More structured than liquid water
   - Adjacent water molecules hydrogen bonding to form hexagonal clusters
   - This causes large vacant spaces within the hexagonal clusters
   - As the volume of ice is greater, it is less dense

[jakesilove]

heeey would someone please be able to tell me if this sounds right:

im comparing the densities of water and ice, but not too sure if this is valid, and our teacher often contradicts sites so idk

   • Structure of Liquid Water
-   Water molecules are randomly orientated
-   Overall structure of water is random
   - Molecules much closer together than in ice (so there is h bonding as h bonding is the strongest)
   -    The more h bonding present, the bigger the density

   • Structure of Ice
     -     More structured than liquid water
   - Adjacent water molecules hydrogen bonding to form hexagonal clusters
   - This causes large vacant spaces within the hexagonal clusters
   - As the volume of ice is greater, it is less dense

Yep, that is 100% correct! Looks like you've got it! Might be worth looking at the actual, relative density of ice/water (so that you can give a sexy looking statistic to back up your theoretical response). However, great job!

[itssona]

Yep, that is 100% correct! Looks like you've got it! Might be worth looking at the actual, relative density of ice/water (so that you can give a sexy looking statistic to back up your theoretical response). However, great job!

Omg thank you Jake!!

[hansolo9]

Is the answer A or B?
Do we assume that sulfuric acid fully ionises both its hydrogens?

[jakesilove]

Is the answer A or B?
Do we assume that sulfuric acid fully ionises both its hydrogens?

Sulfuric acid is a STRONG acid. So, yes it fully ionises in solution. As you say, it produces two hydrogen ions per mole of acid, therefore the concentration of hydrogen ions is 0.2 mol/L. Plug this into the pH formula, and you'll get the answer!

[hansolo9]

Sulfuric acid is a STRONG acid. So, yes it fully ionises in solution. As you say, it produces two hydrogen ions per mole of acid, therefore the concentration of hydrogen ions is 0.2 mol/L. Plug this into the pH formula, and you'll get the answer!

But doesn't sulfuric acid ionise the first hydrogen to become a weak acid? Then is that second hydrogen fully or partially ionised?

[kiwiberry]

But doesn't sulfuric acid ionise the first hydrogen to become a weak acid? Then is that second hydrogen fully or partially ionised?

Yeah the second hydrogen only partially ionises. But either way, H+ concentration will be between 0.1 mol/L and 0.2 mol/L, so pH will be less than 1.

[seventeenboi]

HEYOO,
I have two questions:
1. why is it that CFCs are so unreactive that they take up to 7 years to diffuse into the stratosphere?
2. net effect of ozone depletion by CFC photodissociation is given by this equation: O3 + O* → O2 + O2.. in conjunction with the other equations this suggests the continual regeneration of chlorine radicals which in turn catalyse more and more ozone depletion... then it would make sense that 1 molecule of CFC has the potential to destroy an infinite amount of ozone molecules, but why doesn’t it?

THANKS )

[MisterNeo]

HEYOO,
I have two questions:
1. why is it that CFCs are so unreactive that they take up to 7 years to diffuse into the stratosphere?
2. net effect of ozone depletion by CFC photodissociation is given by this equation: O3 + O* → O2 + O2.. in conjunction with the other equations this suggests the continual regeneration of chlorine radicals which in turn catalyse more and more ozone depletion... then it would make sense that 1 molecule of CFC has the potential to destroy an infinite amount of ozone molecules, but why doesn’t it?

THANKS )

CFC's unreactivity may have something to do with chlorine's electronegativity compared to hydrogen in methane. (probably don't need to know this, you just need to know that it is unreactive)
They are also water insoluble, which means that they don't get washed out by rain. This allows them to survive the journey into the stratosphere where they are decomposed by UV rays into free radicals.

You would assume that the chain reaction goes on forever, but there are millions of chlorine free radicals in affected areas. This means that they will bump into each other and form chlorine gas, which ends the chain reaction. Some free radicals may react with moisture or whatever to form hydrogen chloride, whereas some may dissipate into space. But by the time it does end, each radical would have destroyed countless ozone molecules.

[seventeenboi]

CFC's unreactivity may have something to do with chlorine's electronegativity compared to hydrogen in methane. (probably don't need to know this, you just need to know that it is unreactive)
They are also water insoluble, which means that they don't get washed out by rain. This allows them to survive the journey into the stratosphere where they are decomposed by UV rays into free radicals.

You would assume that the chain reaction goes on forever, but there are millions of chlorine free radicals in affected areas. This means that they will bump into each other and form chlorine gas, which ends the chain reaction. Some free radicals may react with moisture or whatever to form hydrogen chloride, whereas some may dissipate into space. But by the time it does end, each radical would have destroyed countless ozone molecules.

ohhh ok thanks :DD

[bsdfjnlkasn]

Hey there!

Could someone please help out with the following question:

Write a general structural formula for the saponification of vegetable oil. Use symbols such as R1 and R2 to represent the carbon chains in the organic molecules.

Also could someone help me answer:
The equilibrium equation being referred to is attached

Draw a diagram to show the arrangement of (a) water molecules and (b) chloride ions around the cobalt ion. Why do the particles behave this way? Hint: Your answer should use the term “lone pairs of electrons” and coordinate covalent bonds may be involved



Thank you so much

[MisterNeo]

Hey there!

Could someone please help out with the following question:

Write a general structural formula for the saponification of vegetable oil. Use symbols such as R1 and R2 to represent the carbon chains in the organic molecules.

Also could someone help me answer:
The equilibrium equation being referred to is attached

Draw a diagram to show the arrangement of (a) water molecules and (b) chloride ions around the cobalt ion. Why do the particles behave this way? Hint: Your answer should use the term “lone pairs of electrons” and coordinate covalent bonds may be involved



Thank you so much

Hey!
For saponification, the general reaction between an ester and a base to form an alcohol and a salt.

The most common ester used are a group called triglyceride found in fats and oils. Most common one in textbooks is tristearin.
The alcohol produced is usually glycerol, plus the salt is the sodium stearate (soap).


Idk much about the cobalt question, but hydrous compounds have water loosely bonded to the metal in the middle. This is partly due to the water's oxygen having two lone pairs of electrons that can form bonds with cobalt's electrons. Other hydrous compounds can have water hydrogen bond onto the anion of a salt such as sulfates.
But it looks something like this:

[limtou]

Could someone kindly explain the answer to this multiple choice?
Thanks !

[MisterNeo]

Could someone kindly explain the answer to this multiple choice?
Thanks !

Hi!!
A pH increase is a decrease in acidity/hydrogen ion concentration.

So are you familiar with the whole buffer system with hydrogen carbonate?




Adding water
Remember the equation for pH?

The higher the concentration of H+, the lower the pH will be (acidic).
Adding more water will dilute the entire system, thus reducing the concentration. If you were to plug that dilute concentration into the formula, you would get a higher pH (less acidic) than the current concentration.
Increasing temperature
Carbon dioxide is a gas. Increasing the temperature of water will reduce its ability to dissolve gases, and most of the carbon dioxide will be evolved as a gas. This reduces the concentration of aqueous carbon dioxide.
According to LCP, the lost aqueous CO2 will cause the carbonic acid to reverse react to reform aqueous CO2 and water. This will also cause the hydrogen ions to shift to the left to replace the carbonic acid. Ultimately, the hydrogen ion concentration will fall, and become a more dilute solution. As stated in the water section, dilute solutions of H+ will be a higher pH because it is less of an acid and more towards neutral/basic.

Hope this helps 

[limtou]

Hi!!
A pH increase is a decrease in acidity/hydrogen ion concentration.

So are you familiar with the whole buffer system with hydrogen carbonate?




Adding water
Remember the equation for pH?

The higher the concentration of H+, the lower the pH will be (acidic).
Adding more water will dilute the entire system, thus reducing the concentration. If you were to plug that dilute concentration into the formula, you would get a higher pH (less acidic) than the current concentration.
Increasing temperature
Carbon dioxide is a gas. Increasing the temperature of water will reduce its ability to dissolve gases, and most of the carbon dioxide will be evolved as a gas. This reduces the concentration of aqueous carbon dioxide.
According to LCP, the lost aqueous CO2 will cause the carbonic acid to reverse react to reform aqueous CO2 and water. This will also cause the hydrogen ions to shift to the left to replace the carbonic acid. Ultimately, the hydrogen ion concentration will fall, and become a more dilute solution. As stated in the water section, dilute solutions of H+ will be a higher pH because it is less of an acid and more towards neutral/basic.

Hope this helps 

Ahh, thank you