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May 12, 2021, 12:16:53 am

Author Topic: VCE Chemistry Question Thread  (Read 1474882 times)  Share 

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ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #9045 on: February 17, 2021, 04:32:47 pm »
+2
Need a bit of a refresher regarding SF :D If we were calculating the amount in mole of 3.45g of H2, would we only be able to go to 2SF due to the molar mass of H being 1.0 in the data booklet?

When I learnt it I was told to go to 3 sig figs, as 3.45 is being used in the calculations as an experimental value. (Smallest number of sig figs in the question) 1.0 is just a constant, so you donít take that into account. Hope this helps, however, itíll be good to double check with what your teacher wants. 😃

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #9046 on: February 17, 2021, 11:41:11 pm »
+4
Thanks

if someone could help with this question
a. From the graph determine the instantaneous rate when the concentration of o2 is at 8x 10-3M
b. Between 60 seconds and 300seconds, which instantaneous rate was higher? why?


For b would this be right
60 seconds is steeper that means it has a higher gradient than the 300 secs. Higher gradient means higher rate

For c from the graph determine the instantaneous rate at 60 seconds and at 300 seconds

Would this be right
300- 6/500= 0.012
60-13/40= 0.325

Would a be pretty much the same as the 60 secs


Mod edit: Merged Double posts

To work out the instantaneous reaction rate at a given point on the graph you will want to draw a tangent line (a straight line that just touches this particular given point) and then work out the gradient of this tangent line (the gradient of the line is equal to the rate of the reaction - rise/run == change in product/time).  Once the tangent line has been drawn (i have shown a quick example for this at ~320 seconds) you can calculate the gradient by essentially forming a right-angle triangle and obtaining the change in O2 concentration (rise) for a particular time interval (run) and divide this concentration value by the time to determine the change in concentration per second (the reaction rate). I am not entirely sure what you have done in your calculations, but the method i have explained here should give you the correct answers. Its also important to note that the units of the y-axis (the product concentration) are 10^-3M and that the O2 concentration is decreasing over time so the reaction rate should be a negative value.

And when doing question like these, don't worry too much about getting the tangent line 100% perfect - it really is just an estimation and in question like these the answer key will often accept a range of values to account for this error in estimation.


Excuse the scrappiness of the example above - my ms paint skills are lacking



Need a bit of a refresher regarding SF :D If we were calculating the amount in mole of 3.45g of H2, would we only be able to go to 2SF due to the molar mass of H being 1.0 in the data booklet?

On this page here you can find VCAA 'advice to teachers' on sig figs.

For your specific problem, since the molar mass of hydrogen listed in periodic table in the data booklet (or whatever resource you are using) is an approximation of the molar of hydrogen (which is more accurately 1.00784), I think its important to take it into account.
In this particular instance depending on if you use the approximation of hydrogen as 1.0 g/mol compared to the more accurate 1.00784 g/mol, you will get different values for the 3rd significant figure. It is for this exact reason why when using a value approximated to 2 sig figs, the answer should be written to two sig figs. This being said, as colline has pointed out, if this is for a SAC which your teacher is marking, you should definitely ask your teacher what she is looking for.
« Last Edit: February 17, 2021, 11:57:39 pm by Erutepa »
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thenuttyprofessor

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Re: VCE Chemistry Question Thread
« Reply #9047 on: February 18, 2021, 07:28:15 pm »
0
What would the dependent variable be for the following experiment?
Measuring the molar heat of combustion of increasing carbon chain alcohols
I was under the impression it cant be a derived variable (calculated from collected data)
Since the temperature change is constant, would the dv be the change in mass of the final mass? unless I'm totally incorrect.

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #9048 on: February 19, 2021, 11:17:06 am »
+4
What would the dependent variable be for the following experiment?
Measuring the molar heat of combustion of increasing carbon chain alcohols
I was under the impression it cant be a derived variable (calculated from collected data)
Since the temperature change is constant, would the dv be the change in mass of the final mass? unless I'm totally incorrect.

The dependent variable is the data that you are measuring so I would say you are correct In identifying it as the change in mass of the fuel.
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laura_

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Re: VCE Chemistry Question Thread
« Reply #9049 on: February 19, 2021, 03:40:29 pm »
0
Hey!
I'm not sure what I'm missing here but I got a bit stuck on this redox question.

SO2 (g) and Al (s) are mixed in acid. Write the Al(s) half-equation and an overall equation.
she/her

berryland

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Re: VCE Chemistry Question Thread
« Reply #9050 on: February 19, 2021, 05:16:35 pm »
0
Hey what is the difference between q and ΔH

wingdings2791

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Re: VCE Chemistry Question Thread
« Reply #9051 on: February 19, 2021, 05:19:21 pm »
+4
Hey!
I'm not sure what I'm missing here but I got a bit stuck on this redox question.

SO2 (g) and Al (s) are mixed in acid. Write the Al(s) half-equation and an overall equation.

Hello Laura, I'm fairly sure the equations are as follows:
Half: Al(s) -> Al3+(aq) + 3e-
Overall: 4Al(s) + 3O2(g) + 12H+(aq) -> 4Al3+(aq) + 6H2O(l)

Here's a breakdown:
- Mixed in acid means acidic conditions, so no need to balance with OH- afterwards
- If you look at the electrochemical series, you'll see that O2(g) + 4H+(aq) + 4e- <-> 2H2O(l) is higher up than all other appearances of O2(g) (which comes from the SO2(g))
- Therefore, O2(g) is the strongest oxidant and Al(s) is the strongest reductant, so they will react
- To balance the electrons, (O2(g) equation x 3) and (Al(s) equation) x 4

Hope this helps :)
« Last Edit: February 19, 2021, 05:28:51 pm by wingdings2791 »
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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #9052 on: February 20, 2021, 10:38:17 pm »
0
if someone could help with this the products that would be formed in this reaction

wingdings2791

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Re: VCE Chemistry Question Thread
« Reply #9053 on: February 21, 2021, 11:34:58 am »
0
if someone could help with this the products that would be formed in this reaction

Hi, I'm not completely sure of this (especially with the states) but I think the reaction might be:
C7H15Br(l) + KOH(aq) -> C7H15OH(l) + KBr(aq)

I hope that's right  :-\ :)
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wingdings2791

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Re: VCE Chemistry Question Thread
« Reply #9054 on: February 21, 2021, 11:57:53 am »
0
Hello all, I've got two questions:
1) Can combustion possibly occur without O2(g)? TSFX said that this reaction:
3Mg(s) + N2(g) -> Mg3H2(s) ΔH = -ve
is an example of combustion, simply because it's an exothermic redox reaction. Is this true? I was always under the impression that a fuel has to be oxidised by O2(g) to qualify as combustion.

2) For Q5a) of 9.1 from the Heinemann textbook, why does the answer not also include H2O2(aq) + 2H+(aq) + 2e- -> 2H2O(l)? Is it because it's higher on the series than the other reaction that oxidises H2O (the one that's listed)?

Thanks everyone
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ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #9055 on: February 21, 2021, 04:01:27 pm »
+1
Hello all, I've got two questions:
1) Can combustion possibly occur without O2(g)? TSFX said that this reaction:
3Mg(s) + N2(g) -> Mg3H2(s) ΔH = -ve
is an example of combustion, simply because it's an exothermic redox reaction. Is this true? I was always under the impression that a fuel has to be oxidised by O2(g) to qualify as combustion.

2) For Q5a) of 9.1 from the Heinemann textbook, why does the answer not also include H2O2(aq) + 2H+(aq) + 2e- -> 2H2O(l)? Is it because it's higher on the series than the other reaction that oxidises H2O (the one that's listed)?

Thanks everyone

1. I'm not 100% sure about the answer to this question, but I'm pretty sure in VCE chem all combustion reactions are taken to be reactions with fuels that are oxidised with oxygen. However, I do think it can happen with other oxidants, I'm not sure in which cases though. However, it is useful to know that although all combustion reactions are exothermic, not all exothermic reactions are combustion reactions  :)

2. When dealing with aqueous solutions (where there's water present) in electrolysis, the only water equations you look at in the electrochemical series is when water decomposes to hydrogen gas and oxygen gas. [I've attached a pic below]. That equation you've written there cannot happen, as you don't have any hydrogen peroxide present. Water is a reactant in this case.

Hope this helps! Sorry if it doesn't make much sense though.
« Last Edit: February 21, 2021, 04:37:17 pm by ArtyDreams »

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #9056 on: February 21, 2021, 05:23:04 pm »
0
What is the electrolyte in a galvanic cell? Is it just the inert solution in the salt bridge? Or the solvent and/or spectator ions?
Thanks  :P

ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #9057 on: February 21, 2021, 05:47:21 pm »
+2
What is the electrolyte in a galvanic cell? Is it just the inert solution in the salt bridge? Or the solvent and/or spectator ions?
Thanks  :P

The electrolyte is the solutions in the containers, that the electrodes are immersed in. I'm not 100% sure if the solution that makes up the salt bridge is also referred to as an electrolyte.

In other words: The electrolyte are substances that release ions when in solution, and carries an electrical current. It provides a medium for transport of ions, thus allowing charge to be transferred. Also helps in maintaining the electrical neutrality through the salt bridge.

linty_

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Re: VCE Chemistry Question Thread
« Reply #9058 on: February 21, 2021, 07:35:17 pm »
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Hey there, could someone please help me with this question?

If 1.216g of a dampened sample of oxalic acid is titrated with acidified 0.1234M potassium permanganate, and 32.33 mL is required, what is the percentage water in the sample?

Thanks

linty_

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Re: VCE Chemistry Question Thread
« Reply #9059 on: February 21, 2021, 07:35:45 pm »
-1
Hey there, could someone please help me with this question?

If 1.216g of a dampened sample of oxalic acid is titrated with acidified 0.1234M potassium permanganate, and 32.33 mL is required, what is the percentage water in the sample?

Thanks