Heating curve question
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Heating curve question
Why is the second horizontal much longer than the first horizontal line (screenshot below)? In other words, why does it take more heat to change from a liquid to a solid than it does to change from a solid to a liquid?
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Re: Heating curve question
The reason the second horizontal is much longer than the first horizontal is that boiling requires less energy than vaporizing. Boiling only requires 6.01 kJ*mol-1 while vaporizing requires 40.7kJ*mol-1. This elevated level of energy accounts for the differing durations of the state change.
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Re: Heating curve question
I think it has to do with intermolecular bonds. To go from solid to liquid, the molecules aren't fully breaking apart, but rather they are just not tightly, orderly packed together like in solids. However, to go from liquid to gas, the gas molecules themselves are free of the other molecules, so more bonds must be broken to get into the gaseous phase.
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Re: Heating curve question
In liquids, the intermolecular forces present are still somewhat similar to those in a solid since the molecules are still relatively close together but are able to slide past one another. As a result, not as much energy is needed to go from a solid to a liquid since it is really only "loosening" the intermolecular forces. However, in a vapor, the molecules are much farther apart and move randomly in all directions, so the intermolecular forces must be broken. Thus, going from a liquid to a vapor takes much more energy, which is why the second horizontal line is longer than the first.
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Re: Heating curve question
The flat lines in the graph indicate a phase change. The heat continues to increase, however, at the flat lines the heat stops contributing to the temperature and it is now contributing to the phase change, by breaking bonds. Therefore the temperature stays the same at flat lines. At these lines, the thermal motion is enough to break enough interactions to move molecules inside of the substance to then change it to a different state. The reason why the water has a longer line to go from liquid to gas is because it needs to break all the strong hydrogen bonds within them, therefore it requires more heat. In other words, it takes more heat to go from liquid to gas than it would to go from solid to liquid because more hydrogen bonds would need to be broken within the substance.
Re: Heating curve question
The first horizontal line is much shorter than the second because the amount of energy required to cause the phase change from solid to liquid is significantly less than the amount needed to cause the phase change from liquid to gas. The increased length indicates that the energy from the rise in temperature is going towards breaking the intermolecular forces of the water.
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Re: Heating curve question
The phase transition from solid to liquid involves breaking the intermolecular forces that supplied the rigidity of the substance and are relatively weak and don't require much energy to break. Therefore less heat (which is a form of energy) is required for this phase transition. However, the phase transition from liquid to gas involves breaking hydrogen bonds, and physically separating molecules from each other. Therefore, this transition requires much more energy (heat) in order to fully transition and that is why the line is longer.
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Re: Heating curve question
It requires much more heat to turn a liquid to a gas than a solid to a liquid because you have to overcome much stronger intermolecular forces. When going from a solid to a liquid, you just have to disrupt the structure of the ice. However, to overcome the very strong hydrogen bonds between the water molecules, as you would when going from a liquid to a gas, you have to input a lot more energy.
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Re: Heating curve question
Going from a solid to a liquid, the molecules need a bit of energy input. However, they aren't moving all around the container just yet, so they don't need a LOT of energy. For gases, they are moving everywhere at high speeds so they must have high energy. It takes a lot more energy input before molecules can achieve this level of energy compared to the energy needed for molecules to loosen from a solid into liquid.
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Re: Heating curve question
It needs a substantial amount of extra heat to become a liquid to gas in contrast to a solid to a liquid. The reasoning is that it overcomes higher intermolecular forces. When transforming from a liquid to a gas, requires additional energy for the hydrogen bonds in the water molecules.
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Re: Heating curve question
For this question, the answer is due to the different intermolecular forces. In liquids the intermolecular forces are similar to those of solids, so it takes energy for this change, but not a substantial amount. However, going from liquid to gas requires the intermolecular forces to be broken, requiring more energy.
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Re: Heating curve question
The horizontal lines on this graph represent phase changes. This is where heat the heat being added to the system is used for a phase change, so it doesn't change the temperature. Vaporization's longer horizontal line means that more heat is required for the phase change from liquid to vapor.
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Re: Heating curve question
The flat lines indicate phase changes, as other users have mentioned. The reason the second line is longer than the first one is because boiling involves breaking all intermolecular attractions between water molecules, whereas the first line represents melting, where the intermolecular attractions aren't fully broken, only partially. As such, melting takes less heat than boiling does.
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