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June 17, 2019, 08:44:32 pm

Author Topic: Chemistry Question Thread  (Read 468456 times)  Share 

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david.wang28

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Re: Chemistry Question Thread
« Reply #3720 on: March 01, 2019, 07:09:24 pm »
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Why is the answer (B) in this question (from Nesa).
The reason why the answer is B is because the OH ion is a base, meaning H is a conjugate acid. CO2 is an acid, meaning HCO3 is a conjugate base. Hence, this is a Bronsted-Lowry reaction. (Please note: I cannot guarantee this is the correct answer, but I gave it a try!) :)
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Jefferson

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Re: Chemistry Question Thread
« Reply #3721 on: March 01, 2019, 08:12:09 pm »
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The reason why the answer is B is because the OH ion is a base, meaning H is a conjugate acid. CO2 is an acid, meaning HCO3 is a conjugate base. Hence, this is a Bronsted-Lowry reaction. (Please note: I cannot guarantee this is the correct answer, but I gave it a try!) :)
yeah, but that doesn't quite eliminate A and D.

sarrahbarodawala

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Re: Chemistry Question Thread
« Reply #3722 on: March 02, 2019, 01:41:46 pm »
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Hello!
I was working through the chemistry topic test book and got confused on how to answer this question about combustion reactions. The question is
"Combustion reactions are typically considered to be irreversible reaction. To what extent is this conception a genuine description of the system, as opposed to simply a useful way of thinking about the reactions?"
I understood why they aren't reversible, but the answer at the back said something about the increase in entropy, which i didn't get. Someone please explain how they are related.
Thank you :)

annabeljxde

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Re: Chemistry Question Thread
« Reply #3723 on: March 02, 2019, 03:45:28 pm »
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Hello!
I was working through the chemistry topic test book and got confused on how to answer this question about combustion reactions. The question is
"Combustion reactions are typically considered to be irreversible reaction. To what extent is this conception a genuine description of the system, as opposed to simply a useful way of thinking about the reactions?"
I understood why they aren't reversible, but the answer at the back said something about the increase in entropy, which i didn't get. Someone please explain how they are related.
Thank you :)


Recall what entropy means. Entropy is the degree of disorder in a system and generally refers to the number of possible arrangements. Entropy tends to increase in reactions where:
- number of particles increase
- number of gaseous particles increase

Combustion reactions are always spontaneous, meaning they do not require a continuing supply of energy for the reaction to occur. Recall also that the symbol used to represent this fact is ΔG < 0, or ΔH - TΔS < 0. Since combustion reactions are exothermic, there is a release of energy and therefore, ΔS of the surroundings increase (release of gaseous particles increases the number of possible arrangements). Putting this into the equation:

ΔH = negative
ΔS = positive

ΔH - TΔS --> (negative) - (positive) which will always be negative. Therefore ΔG < 0 and the reaction is spontaneous.

The question asks you to discuss whether the fact that knowing combustion reactions are irreversible is a sufficient description of the nature of the system, and in short, it is. The high activation energy of the reverse reaction (since combustion reactions are exothermic, therefore, Ea of reverse is greater than the forward reaction) reduces the chance that the products will recombine to form reactants. And since they occur in an open system, the products cannot recombine and therefore an equilibrium is impossible. (Here you can talk about entropy)

I hope this helps :) (I'm also answering you to help myself as well!)
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sarrahbarodawala

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Re: Chemistry Question Thread
« Reply #3724 on: March 02, 2019, 04:04:18 pm »
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Recall what entropy means. Entropy is the degree of disorder in a system and generally refers to the number of possible arrangements. Entropy tends to increase in reactions where:
- number of particles increase
- number of gaseous particles increase

Combustion reactions are always spontaneous, meaning they do not require a continuing supply of energy for the reaction to occur. Recall also that the symbol used to represent this fact is ΔG < 0, or ΔH - TΔS < 0. Since combustion reactions are exothermic, there is a release of energy and therefore, ΔS of the surroundings increase (release of gaseous particles increases the number of possible arrangements). Putting this into the equation:

ΔH = negative
ΔS = positive

ΔH - TΔS --> (negative) - (positive) which will always be negative. Therefore ΔG < 0 and the reaction is spontaneous.

The question asks you to discuss whether the fact that knowing combustion reactions are irreversible is a sufficient description of the nature of the system, and in short, it is. The high activation energy of the reverse reaction (since combustion reactions are exothermic, therefore, Ea of reverse is greater than the forward reaction) reduces the chance that the products will recombine to form reactants. And since they occur in an open system, the products cannot recombine and therefore an equilibrium is impossible. (Here you can talk about entropy)

I hope this helps :) (I'm also answering you to help myself as well!)

Thank you very much!!!!

annabeljxde

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Re: Chemistry Question Thread
« Reply #3725 on: March 02, 2019, 05:42:32 pm »
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Hey!

I'm super confused about what refers to what in statements like: strong acid titrated against a weak base. Is the strong acid the aliquot and the weak base the standardised solution? Or is it the other way around?

How do you know which is which when you are given questions like this in an exam?
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milie10

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Re: Chemistry Question Thread
« Reply #3726 on: March 13, 2019, 09:33:30 pm »
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Hi!

I'm currently doing my year11 chemistry depth study, and I'm struggling with ideas- our teachers say it has to be more complicated than a year 8-10 experiment, but we have to be able to buy the chemicals ourselves. It's based on consumer chemistry. Any thoughts on what I could do?

Thanks!! :D

myopic_owl22

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Re: Chemistry Question Thread
« Reply #3727 on: March 16, 2019, 02:58:43 pm »
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Hey!

I'm super confused about what refers to what in statements like: strong acid titrated against a weak base. Is the strong acid the aliquot and the weak base the standardised solution? Or is it the other way around?

How do you know which is which when you are given questions like this in an exam?

Hello,
I've always considered the 'against' as to mean 'with reference to'. You're right about the acid being the aliquot (or analyte or titrand - whatever's in the conical flask) and the base being the standardised solution for the burette. If you like, you can remember that if A is titrated against B, B is the burette and A is the analyte.

However, for most calculation questions you'd be able to tell which one goes where as, typically, you'd be given a known concentration for whatever is in the burette (standardised solution) and the analyte will just have a volume. If you're feeling brave, though, it is possible to switch them around (i.e. put the analyte in the burette and standard solution in the flask) and still get the right answer.

Hope this helps!
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david.wang28

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Re: Chemistry Question Thread
« Reply #3728 on: March 18, 2019, 05:50:23 pm »
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Hello,
I have trouble with question 2.6 and 2.1 in the link below. Can anyone please help me out? Thanks :)
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InnererSchweinehund

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Re: Chemistry Question Thread
« Reply #3729 on: March 26, 2019, 10:21:35 am »
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Hi! I was wondering if anyone could help me with the following questions, based off a student investigation to determining the Equilibrium Constant using Colorimetry.

Fe3+(aq) + SCN–(aq) ⇋ FeSCN2+(aq)

1. Why do you think the equilibrium constant remained virtually constant, within experimental error, even though you were changing the concentrations?
 
2. Beer's Law (the linear relationship between concentration and absorbance) is accurate to about A = 1.5. How would you modify the experiment if the absorbance readings were higher than 1.5?

Finally, if someone could explain how to write a conclusion for the a practical investigation, that would be great!!
Thanks!!



e2503

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Re: Chemistry Question Thread
« Reply #3730 on: March 27, 2019, 11:49:44 pm »
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Hi  :D

I was just wondering whether enthalpy has a significant effect on the thermal stability of compounds ( specifically metal CO3 ). Ultimately, I'm trying to figure out whether the thermal decomposition temperatures of metal carbonates are affected by enthalpy but I am a little lost. :'(  If anyone could send some help, it would be very much appreciated. Thank you in advance  :)

e2503

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Re: Chemistry Question Thread
« Reply #3731 on: March 28, 2019, 12:07:28 am »
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Hi! I was wondering if anyone could help me with the following questions, based off a student investigation to determining the Equilibrium Constant using Colorimetry.

Fe3+(aq) + SCN–(aq) ⇋ FeSCN2+(aq)

1. Why do you think the equilibrium constant remained virtually constant, within experimental error, even though you were changing the concentrations?
 
2. Beer's Law (the linear relationship between concentration and absorbance) is accurate to about A = 1.5. How would you modify the experiment if the absorbance readings were higher than 1.5?

Finally, if someone could explain how to write a conclusion for the a practical investigation, that would be great!!
Thanks!!

Hi there :)
The conclusion of your practical investigation should consist of a few sentences which summarise your experiment. You should re-address your hypothesis to either accept or reject it providing clear reasons based on the implications made from your major findings. The key is to keep the conclusion concise. I hope this was helpful  :D :D

InnererSchweinehund

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Re: Chemistry Question Thread
« Reply #3732 on: March 28, 2019, 08:01:31 am »
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Hi there :)
The conclusion of your practical investigation should consist of a few sentences which summarise your experiment. You should re-address your hypothesis to either accept or reject it providing clear reasons based on the implications made from your major findings. The key is to keep the conclusion concise. I hope this was helpful  :D :D

Thank you!!!

maxinenicole

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Re: Chemistry Question Thread
« Reply #3733 on: April 04, 2019, 06:22:11 pm »
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Hi I have no clue how to work this question out:

NaOH solution has a pH of 12.0. a 60.00mL volume of NaOH solution is added to 4.0 x 10-3 mol in HCl in water. What volume of 0.200mol L-1 KOH must be added to bring the pH of the solution to 7.0?

Thanks

InnererSchweinehund

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Re: Chemistry Question Thread
« Reply #3734 on: April 11, 2019, 09:09:43 am »
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Hi,
I was wondering if someone could tell me if my explanation to this question is correct, and if not, could you please explain the concept??

Q: Beer's Law (the linear relationship between concentration and absorbance) is accurate to about A = 1.5. How would you modify the experiment if the
    absorbance readings were higher than 1.5?


A: According to Beer’s Law, if absorbance readings are higher than 1.5 they are usually inaccurate, as it means that most of the light was absorbed by the sample, and only a small amount of light was detected by the detector. When this occurs, the experiment should be modified by diluting each of the samples equally, usually by a factor of 10. This will enable the new concentrations to be accurately re-calculated, and can be repeated until the absorbance measured produces more accurate readings. It is important that the final concentrations and their measured absorbance have a linear relationship in order for the equilibrium constant to be calculated.

Thanks!!