2Al(s) Fe2O3(s) → Al2O3(s) 2Fe(s), struggling with this question

You have been asked to calculate the enthalpy change in the following reaction: 2Al(s) Fe2O3(s) → Al2O3(s) 2Fe(s) The reaction occurs in the following stages: Chemical Change Assignment 2 1. 2Al(s) Fe2O3(s) → Al2O3(s) 2Fe(l) ΔH = -732.5kJ mol–1 2. Al2O3(s) 2Fe(l) → Al2O3(s) 2Fe(s) ΔH = -27.6kJ mol–1 3. Al2O3(s) 2Fe(l) → Al2O3(s) 2Fe(s) ΔH = -91kJ mol–1 Note stages 2 and 3 look the same, but the reaction is cooling from 1700oC to 25oC and hence the enthalpy change there. Calculate the overall enthalpy change for the reaction. You must show your workings within your submission, and explain your reasoning. Your explanation should be a maximum of 300 words.I have been advised by my tutor to use Hess' law to calculate the overall enthalpy of the reaction given the individual steps. can anyone help, can't get my head around it at all. thanks :smile:

Reply 1

AC79

1F90D97F-4E1D-4682-9EBF-0F069845B0E8.jpg.jpeg

4CA80B37-B3F9-496C-A7EA-F94EAD9DC84E.jpg.jpeg

Here’s how I did it. Let me know if you have any questions :smile:

Reply 2

Vicky09

Hi can I ask what was the reason for x2 of the second equation and flip of the 3rd?

Reply 3

AC79

Original post by Vicky09

Hi can I ask what was the reason for x2 of the second equation and flip of the 3rd?

Hi,
The second equation must be doubled so Al2SO3(s) and 2Fe(l) can be cancelled in equations 1 and 2 on the RHS. It also allows Al2O3(s) to be cancelled on LHS equation 3. Allows only two of Fe(s) to be cancelled on RHS of equation 2 by 2Fe(s) on the LHS of equation 3 - as it is seen in the equation wanted that only 2Fe(s) is wanted on the RHS not 4Fe(s).

Reversing the third allows correct cancellations to be made to end up at the desired equation* - in other cases the reversing can be for getting compounds in the correct place for the desired equation.
* ie if it was not flipped then Al2O3(s) (equation 3) would not be able to cancel 2Al2O3(s) in equation 2 to Al2O3(s) as they would not be on opposite sides. A compound/element can only cancel another of its kind** if it is in another equation from it on the opposite sides so the left side of equation 1 can only cancel the right side of equation 2 and 3.
** Compounds/elements can only cancel the exact same version of it, ie Fe(s) cannot cancel Fe(l) as one is in solid state and one is in liquid state but Fe(s) could cancel Fe(s).

RHS = Right hand side
LHS = Left hand side

Reply 4

Vicky09

Thank you very much for your response it has clarified what I thought.

Reply 5

AC79

Original post by Vicky09

Thank you very much for your response it has clarified what I thought.

no worries :smile:

Reply 6

Vicky09

Original post by AC79

no worries :smile:

I hope you don't think I'm being too cheeky but is there any chance you could explain this to me I cant seem to get my head around it?

Thank you

(A) After experimentation you have determined the following rate equations:
1. Rate = k

[NO]2
2. Rate = k

2 [O2]
3. Rate = k

[NO]

Identify the order of reaction for each of these rate equations. You must show your workings within your submission, and explain your reasoning. Your explanation should be a maximum of 300 words.

(B) Through experimentation, you note that at high temperatures ethyl chloride produces HCl and ethylene by the following reaction:

CH3CH2Cl(g) → HCl(g) + C2H4(g)

Using the rate data for the reaction at 650°C presented in the following table, calculate the reaction order with respect to the concentration of ethyl chloride (CH3CH2Cl). You must show your workings within your submission, and explain your reasoning. Your explanation should be a maximum of 300 words.
Experiment [CH3CH2Cl]ₒ (M) Initial Rate (M/s)
1 0.010 1.6 x 10−8
2 0.015 2.4 x 10−8
3 0.030 4.8 x 10−8
4 0.040 6.4 x 10−8

Reply 7

AC79

Original post by Vicky09

[NO]2
2. Rate = k

2 [O2]
3. Rate = k

Hi, this is my workings. I’m not so sure about B) but that is how I would do it. 4A58F4D4-4E63-45A6-BA6E-5344791B32F2.jpg.jpeg

38F35C77-C13C-4C13-A50B-1CECE44EA903.jpg.jpeg

Hope it helps!

Reply 8

Vicky09

Original post by AC79

Hi, this is my workings. I’m not so sure about B) but that is how I would do it. 4A58F4D4-4E63-45A6-BA6E-5344791B32F2.jpg.jpeg

Hope it helps!

Thank you very much this has helped massively i have been able to complete my assignment with your help. I will let you know how i get on.

Reply 9

AC79

Original post by Vicky09

Thank you very much this has helped massively i have been able to complete my assignment with your help. I will let you know how i get on.

Glad to hear! :smile:

Reply 10

Victoria2021

Original post by AC79

Hello,

Hope you got on well with your access course.

I also got stuck on a question similar to this and this post really helped me.

I had a quick question on the workings out of the enthalpy change
Because the bottom 2 equations are the same equation i understand the reverse and x2 but I am confused how you know which one to reverse?

For example you can flip the doubled equation or the single equation and they will still cancel out but by changing them you get different answers because of the different enthalpies?

Hope you can help .

Thanks

Reply 11

AC79

Original post by Victoria2021

Hi, sorry for the late reply, I haven't been on here for quite a while and I haven't done this type of chemistry for 2 years now! Luckily this was one of my favourite equations to solve in Chemistry so I remember it quite well. Also, I'm really glad to hear my previous posts helped you before :smile:

I've gone through the equation the way you stated - flipping 2) and x2 3). My final answer is -886.9 kJmol-1. I'm afraid I can't remember exactly why I chose it that way, I know there is a reason that our teacher told us at the time, but as I said it's been almost 2 years now since I've done this. I can't give an exact answer but here are some potential ideas you may be able to figure out (as your knowledge will be fresher than mine) or ask a teacher -

1. In comparison to answer -696.7kJmol-1, my second answer (-886.9kJmol-1) is a lot lower but both are exothermic. Exothermic --> energy released. (I think) this means answer 2 loses more heat, answer 1 is more sustainable?
2. Enthalpies of equation 2) and 3) are -27.6 and -91 - again shows difference between the two, therefore is it more sustainable to use 2) as less energy/ heat loss?

I'm sorry I couldn't give you a more precise answer and hope you manage to get it

(edited 3 years ago)

Reply 12

Lucasdahlin

Hello,
funnily enough I've been stumped by the same question.
I found this website that shows the exact same reaction with the same values, but the difference is, they just add all the steps together as its asking to calculate the overall enthalpy change.

https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_A_Molecular_Approach_(Tro)/06%3A_Thermochemistry/6.08%3A_Relationships_Involving_Enthalpy_of_Reactions

Is this correct? As I'm not understanding why we are changing the reaction when its just stating the exact reaction we need

Reply 13

AC79

Original post by Lucasdahlin

Hi, I'm not 100% sure I understand your question but from what I can interpret -

In the original question we had to 'change' the equations given in order to create the final equation being asked. When we change the given equations this affects the enthalpy changes so we cannot add the enthalpies together until the equations are correct overall in order to form the final answer. If we add the enthalpies prior to changing the equations we would be given an incorrect answer as they would not overall create the final equation we are needing.

I hope this helps :smile:

Reply 14

NaiNai90

Original post by Lucasdahlin

Hi,

Can I ask whether you got clarity about this? I also added the provided enthalpy changes to calculate the overall enthalpy but have now become confused as everyone else is doing long complex calculations!