AS Chemistry Edexcel - Compound Stability Problem

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OmarEdExcel
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Hello TSR,

I hope you are all alright. Recently, I have been concerned about a question talking about the relationship between the stability of magnesium chloride to it's different forms.

Please consider following the link below :

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* This is Unit 1 AS Chemistry Question 13 January 2017 *

According to the mark schemes, the answer is C. Why is this?

Thank you in advance, your cooperation will be highly appreciated.

Best Regards,
OmarEdExcel
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BobbJo
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(Original post by OmarEdExcel)
Hello TSR,

I hope you are all alright. Recently, I have been concerned about a question talking about the relationship between the stability of magnesium chloride to it's different forms.

Please consider following the link below :

http://prntscr.com/lwv41i

* This is Unit 1 AS Chemistry Question 13 January 2017 *

According to the mark schemes, the answer is C. Why is this?

Thank you in advance, your cooperation will be highly appreciated.

Best Regards,
OmarEdExcel
An energy level diagram is highly useful

Draw a diagram with energy on vertical axis

First bullet point tells us that Mg and Cl2 are at a higher energy than MgCl. We deduce that the change from Mg + Cl2 to MgCl is exothermic. The enthalpy of formation of MgCl is exothermic.

Second bullet tells us MgCl is at a higher energy than MgCl2 + Mg. So the change from MgCl to MgCl2 is exothermic. [By extension, the change from Mg + Cl2 to MgCl2 is exothermic but the question does not require this] The enthalpy of formation of MgCl2 is exothermic.

Third bullet tells us that MgCl3 is at a higher energy than Mg + Cl2. The change from Mg + Cl2 to MgCl3 is endothermic. The enthalpy of formation of MgCl3 is endothermic.

Hence C
Last edited by BobbJo; 1 year ago
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OmarEdExcel
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Hello BobbJo, thank you once again for the instant replies.

However, in the question, I believe that somehow the key word is stable/unstable. Why the same word meant that the starters of the 3 bullet points are at a higher energy?

In the first bullet point, MgCl is stable with respect to magnesium and chlorine : I assume this means that the attractive forces between the oppositely charged ions hold the structure firm ( giant ionic lattice ). However, does being stable mean that it releases energy? I don't think so however I am willing to listen always but I do have this opinion because I would believe it would release energy if it is standard enthalpy of atomisation as gases are formed just like combustion releases energy but the question states the compounds in solid states. Kindly explain this.

In the second bullet point, although MgCl is unstable, you have still stated that it releases energy despite the question mentioning that MgCl is unstable - why a different word was given the same attribute?

In the 3rd bullet point, why the same word ( unstable ) was given a different attribute compared to the 2nd bullet point ( and 1st bullet point ) This time you said it's endothermic.

Thank you once again BobbJo, would always love to have your explanations.
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BobbJo
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(Original post by OmarEdExcel)
Hello BobbJo, thank you once again for the instant replies.

However, in the question, I believe that somehow the key word is stable/unstable. Why the same word meant that the starters of the 3 bullet points are at a higher energy?

In the first bullet point, MgCl is stable with respect to magnesium and chlorine : I assume this means that the attractive forces between the oppositely charged ions hold the structure firm ( giant ionic lattice ). However, does being stable mean that it releases energy? I don't think so however I am willing to listen always but I do have this opinion because I would believe it would release energy if it is standard enthalpy of atomisation as gases are formed just like combustion releases energy but the question states the compounds in solid states. Kindly explain this.

In the second bullet point, although MgCl is unstable, you have still stated that it releases energy despite saying that MgCl is unstable - why a different word was given the same attribute?

In the 3rd bullet point, why a different word was given a different attribute? This time you said it's endothermic.

Thank you once again BobbJo, would always love to have your explanations.

A system is thermodynamically stable if it is in its lowest energy configuration

1st bullet: MgCl is stable wrt to Mg and Cl2
means MgCl is at a lower energy than Mg + Cl2
so the change Mg + Cl2 to MgCl is exothermic
energy(MgCl) - energy(Mg+Cl2) < 0

2nd: MgCl is unstable wrt to MgCl2
MgCl is at a higher energy than MgCl2

3rd: MgCl3 is unstable wrt to Mg + Cl2
MgCl3 is at a higher energy than Mg + Cl2

Represent this on an energy level diagram
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BobbJo
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(Original post by OmarEdExcel)
In the first bullet point, MgCl is stable with respect to magnesium and chlorine : I assume this means that the attractive forces between the oppositely charged ions hold the structure firm ( giant ionic lattice ). However, does being stable mean that it releases energy? I don't think so however I am willing to listen always but I do have this opinion because I would believe it would release energy if it is standard enthalpy of atomisation as gases are formed just like combustion releases energy but the question states the compounds in solid states. Kindly explain this.
Being stable means it is in a lower energy configuration

A being thermodynamically stable wrt B means the change from B to A releases energy.
A is at a lower energy level than B.

The enthalpy of atomisation is always positive and absorbs energy.

Combustion releases energy
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OmarEdExcel
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(Original post by BobbJo)
A system is thermodynamically stable if it is in its lowest energy configuration

1st bullet: MgCl is stable wrt to Mg and Cl2
means MgCl is at a lower energy than Mg + Cl2
so the change Mg + Cl2 to MgCl is exothermic
energy(MgCl) - energy(Mg+Cl2) < 0

2nd: MgCl is unstable wrt to MgCl2
MgCl is at a higher energy than MgCl2

3rd: MgCl3 is unstable wrt to Mg + Cl2
MgCl3 is at a higher energy than Mg + Cl2

Represent this on an energy level diagram
(Original post by BobbJo)
A system is thermodynamically stable if it is in its lowest energy configuration

1st bullet: MgCl is stable wrt to Mg and Cl2
means MgCl is at a lower energy than Mg + Cl2
so the change Mg + Cl2 to MgCl is exothermic
energy(MgCl) - energy(Mg+Cl2) < 0

2nd: MgCl is unstable wrt to MgCl2
MgCl is at a higher energy than MgCl2

3rd: MgCl3 is unstable wrt to Mg + Cl2
MgCl3 is at a higher energy than Mg + Cl2

Represent this on an energy level diagram
Yes! I got it, thanks.

Truly the key term is the word stable/unstable. However, what's more practical is drawing the energy level diagams. When I did draw them, the MgCl3 tends to appear at a higher position than MgCl. However, what I finally want you to confirm and that will truly sum it up :

Is MgCl considered negative as an answer because it is lower down when represented with Mg + Cl2 in an energy diagram compared to the position of MgCl3 which is positioned higher up in it's own energy diagram with Cl2 + Mg being lower in terms of position and energy?

Thank you again BobbJo.
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