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March 28, 2024, 09:14:32 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040527 times)  Share 

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anotherworld2b

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Re: Chemistry Question Thread
« Reply #480 on: July 25, 2016, 01:10:39 am »
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CH3COOH(aq) is acetic acid. I disagree with the answer because the fact it is acetic acid means that it is definitely dissociated.

Unless the question is being pedantic. Their answer isn't "completely" unjustified because some unsplit CH3COOH is in there as well. So I can see where they're coming from. But the question should've made that clearer - I don't like these ambiguous questions.
___________________

Q8 is combining equations. You have to do some maths here.

a) Equation 2 has just one CO in the reactants. But equation 1 has two COs in the products
So we have to multiply equation 2 by two to merge the equations:

2 CO(g) + 2 FeO(s) -> 2 Fe(l) + 2 CO2(g)

Substitute equation 1 in there to get:

C(s) + CO2(g) + 2 FeO(s) -> 2 Fe(l) + 2 CO2(g)

Cancel out the duplicated carbon dioxide:

C(s) + 2 FeO(s) -> 2 Fe(l) + CO2(g)

It's sort of like simultaneous equations: Perform (1) + 2*(2)

I'll let you have a try at the other two first. If you're still stuck, however (especially with 8c - that one is trickier), just post again

For the carbon dioxide in q8 a how do you know to cancel it out? Also does it matter which equation you subsitute?

I am not sure how to do part c  :-[

RuiAce

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Re: Chemistry Question Thread
« Reply #481 on: July 25, 2016, 09:17:50 am »
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For the carbon dioxide in q8 a how do you know to cancel it out? Also does it matter which equation you subsitute?

I am not sure how to do part c  :-[
I didn't. It appeared on both the products AND the reactants once I did simultaneous equations, so that's when I knew to cancel them out.

c) seems to have two correct answers. I'm not sure which direction to head either, actually. Can we please have the answers?
(It's because I'm not sure whether we want Cl to cancel out, or ClO to cancel out.)
« Last Edit: July 25, 2016, 09:24:16 am by RuiAce »

leila_ameli

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Re: Chemistry Question Thread
« Reply #482 on: July 25, 2016, 01:35:03 pm »
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heyy
I need a some help with this question:

Bond energy is the energy needed to break the chemical bond between two atoms. The bond energies for each of the two allotropes of oxygen are shown in the table.
                                                            Oxygen                                       Ozone
Bond Energy (kJ mol-1)                         498                                             364
Account for the difference in bond energy for these two allotropes of oxygen.

Aliceyyy98

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Re: Chemistry Question Thread
« Reply #483 on: July 25, 2016, 01:48:44 pm »
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Hi guys,

Could someone please the concept of buffers? with an example and preferably equations to illustrate? thanks!

Maz

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Re: Chemistry Question Thread
« Reply #484 on: July 25, 2016, 04:01:01 pm »
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Hi guys,

Could someone please the concept of buffers? with an example and preferably equations to illustrate? thanks!
Hey human
Hopefully, I can help a bit...
Buffers are solutions with the ability to resist pH change when small quantities of acid/base are added to them. Buffers are generally a solution which contains a weak acid-base conjugate pair (a weak acid and its conjugate base or vice-versa). These acids and bases and co-exist without neutralising each other, as neutralisation generally occurs in an acid-base reaction, yet they can still react to neutralise any strong acid or strong base added to the buffer.
An example of a buffer solution could include a 2mol ethanoic acid solution, and 2 mol of sodium ethanoate; then making the solution u to 1L. The buffer contains a high concentration of both weak acid CH3COOH and its conjugate weak base, CH3COO-.
The equilibrium reaction is shown below:
CH3COOH (aq) + H20 (l)   <---->   CH3COO- +H3O+(aq)
Buffer behaviour can be predicted via Le Chateliers Principle (LCP). Eg, if you add a base to the buffer will neutralise some of the H3O+ present, causing its concentration to fall. This falling concentration causes the equilibrium reaction to shift right, as explained via LCP, (forward reaction favoured) o replace some of the lost hydronium concentration, and preventing too much of a significant fall. This is the property of a buffer being able to resist some changes in pH. If you flood the buffer with acid/base this will not work, and a pH change will result (each buffer has a buffer capacity)
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jakesilove

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Re: Chemistry Question Thread
« Reply #485 on: July 25, 2016, 04:25:06 pm »
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heyy
I need a some help with this question:

Bond energy is the energy needed to break the chemical bond between two atoms. The bond energies for each of the two allotropes of oxygen are shown in the table.
                                                            Oxygen                                       Ozone
Bond Energy (kJ mol-1)                         498                                             364
Account for the difference in bond energy for these two allotropes of oxygen.

Hey!

Essentially, all I would be talking about to answer this question is the reactivity of each alloptrope. As Ozone has a lower bond energy, it will be easier to break/react. Oxygen will be more difficult to break/react. Clearly, Ozone is more reactive than Oxygen. We can explain this by saying that coordinate covalent bonds are easier to break than normal covalent bonds. This explains why Ozone only really forms in higher levels of the atmosphere, where even more reactive Oxygen free radicals are present to react. I think this is as much as an answer would require in the HSC!

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MysteryMarker

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Re: Chemistry Question Thread
« Reply #486 on: July 25, 2016, 09:03:03 pm »
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Industrial Chemistry Question:

During the electrolysis of dilute NaCl, why is the water oxidised instead of the Chlorine ions? Because in the concentrated NaCl solution, the chlorine ions do get oxidised, however in the very dilute solution they do not. Just curious as to why this is.

Cheers.

marynguyen18

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Re: Chemistry Question Thread
« Reply #487 on: July 25, 2016, 09:11:07 pm »
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Hey i was wondering if anyone has studied/currently studying shipwrecks that could help me understand the difference between a galvanic cell and an electrolytic cell, because I'm getting confused with the charge of the cathode and anode are.
« Last Edit: July 25, 2016, 09:14:51 pm by marynguyen18 »

RuiAce

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Re: Chemistry Question Thread
« Reply #488 on: July 25, 2016, 09:23:15 pm »
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Industrial Chemistry Question:

During the electrolysis of dilute NaCl, why is the water oxidised instead of the Chlorine ions? Because in the concentrated NaCl solution, the chlorine ions do get oxidised, however in the very dilute solution they do not. Just curious as to why this is.

Cheers.
Compare the Eo values of both oxidations:

2 Cl- ⇌ Cl2(g) + 2 e-                  Eo=-1.36 V
2 H2O(l) ⇌ O2(g) + 4 H+ + 4 e-   Eo=-1.23 V

Notice how the two values are very similar. This is opposed to the reduction, where the voltage required to cause sodium to reduce is significantly larger to water reducing.

Because the voltage is so similar, the EMF supplied will seek out the more abundant substance as about a similar amount of voltage is consumed anyway for either reaction. In concentrated sodium chloride solution, there is actually a sufficient amount of chloride ions for their oxidation to be favoured. In dilute solution, there really isn't that many, and water will be oxidised anyway.
Hey i was wondering if anyone has studied/currently studying shipwrecks that could help me understand the difference between a galvanic cell and an electrolytic cell, because I'm getting confused with the charge of the cathode and anode are.
Fortunately this overlaps into industrial chemistry.

Galvanic cells and electrolytic cells function oppositely.

In a galvanic cell, a chemical reaction takes place producing a voltage, thereby chemical energy is converted to electrical energy
In an electrolytic cell, electricity is passed through a cell to reverse the effect of the galvanic cell, thereby electrical energy is converted to galvanic energy.
(The EMF value for a galvanic cell is positive, whereas that of an electrolytic cell is negative.)

Effectively, electrolytic cells can be used as battery rechargers.

The mnemonic RED-CAT AN-OX still takes play. Reduction happens at the cathode and oxidation happens at the anode.

However, the polarity reverses in an electrolytic cell.

For a galvanic cell, the anode is negative and the cathode is positive.
Cathode: Cu2+ + 2 e- -> Cu(s)
Anode: Zn(s) -> Zn2+ + 2 e-

For an electrolytic cell, the anode is positive and the cathode is negative.
Cathode: Zn2+ + 2 e- -> Zn(s)
Anode: Cu(s) -> Cu2+ + 2 e-

Deleted last bit. Forgot the why. JAKE!
« Last Edit: July 25, 2016, 11:44:29 pm by RuiAce »

marynguyen18

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Re: Chemistry Question Thread
« Reply #489 on: July 25, 2016, 09:37:25 pm »
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thank you so much! but with the half equations why is that some of them are either producing H+ or OH- if that makes sense?

RuiAce

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Re: Chemistry Question Thread
« Reply #490 on: July 25, 2016, 09:39:01 pm »
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thank you so much! but with the half equations why is that some of them are either producing H+ or OH- if that makes sense?
Not quite

All I can respond to out of that is that one of them is an oxidation whilst the other is a reduction

onepunchboy

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Re: Chemistry Question Thread
« Reply #491 on: July 25, 2016, 09:56:57 pm »
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Hey guys im not sure to do q 15 hehe thanks

MysteryMarker

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Re: Chemistry Question Thread
« Reply #492 on: July 25, 2016, 10:03:00 pm »
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Compare the Eo values of both oxidations:

2 Cl- ⇌ Cl2(g) + 2 e-                  Eo=-1.36 V
2 H2O(l) ⇌ O2(g) + 4 H+ + 4 e-   Eo=-1.23 V

Notice how the two values are very similar. This is opposed to the reduction, where the voltage required to cause sodium to reduce is significantly larger to water reducing.

Because the voltage is so similar, the EMF supplied will seek out the more abundant substance as about a similar amount of voltage is consumed anyway for either reaction. In concentrated sodium chloride solution, there is actually a sufficient amount of chloride ions for their oxidation to be favoured. In dilute solution, there really isn't that many, and water will be oxidised anyway.Fortunately this overlaps into industrial chemistry.


Cheers man, another question for industrial chemistry:

Within a diaphragm cell, why is it more important to stop the migration of hydroxide ions into the anode compartment then it is to stop the migration of chloride ions into the cathode compartment?


anotherworld2b

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Re: Chemistry Question Thread
« Reply #493 on: July 25, 2016, 10:51:34 pm »
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I didn't. It appeared on both the products AND the reactants once I did simultaneous equations, so that's when I knew to cancel them out.

c) seems to have two correct answers. I'm not sure which direction to head either, actually. Can we please have the answers?
(It's because I'm not sure whether we want Cl to cancel out, or ClO to cancel out.)

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #494 on: July 25, 2016, 11:30:10 pm »
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Cheers man, another question for industrial chemistry:

Within a diaphragm cell, why is it more important to stop the migration of hydroxide ions into the anode compartment then it is to stop the migration of chloride ions into the cathode compartment?

Hey Mystery Marker!

Both are equally just as important as each other. By preventing OH- from entering the anode we are ensuring that all of the Na+ from the brine are reacting with OH-. This is important because after all, it takes energy to reduce water to produce hydroxide and therefore any waste of OH- (i.e. those that dont end up participating in the formation of NaOH) is undesirable as it is wasting the energy input. We also dont want Cl- to migrate into cathode compartment because we dont like the formation of NaCl in our final product, all we want is NaOH. A major concern that may answer your question is that we can get Na+ and Cl- very easily, just from natural sea water (brine) but to get OH- we need quite an intensive amount of energy (diaphragm cell needs about 2750kwh energy). So it's less of a concern to form NaCl than to not form NaOH.

Best regards
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