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Chemistry - melting point

How does branching decrease melting point?
Original post by samiul.tr
How does branching decrease melting point?


decreases surface areas of contact so weaker london forces so less energy needed to overcome bonds
Reply 2
increase* (Original post by NotI'mA)decreases surface areas of contact so weaker london forces so less energy needed to overcome bonds
Reply 3
Original post by NotI'mA
decreases surface areas of contact so weaker london forces so less energy needed to overcome bonds


How does branching increase* melting point
Original post by samiul.tr
How does branching increase* melting point

Basically, the branching makes the molrecule 'wider', if you will, because parts stick out more rather than just being a straight line.
This pushes the molecules further away from each other, as they are wider.
This means the London forces (do you know what these are?) are weaker as the molecules interact less due to being further away.
This means there are fewer London forces, and the ones that are present, are weaker.
This means less energy is needed to break the bonds, when melting, so the melting point (or boiling point as well) is less.
Hope this helps!
Reply 5
Original post by Emmmaaaa...
Basically, the branching makes the molrecule 'wider', if you will, because parts stick out more rather than just being a straight line.
This pushes the molecules further away from each other, as they are wider.
This means the London forces (do you know what these are?) are weaker as the molecules interact less due to being further away.
This means there are fewer London forces, and the ones that are present, are weaker.
This means less energy is needed to break the bonds, when melting, so the melting point (or boiling point as well) is less.
Hope this helps!


Online, it's stated that branching causes an increase in melting point but decrease in boiling point. Your explanation adequately explains that decrease in boiling point as you're essentially saying less points of contact between 2 molecules so less van der waal (London forces) (much appreciated for your reply btw). But according to: https://www.masterorganicchemistry.com/2010/07/09/branching-melting-boiling-points/#:~:text=Starting%20with%20the%20simplest%20branched,but%20decrease%20the%20boiling%20point. - going from unbranched to branched results in a decrease in melting & boiling point but going from branched to highly branched results in a spherical structure which is more compact (smallest surface area of all shapes) resulting in a greater melting point but the same decreasing boiling point - hope that clarifies the topic as much as it did for me!
Original post by samiul.tr
Online, it's stated that branching causes an increase in melting point but decrease in boiling point. Your explanation adequately explains that decrease in boiling point as you're essentially saying less points of contact between 2 molecules so less van der waal (London forces) (much appreciated for your reply btw). But according to: https://www.masterorganicchemistry.com/2010/07/09/branching-melting-boiling-points/#:~:text=Starting%20with%20the%20simplest%20branched,but%20decrease%20the%20boiling%20point. - going from unbranched to branched results in a decrease in melting & boiling point but going from branched to highly branched results in a spherical structure which is more compact (smallest surface area of all shapes) resulting in a greater melting point but the same decreasing boiling point - hope that clarifies the topic as much as it did for me!

Aha I didn't know this. Thanks!
Reply 7
Original post by samiul.tr
Online, it's stated that branching causes an increase in melting point but decrease in boiling point. Your explanation adequately explains that decrease in boiling point as you're essentially saying less points of contact between 2 molecules so less van der waal (London forces) (much appreciated for your reply btw). But according to: https://www.masterorganicchemistry.com/2010/07/09/branching-melting-boiling-points/#:~:text=Starting%20with%20the%20simplest%20branched,but%20decrease%20the%20boiling%20point. - going from unbranched to branched results in a decrease in melting & boiling point but going from branched to highly branched results in a spherical structure which is more compact (smallest surface area of all shapes) resulting in a greater melting point but the same decreasing boiling point - hope that clarifies the topic as much as it did for me!

i'm looking at an exam question that's saying compound a has a lower melting point than compound b, suggest why and compound a has a branch and compound b doesn't. so doesn't that then mean that the melting point subsequently reduces with branching?
Original post by RT23
i'm looking at an exam question that's saying compound a has a lower melting point than compound b, suggest why and compound a has a branch and compound b doesn't. so doesn't that then mean that the melting point subsequently reduces with branching?

I see some confusion in this thread.
Increased branching reduces the melting point.
To understand this compare the surface area to volume ratio of various geometric shapes. The shape with the lowest surface area to volume ratio is the sphere.
More branching means closer to spherical and less surface area to volume ratio.
This means that there is relatively less surface for London dispersion forces to act and the inter-particular forces are weaker so the substance is more volatile and has a lower boiling point.
Compare boiling points:

pentane: 31.6ºC

methylbutane: 27.8 ºC

dimethylpropane: 9.5 ºC


all have the same volume (same molecular formula), but the boiling point decreases with the surface area.
I see some confusion in this thread.
Increased branching reduces the melting point.
To understand this compare the surface area to volume ratio of various geometric shapes. The shape with the lowest surface area to volume ratio is the sphere.
More branching means closer to spherical and less surface area to volume ratio.
This means that there is relatively less surface for London dispersion forces to act and the inter-particular forces are weaker so the substance is more volatile and has a lower boiling point.
Compare boiling points:

pentane: 31.6ºC

methylbutane: 27.8 ºC

dimethylpropane: 9.5 ºC


all have the same volume (same molecular formula), but the boiling point decreases with the surface area.

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