VCE Chemistry Question Thread

[jazzaa36]

For 2015 chem exam, SA question 10 b) part ii. why is NiOOH on the positive electrode and MH on the negative (during discharging) ?? Getting real confused, as they have stated that for the anode the recharging eq was Ni(OH)2 + OH -- NiOOH + H20 + e (why does NiOOH go to the postive electrode when its given it at the anode? )

[bedigursimran]

Hey guys, on a follow up. Why is the answer to the attached file A? Why is Y disulfide bonds?

[The Usual Student]

Hey guys, on a follow up. Why is the answer to the attached file A? Why is Y disulfide bonds?

kinda outside of the course but its between two cystines which are typical of disulfide bridges.

[edebi004]

Does anyone know if the textbook is the same for next year?

[Elizawei]

Does anyone know if the textbook is the same for next year?

If you're talking about 3/4 textbooks, then it's definitely changed.  RIP 2013-2016 study design.

Don't buy any textbook from people who did 3/4 chem this year

[Gogo14]

For volumetric analysis,
Why is the equivilance point not always at ph7?

[Sine]

For volumetric analysis,
Why is the equivilance point not always at ph7?

for strong acid vs strong base equivalence point is at pH 7 since both the products: conjugate base and conjugate acid is weak.

for strong acid vs weak base the the products: conjugate base is weak and conjugate acid is strong hence the pH less than 7

for weak acid vs strong base the products :  conjugate base is strong and conjugate acid is weak therefore pH greater than 7

for weak acid vs weak base (not really done since their won't be a sharp end point) products will be strong acid +  strong base and pH7.

[Adequace]

http://m.imgur.com/a/q71SX

For Q3) I've completely forgotten how to do this and my notes don't help. Is this because the KNO3 will dissolve in water and therefore be ions? So it'll form ion-dipole bonds to the water's dipoles?

For Q8) I'm so confused how to read the mole ratios from the equation but observation. For option A, I proved it was wrong but by looking at it doesn't it say 2 moles of C8H18 equals 16 moles of CO2..? I'm pretty sure you're supposed to do this by observation but I'm not confident after getting A instead of C..

[Syndicate]

http://m.imgur.com/a/q71SX

For Q3) I've completely forgotten how to do this and my notes don't help. Is this because the KNO3 will dissolve in water and therefore be ions? So it'll form ion-dipole bonds to the water's dipoles?

For Q8) I'm so confused how to read the mole ratios from the equation but observation. For option A, I proved it was wrong but by looking at it doesn't it say 2 moles of C8H18 equals 16 moles of CO2..? I'm pretty sure you're supposed to do this by observation but I'm not confident after getting A instead of C..

Q3) KNO3 is an ionic compound, which dissociates and forms ion-dipole bonds with H2O. The hydrogen atoms will be more attracted to the NO3- ion, whereas the oxygen atoms from the H2O molecule will be more attracted to the K+ ions.

Only acids and bases ionise in water, so A is incorrect.

H2O cannot form hydrogen bonds with K and NO3 ions (self explanatory), so B is incorrect.

I believe inter-particle forces refers to interatomic, which is also wrong, as only intermolecular forces are responsible for the dissolution on KNO3.

Therefore D is the correct answer.
Q8)

Molar ratio: Unknown reactant's mole/ known reactant's mole : Unknown ration/ known ratio

n(CO2)/ n(H2O) : 16/ 18 (8/9)

[sweetcheeks]

http://m.imgur.com/a/q71SX

For Q3) I've completely forgotten how to do this and my notes don't help. Is this because the KNO3 will dissolve in water and therefore be ions? So it'll form ion-dipole bonds to the water's dipoles?

For Q8) I'm so confused how to read the mole ratios from the equation but observation. For option A, I proved it was wrong but by looking at it doesn't it say 2 moles of C8H18 equals 16 moles of CO2..? I'm pretty sure you're supposed to do this by observation but I'm not confident after getting A instead of C..

Q3) You are correct. The KNO3 will dissociate into K+ and NO3- ions, forming intermolecular bonds with water molecules. The δ negative O will form a bond with the K+ and the δ positive hydrogens will bond with NO3-.

Q8) This was difficulty to comprehend initially. It can be viewed as an algebraic expression, trying to get the same number on both sides. Lets assume that n is equal to 1, as it makes it simpler to explain.
To start with I will show why C is the correct answer. n(CO2)=8/9n(H2O)  this becomes 1x16=8/9x1x(18) becoming 32=32. That means that the amount of CO2 produced in the reaction is equal to 8/9 the amount of H2O produced, which can be seen in the reaction (16 is 8/9 of 18)
A would be 1x2=1x8x16 which is not equal on both sides. What that is saying is that n(octane) is equal to 8 times n(CO2) which is incorrect.
B is saying that the amount of oxygen produced is equal to 2/25 of the amount of octane used, which is again incorrect (there is 2/25 times more oxygen produced)
D claims that the number of mole of water is 25/18 times the amount of oxygen. It is the opposite way around

[Adequace]

Thanks for the help both of you!

[Adequace]

Also, why isn't H2CO3 considered an amphiprotic species?

[keltingmeith]

Also, why isn't H2CO3 considered an amphiprotic species?

Because it can't accept protons, only donate them. Remember - for something to be AMPHIprotic, it must do both. You might be confusing this with POLYprotic, which means it can donate more than one proton.

[Sine]

Also, why isn't H2CO3 considered an amphiprotic species?

Basically an amphiprotic species can act as an acid or a base. It can accept a proton (H+) or donate a proton. Most of the ones you will see in VCE are those with a hydrogen and a negative charge, i.e HCO3-  H2PO4-  HPO42- and probably water is the only exception to this in VCE.

[Adequace]

http://m.imgur.com/LlQcYek

For this, is the reluctant O2- or H2O? Answer said H2O, don't get why..