Melting and Potential Energy
When ice melts into water, the open structure of ice collapses, so the molecules are more closely packed in liquid state than in solid state, but potential energy increases during the melting process. Does this feel a bit contradictory? My understanding about potential energy is basically the distance between molecules, are there any misconceptions?
Thank you.
Thank you.
Reply 1
Original post by lululemontea
When ice melts into water, the open structure of ice collapses, so the molecules are more closely packed in liquid state than in solid state, but potential energy increases during the melting process. Does this feel a bit contradictory? My understanding about potential energy is basically the distance between molecules, are there any misconceptions?
Thank you.
Thank you.
Yes, water is a special case here since it has a larger volume in the solid phase than in the liquid phase. This is due to Hydrogen Bonding having a longer bond length than id-id forces (your usual intermolecular forces that hold the particles together as a solid). Hydrogen bonding is stronger and acts at a way longer range than id-id forces (I will explain why later) as well as having polarity, hence why water expands when it freezes and does so at an unusually high temperature.
Potential energy does indeed increase when ice melts. In each type of "bond" there are two forces at play, some repulsive force and some attractive force. The attractive force here is clearly the electrostatic attraction between partial charges on the Oxygen and Hydrogen atoms, which obey Coulomb's Law and it's strength is inversely proportional to distance squared. There must also be a repulsive force since if not the electron would just fall into one of the atoms (we're ignoring all of the quantum mechanics jiggery pokery here). The exact details of this "repulsive force" I won't go into detail here (I'm running out of time to answer this) but there is a point where the forces just cancel out. This is where it is most stable (hence lower potential energy) and the distance is the bond length. It just so happens that because the attractive force acts at a longer range, this bond length is longer than usual.
Ice forms a hexagonal structure (due to the polar nature of Hydrogen bonding) also because it is at a local minimum of potential energy; the key idea here is by heating the ice, you are giving the particles more kinetic energy to be used to move up the potential barrier; so it can find another local minimum, such as a certain configuration as a liquid.
Reply 2
Original post by lululemontea
When ice melts into water, the open structure of ice collapses, so the molecules are more closely packed in liquid state than in solid state, but potential energy increases during the melting process. Does this feel a bit contradictory? My understanding about potential energy is basically the distance between molecules, are there any misconceptions?
Thank you.
Thank you.
It seems absurd, however, that as the ice melts, the molecules move closer together, gaining potential energy.
Here's the thing: potential energy is not simply a matter of distance.
A molecule has potential energy due to its position and arrangement with other molecules. Electrical forces and attractions are important factors.
The process involves the tiny crystalline arrangement of molecules in the ice, which gain energy as the ice melts and changes from a solid to a liquid state. This creates potential energy because the molecules can interact in many other ways.
Think of:
-Charges that attract or repel each other.
-Hydrogen bonds.
-Orientation of molecules in space.
In ice, molecules are in a configuration that minimises potential energy, supported by weak bonds.
Melting interrupts this arrangement, freeing the molecules and allowing them to break free and arrange themselves in parallel liquid threads. Disrupting the order (entropy: Delta S >0) adds energy (with potential energy) to the system. Disorder adds energy (with potential energy) to the system. The proximity and the way the structure change the interactions between the molecules.
In water, the molecules are closer together than in ice, but due to orientation and interactions, they have a higher potential energy.
Kind regards from Italy,
Sandro
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