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March 28, 2024, 09:08:58 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040526 times)  Share 

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jakesilove

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Re: Chemistry Question Thread
« Reply #705 on: September 04, 2016, 03:14:03 pm »
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Hey guys, i was doing a question for my option topic (industrial chem) and the question was to find the equilibrium constant (K). However the question didn't give the concentrations of the reactants and products in mol/L but rather gave it in kPa. When instructed to find K, the answers just substituted these kPa values into the formula. I was just confused, i thought you could only sub the concentration that are in mol/L. :S

Since the formula relies on ratios, if everything is in a gaseous form, the relative pressures will be equivalent to concentration. This assumes ideal gas etc etc. but we can assume all of that in a question like this. For instance, if I combined 1kPa of Nitrogen and 2kPa of Hydrogen, because the pressure is across a specific unit area (which is the same in both measurements), and because Gas will occupy a constant space per mole regardless of the chemical composition, we can assume that Hydrogen is 'twice as concentrated'. I didn't do Industrial, and I'm sure there are way more complicated reasons why this trick works, but basically if you ever see another question like this just use the kPa measurements! May not make 100% sense, just turns out to be what you need to do.
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RuiAce

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Re: Chemistry Question Thread
« Reply #706 on: September 04, 2016, 03:19:45 pm »
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Since the formula relies on ratios, if everything is in a gaseous form, the relative pressures will be equivalent to concentration. This assumes ideal gas etc etc. but we can assume all of that in a question like this. For instance, if I combined 1kPa of Nitrogen and 2kPa of Hydrogen, because the pressure is across a specific unit area (which is the same in both measurements), and because Gas will occupy a constant space per mole regardless of the chemical composition, we can assume that Hydrogen is 'twice as concentrated'. I didn't do Industrial, and I'm sure there are way more complicated reasons why this trick works, but basically if you ever see another question like this just use the kPa measurements! May not make 100% sense, just turns out to be what you need to do.
Oh dear. Seems unfair to throw that on them. Yeah that's abnormal for the HSC

massive

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Re: Chemistry Question Thread
« Reply #707 on: September 04, 2016, 04:03:47 pm »
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Thanks Jake, just one more q, Why is it when pressure is increased the concentrations of everything increases instantaneously but then equilibrium shifts to side with fewer moles of gases. Also does this instant increase happen with any of the other variables (e.g. temperature or concentration). What i mean is, if the temperature is increased, does everything else increase suddenly then equilibrium shifts right or left (depending if exo/endo) and everything decreases or increases according to this new equilibrium position?

RuiAce

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Re: Chemistry Question Thread
« Reply #708 on: September 04, 2016, 04:09:27 pm »
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Thanks Jake, just one more q, Why is it when pressure is increased the concentrations of everything increases instantaneously but then equilibrium shifts to side with fewer moles of gases. Also does this instant increase happen with any of the other variables (e.g. temperature or concentration). What i mean is, if the temperature is increased, does everything else increase suddenly then equilibrium shifts right or left (depending if exo/endo) and everything decreases or increases according to this new equilibrium position?
We had this discussion on pressure already.
The concentration of the solids are constant, only (aq) and (g) are in the equilibrium expressions. You can exclude pure solids and pure liquids from the equilibrium expressions.
What I thought. I remember being taught this but it was a blur.When you add more moles of solid, you add the same amount of volume of the solid to counteract a potential increase in concentration. Given C=n/V, if n and V go up in the exact same proportion, the concentration is not affected.

Note that it is the CONCENTRATION of the substance which shifts the equilibrium, not the actual quantity (moles)
Temperature is different. Temperature depends on your enthalpy change value, that is, your value for ΔH. The value for ΔH has nothing to do with the state your substances are in, be it solid, liquid or gaseous.


If you mean the actual concentrations, which are best depicted on a graph, then no, only changes in concentrations spike the graph up. This is because a change in temperature is nothing but a change in temperature - we are not introducing or extracting a substance relative to the volume of the vessel. Pressure or increases in concentration disturbs the ratio of substances (moles) to the volume of the vessel (volume, and note C=n/V), whereas temperature does not.

Note that the more gradual changes later are just a consequence of Le Chatelier's principle. The spikes are when we suddenly change concentrations in a matter of a second, whereas what comes next is the system trying to shift it's equilibrium to counter the change. (Once again - temperature does NOT forcibly disturb what's in there immediately, as opposed to e.g. taking out 2L of hydrogen.)
« Last Edit: September 04, 2016, 04:12:59 pm by RuiAce »

massive

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Re: Chemistry Question Thread
« Reply #709 on: September 04, 2016, 04:15:16 pm »
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We had this discussion on pressure already.Temperature is different. Temperature depends on your enthalpy change value, that is, your value for ΔH. The value for ΔH has nothing to do with the state your substances are in, be it solid, liquid or gaseous.


If you mean the actual concentrations, which are best depicted on a graph, then no, only changes in concentrations spike the graph up. This is because a change in temperature is nothing but a change in temperature - we are not introducing or extracting a substance relative to the volume of the vessel. Pressure or increases in concentration disturbs the ratio of substances (moles) to the volume of the vessel (volume, and note C=n/V), whereas temperature does not.

Note that the more gradual changes later are just a consequence of Le Chatelier's principle. The spikes are when we suddenly change concentrations in a matter of a second, whereas what comes next is the system trying to shift it's equilibrium to counter the change. (Once again - temperature does NOT forcibly disturb what's in there immediately, as opposed to e.g. taking out 2L of hydrogen.)
OHHH thanks man i get it now! btw, if the question gives the concentration of one substance we can't do mole ratio comparison using the balanced equation to get the concentration of the other species right?

RuiAce

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Re: Chemistry Question Thread
« Reply #710 on: September 04, 2016, 04:17:40 pm »
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OHHH thanks man i get it now! btw, if the question gives the concentration of one substance we can't do mole ratio comparison using the balanced equation to get the concentration of the other species right?
Well technically since n=CV, if you just leave V as something you don't know (like x in maths) you could carry from there.

But in general, no. Because such a method is asking too much for HSC chemistry students.

massive

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Re: Chemistry Question Thread
« Reply #711 on: September 04, 2016, 04:27:16 pm »
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ohk nws, thankyou. I just have one last question (hopefully)

For the equilibrium reaction below, would adding acid or base affect the equilibrium position at all?

N2O4 <---> 2NO2

RuiAce

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Re: Chemistry Question Thread
« Reply #712 on: September 04, 2016, 04:31:58 pm »
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ohk nws, thankyou. I just have one last question (hopefully)

For the equilibrium reaction below, would adding acid or base affect the equilibrium position at all?

N2O4 <---> 2NO2
That equilibrium is between two gases. Highly doubt an acid or base will do anything to it

jakesilove

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Re: Chemistry Question Thread
« Reply #713 on: September 04, 2016, 04:46:10 pm »
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That equilibrium is between two gases. Highly doubt an acid or base will do anything to it

Ditto. It would only have an effect if the substance (namely, Nitrogen Dioxide) was in aqueous form, which would have a different equilibrium anyway.
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massive

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Re: Chemistry Question Thread
« Reply #714 on: September 04, 2016, 11:45:08 pm »
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GUYS, for dehydration of ethanol and hydration of ethylene what are the states for each species. There are some notes that say that water is gas and others that say it's liquid...so confused D:

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Re: Chemistry Question Thread
« Reply #715 on: September 04, 2016, 11:49:35 pm »
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GUYS, for dehydration of ethanol and hydration of ethylene what are the states for each species. There are some notes that say that water is gas and others that say it's liquid...so confused D:
water is a gas as the reaction requires such a high temperature. In VCE examiners still accept liquid though. i'm not aware of how it's marked in HSC.

EDIT: minor fix
« Last Edit: September 05, 2016, 12:01:55 am by Sine »

jakesilove

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Re: Chemistry Question Thread
« Reply #716 on: September 05, 2016, 12:00:04 am »
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water is a gas as the reaction requires such a high temperature. In VCE examiners still accept liquid though. i'm not aware of how it's mark in HSC.

Same with the HSC. Basically, whether you put gas or liquid doesn't matter (although generally liquid is safer). Just as long as you include a state!
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Re: Chemistry Question Thread
« Reply #717 on: September 05, 2016, 05:15:30 pm »
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Is HPO-4 and acid? Shouldn't it be a base, becuase H2SO4 is a strong acid and HPO-4 is its conjugate base ??

RuiAce

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Re: Chemistry Question Thread
« Reply #718 on: September 05, 2016, 05:38:25 pm »
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Is HPO-4 and acid? Shouldn't it be a base, becuase H2SO4 is a strong acid and HPO-4 is its conjugate base ??
Phosphoric acid and sulfuric acid are two different things. Sulfuric acid being acidic doesn't have anything to do with the nature of the hydrogen phosphate ion.

Presumably you meant HSO4 or something. Please clarify and then we'll get back to you

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Re: Chemistry Question Thread
« Reply #719 on: September 05, 2016, 06:09:48 pm »
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Phosphoric acid and sulfuric acid are two different things. Sulfuric acid being acidic doesn't have anything to do with the nature of the hydrogen phosphate ion.

Presumably you meant HSO4 or something. Please clarify and then we'll get back to you

Yeah that's what i meant, my bad!