Defence11
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Hey pls help. Question 9.4 http://filestore.aqa.org.uk/resource...84621H-SQP.PDF
Mark scheme http://filestore.aqa.org.uk/resource...84621H-SMS.PDF
I don’t get the last bit of the mark scheme in the conclusion where it says that
Conclusions
• if C−Cl bond changes less, then less exothermic
• if C−Cl bond changes more then more exothermic
• can’t tell how overall energy change will differ as do not

Surely if C-Cl bonds are stronger as it states in the question then the C-Cl bonds can’t change “less” so I don’t really get the first bullet point of the conclusion
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Anonymouspsych
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(Original post by Defence11)
Hey pls help. Question 9.4 http://filestore.aqa.org.uk/resource...84621H-SQP.PDF
Mark scheme http://filestore.aqa.org.uk/resource...84621H-SMS.PDF
I don’t get the last bit of the mark scheme in the conclusion where it says that
Conclusions
• if C−Cl bond changes less, then less exothermic
• if C−Cl bond changes more then more exothermic
• can’t tell how overall energy change will differ as do not

Surely if C-Cl bonds are stronger as it states in the question then the C-Cl bonds can’t change “less” so I don’t really get the first bullet point of the conclusion
What the last bit of the mark scheme is trying to get at is that because both the C-Cl bond and the Cl-Cl bond will be stronger, there are two variables that are changing. So if you refer back to 9.3 and think about the the overall enthalpy change is calculated (sum of bond enthalpies of reactants- sum of bond enthalpies of products ), the overall energy change with the reaction of chlorine and ethene will both depend on the C-Cl bond and Cl-Cl bond enthalpies.

So theoretically if both of the bond enthalpies of C-Cl and Cl-Cl increase by the same amount from that of C-Br and Br-Br, then the enthalpy change will be exactly the same as that in 9.3. But if say the bond enthalpy of C-Cl increased by a greater amount than the bond enthalpy of Cl-Cl, then the enthalpy change will be more negative and hence more exothermic. On the other hand if the bond enthalpy of the Cl-Cl bond was greater than the C-Cl bond enthalpy, then the overall enthalpy change would be less exothermic and less negative.

So the idea the conclusion is getting at is because there are two things changing simultaneously, you can't for certain say how the overall energy change will differ unless you know how one of the bond enthalpies changes relative to the other bond enthalpy.
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Defence11
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(Original post by Anonymouspsych)
What the last bit of the mark scheme is trying to get at is that because both the C-Cl bond and the Cl-Cl bond will be stronger, there are two variables that are changing. So if you refer back to 9.3 and think about the the overall enthalpy change is calculated (sum of bond enthalpies of reactants- sum of bond enthalpies of products ), the overall energy change with the reaction of chlorine and ethene will both depend on the C-Cl bond and Cl-Cl bond enthalpies.

So theoretically if both of the bond enthalpies of C-Cl and Cl-Cl increase by the same amount from that of C-Br and Br-Br, then the enthalpy change will be exactly the same as that in 9.3. But if say the bond enthalpy of C-Cl increased by a greater amount than the bond enthalpy of Cl-Cl, then the enthalpy change will be more negative and hence more exothermic. On the other hand if the bond enthalpy of the Cl-Cl bond was greater than the C-Cl bond enthalpy, then the overall enthalpy change would be less exothermic and less negative.

So the idea the conclusion is getting at is because there are two things changing simultaneously, you can't for certain say how the overall energy change will differ unless you know how one of the bond enthalpies changes relative to the other bond enthalpy.
I don’t understand ur middle paragraph. How can C-Cl be increased by greater amount than Cl-Cl and vice versa.
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Defence11
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(Original post by Anonymouspsych)
What the last bit of the mark scheme is trying to get at is that because both the C-Cl bond and the Cl-Cl bond will be stronger, there are two variables that are changing. So if you refer back to 9.3 and think about the the overall enthalpy change is calculated (sum of bond enthalpies of reactants- sum of bond enthalpies of products ), the overall energy change with the reaction of chlorine and ethene will both depend on the C-Cl bond and Cl-Cl bond enthalpies.

So theoretically if both of the bond enthalpies of C-Cl and Cl-Cl increase by the same amount from that of C-Br and Br-Br, then the enthalpy change will be exactly the same as that in 9.3. But if say the bond enthalpy of C-Cl increased by a greater amount than the bond enthalpy of Cl-Cl, then the enthalpy change will be more negative and hence more exothermic. On the other hand if the bond enthalpy of the Cl-Cl bond was greater than the C-Cl bond enthalpy, then the overall enthalpy change would be less exothermic and less negative.

So the idea the conclusion is getting at is because there are two things changing simultaneously, you can't for certain say how the overall energy change will differ unless you know how one of the bond enthalpies changes relative to the other bond enthalpy.
Could u say the same about bromine then. So if the C-Br is stronger than Br-Br the reaction would be more exothermic and vice versa. Is that right
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Anonymouspsych
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(Original post by Defence11)
I don’t understand ur middle paragraph. How can C-Cl be increased by greater amount than Cl-Cl and vice versa.
Well you know that both the C-Cl and Cl-Cl bond will be stronger than the C-Br and Br-Br bond as chlorine has less electrons. But what you don't know is if the C-Cl bond is stronger than the Cl-Cl bond or if its the other way round. So if the C-Cl bond has a stronger bond enthalpy than the Cl-Cl bond the total enthalpy change will be more negative and hence more exothermic (and vice versa if Cl-Cl bond enthalpy increased from the Br-Br bond by a greater amount than C-Cl bond enthalpy from the C-Br bond enthalpy, then the overall enthalpy change is less negative and less exothermic.)

So because you don't know if the C-Cl or Cl-Cl bond enthalpy is stronger you can't determine how the overall enthalpy change will differ from the one you calculated in 9.3
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Anonymouspsych
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(Original post by Defence11)
Could u say the same about bromine then. So if the C-Br is stronger than Br-Br the reaction would be more exothermic and vice versa. Is that right
yes
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Defence11
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(Original post by Anonymouspsych)
Well you know that both the C-Cl and Cl-Cl bond will be stronger than the C-Br and Br-Br bond as chlorine has less electrons. But what you don't know is if the C-Cl bond is stronger than the Cl-Cl bond or if its the other way round. So if the C-Cl bond has a stronger bond enthalpy than the Cl-Cl bond the total enthalpy change will be more negative and hence more exothermic (and vice versa if Cl-Cl bond enthalpy increased from the Br-Br bond by a greater amount than C-Cl bond enthalpy from the C-Br bond enthalpy, then the overall enthalpy change is less negative and less exothermic.)

So because you don't know if the C-Cl or Cl-Cl bond enthalpy is stronger you can't determine how the overall enthalpy change will differ from the one you calculated in 9.3
Ok thanks
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