VCE Chemistry Question Thread

[jyce]

Hey guys, with the attached question(s):
1) The first attachment - The mol ratios won't always be 1:1 will it? And why can we get the conc. of the two from the mol? i.e. [H3O^+] and [CH3COO^-]
2) Second attachment - Do we need to know this for the end of year exam?
Thank you
Edit: I worded my question weirdly, I mean why can we say that because we have 1 mol of H3O... etc. we can say their conc. are equal? I think I may have just answers my question while typing this but I would still like confirmation

You'll always see a 1:1 mole ratio for the products of the ionisation of an acid. At least that's been my experience.

We can assume that the products have equal concentration because:
- they are produced in a 1:1 mole ratio
- their amounts are initially zero (or close enough to zero)
- the extra H₃O⁺ produced by the self-ionisation of water is negligible (as K_w is very small).

And no, you don't need to know about basicity constants, just acidity constants.

[blacksanta62]

Thanks jyce

[jazzaa36]

Hello, i need some urgent help for a few questions..

- A syringe contains the gases nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) in quilibrium:

                              N204 (g) <=> 2NO2 (g)

1. Would the addition of krypton have any affect on the concentration or equilibrium?

-for these two equilibriums
Fe3 + SCN <=> Fe(SCN)2+
N204(g) <=> 2NO2(g)

2. why is platinum catalyst used? Why specifically platinum?

[Maz]

Hey humans,
My class recently did an experiment on the reactivity of alcohols. We reacted primary, secondary and tertiary alcohols with dichromate, permanganate, and sodium...I have formed the reaction redox equations. We have a test on it on Friday and I was wandering if you had any idea about some of the questions that may come up/any questions that I should really study?
Also one more thing please...For household cleaning would it be better to have a primary alchohol, or secondary? In a primary one there is more solubility, but less reactivity. In a secondary one there would be slightly less solubility, and a bit more reactivity. I'm thinking secondary...but I'm not really sure?
Any help would be really appreciated

Thankyou,
Maryam

[zsteve]

Hey humans,
My class recently did an experiment on the reactivity of alcohols. We reacted primary, secondary and tertiary alcohols with dichromate, permanganate, and sodium...I have formed the reaction redox equations. We have a test on it on Friday and I was wandering if you had any idea about some of the questions that may come up/any questions that I should really study?
Also one more thing please...For household cleaning would it be better to have a primary alchohol, or secondary? In a primary one there is more solubility, but less reactivity. In a secondary one there would be slightly less solubility, and a bit more reactivity. I'm thinking secondary...but I'm not really sure?
Any help would be really appreciated

Thankyou,
Maryam

Not sure about your syllabus, but I think some things worth studying are the reactions of these alcohols to form ketones (from secondary alkanol), aldehydes (from primary alkanol), carboxylic acids (primary alkanols), and alkoxides (reaction of alkanols with sodium to form sodium alkoxides + H2 gas)

Remember that oxidation of primary alkanols is two-step to form first an aldehyde, followed by further oxidation to form the carboxylic acid.

[Adequace]

http://m.imgur.com/a/kgQo8, need some help with these 2 Qs.

Just to double check for Q1, at 60 degrees, the solubility of oxygen is 0.023. This is the amount of oxygen still remaining in the solution, so the other 0.01g has been lost. Hence D, right?

Q6d) the answer has 0.78 mol whereas I got 7.8 * 10^-4? Am I wrong?

Thanks

[blacksanta62]

http://m.imgur.com/a/kgQo8, need some help with these 2 Qs.

Just to double check for Q1, at 60 degrees, the solubility of oxygen is 0.023. This is the amount of oxygen still remaining in the solution, so the other 0.01g has been lost. Hence D, right?

Q6d) the answer has 0.78 mol whereas I got 7.8 * 10^-4? Am I wrong?

Thanks

Hey man, with the second question they didn't convert ml ---> L. That's why they got 0.78. Your answer is right.
With the first question, yes, we will lose 0.010 g of O₂

[blacksanta62]

Can anyone help me with these questions? I can't seem to establish a link to find the conc. of H⁺
Thanks
Edit: C wants us to find the pH as well

[vox nihili]

Can anyone help me with these questions? I can't seem to establish a link to find the conc. of H⁺
Thanks
Edit: C wants us to find the pH as well

Not going to do the questions for you. But here are some hints.

9. The equation pH=-log[H⁺] should be handy. You can work out the concentrations of hydrogen ions in each solution this way and then work out what's going on when you mix them together.

c. The concentration of H⁺ in the HCl solution is equal to the concentration of HCl, likewise the concentration of OH^- is equal to the concentration of NaOH in the NaOH solution. Now it's just a matter of working out how much of each is left over when you mix them.

[jazzaa36]

Hello, i need some urgent help for a few questions..

- A syringe contains the gases nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) in quilibrium:

                              N204 (g) <=> 2NO2 (g)

1. Would the addition of krypton have any affect on the concentration or equilibrium?

-for these two equilibriums
Fe3 + SCN <=> Fe(SCN)2+
N204(g) <=> 2NO2(g)

2. why is platinum catalyst used? Why specifically platinum?

[lzxnl]

Hello, i need some urgent help for a few questions..

- A syringe contains the gases nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) in quilibrium:

                              N204 (g) <=> 2NO2 (g)

1. Would the addition of krypton have any affect on the concentration or equilibrium? Nope. Krypton doesn't do anything to either side of the reaction and doesn't change the reaction rate because it doesn't change the partial pressures of either species.

-for these two equilibriums
Fe3 + SCN <=> Fe(SCN)2+
N204(g) <=> 2NO2(g)

2. why is platinum catalyst used? Why specifically platinum? The real reason why they're using platinum (btw, it's most likely NOT metallic platinum; almost certainly a coordinatively unsaturated platinum complex, but I've forgotten all the third year chem I've learnt so yeah) is to speed up the rate equilibrium is attained, but the exact mechanism wouldn't be needed for VCE. The details are not needed for VCE, and I've never seen platinum needed as a catalyst for these reactions, but if platinum was indeed needed for the second reaction, the platinum would bind to either nitrogen species and weaken the bonds in the molecules, making bond-breaking easier.

[Adequace]

Hey,

For units of concentration are there any other necessary units frequently used apart from g/L, ppm and mol/L? (Pretty much just looking to memorise the ones used in 3/4 lol)

Thanks

[jyce]

Hey,

For units of concentration are there any other necessary units frequently used apart from g/L, ppm and mol/L? (Pretty much just looking to memorise the ones used in 3/4 lol)

Thanks

Percentage by mass (% w/w) also pops up quite often.

[Callum@1373]

Hey, Could someone please help me with these questions?

[HighTide]

Hey, Could someone please help me with these questions?

Hey Callum@1373
Redox potential= ability of a substance to be reduced. Hence higher redox potential = more reduction= less oxidation. We're looking for decreasing order of reductive activity. So the strongest reductant, should be the weakest oxidant, and thus, the lowest redox potential. So D?
In half cell 1, the species being reduced is both aqueous, thus you need a platinum or graphite electrode. In half cell 2, you can have a chromium electrode since it'll be oxidised. So B