chemistry question #1206



Peter Griffin, a 19 year old male from Beaufort asks on January 22, 2003,

Q:

How does the type of liquid effect the way it holds heat?

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the answer

Barry Shell answered on January 27, 2003, A:

Different liquids can hold different amounts of heat per unit weight or volume depending on the heat capacity or specific heat of the liquid. There's a list of heat capacities and a bit of an explanation at Engineering Toolbox. The heat carrying ability of a liquid depends more or less on the way atoms of a liquid are related to each other within the substance, and this usually has to do with the geometry of the particular molecules that make up the liquid and how that geometry causes them to interact with each other.

Muhammad Asif Arif, molecular biology graduate student, Lahore, Pakistan answered on September 1, 2003, A:

Besides the shape or geometry of the molecules of the liquid there are many other factors involved in the heat-carrying capacity of liquids. One of the most important is the cohesive forces between molecules arising from hydrogen bonding and other weak molecular interactions.

Hydrogen bonds can be formed when a hydrogen atom is bound to a highly electronegative atom such as nitrogen, oxygen, or fluorine. The hydrogen "end" of the atom thus has a partial positive charge and can interact with the electronegative "end" of a different molecule (the N, O, or F "side" of the molecule). This results in a stabilizing interaction that loosely binds the two molecules together. A common example is H20---H-O-H, where the three dashes represent the hydrogen bond. Hydrogen bonds are found throughout nature. They give water its unique properties, which allowed life to develop on Earth. Hydrogen bonds are also the intermolecular force that binds together the two strands in a molecule of DNA. They are responsible for numerous other biochemical phenomena.

Dipole-Dipole interactions are another kind of weak force that occurs between two molecules with permanent dipoles (dipoles are like little magnets with a positive and negative end). These work in a similar manner to ionic interactions but are weaker because only partial charges are involved. An example of this can be seen in hydrochloric acid.
(+)(-) (+) (-)
H-Cl-----H-Cl

London Dispersion Forces, also called London forces or Van der Waals forces involve the attraction between temporaily induced dipoles in nonpolar molecules. This polarization can be induced either by a polar molecule or by the repulsion of negatively charged electron clouds in nonpolar molecules. An example of the former is chlorine dissolving in water:
(+)(-)(+) (-) (+)
[Permanent Dipole] H-O-H-----Cl-Cl [Induced Dipole]
An example of the second scenario is found in molecular chlorine:
(+) (-) (+) (-)
[Induced Dipole]Cl-Cl------Cl-Cl [Induced Dipole]

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