Why do intramolecular forces weaken due to heat?
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Intramolecular forces (bonding forces) exist within molecules and influence the chemical properties. Intermolecular forces exist between molecules and influence the physical properties. We can think of H2O in its three forms, ice, water and steam. In all three cases, the bond angles are the same, the dipole moment is the same, the molecular shape is the same and the hybridization of the oxygen is the same. However, the physical properties of H2O are very different in the three states. As solid ice, H2O possesses a definite shape and volume. It is incompressible. Liquid water possesses a definite volume, but will assume the shape of its container. It is slightly compressible. Steam will assume both the shape and volume of its container and is extremely compressible. Intermolecular forces (IMF) are the forces which cause real gases to deviate from ideal gas behavior. They are also responsible for the formation of the condensed phases, solids and liquids.
The IMF govern the motion of molecules as well. In the gaseous phase, molecules are in random and constant motion. Each gas molecule moves independently of the others. In liquids, the molecules slide past each other freely. In solids, the molecules vibrate about fixed positions.
Heating Curves
The transitions between the phases, phase changes, can be viewed in terms of a Heating Curve, like the one shown below, for water. It is a plot of time versus temperature. The time axis represents the addition of heat as a function of time. The longer the time span, the more heat has been added to the system. In this Heating Curve, we are starting with ice at -20oC.

As we add heat, we raise the temperature of the ice. The heat that we are adding is increasing the Kinetic Energy of the system (KE = ½ mu2) which is proportional to the absolute temperature (K) of the system. In the solid phase, the allowed motions are in vibrational movements within the molecules. In the case of water, the O-H bonds are stretching and bending. The bond lengths and angles are oscillating around the predicted values.

The amount of heat required to raise the temperature of the ice is determined by the heat capacity of ice, the heat required to change the temperature of 1 gram of ice by 1oC. The heat capacity of each phase of each substance is unique, and depends on the chemical nature of the substance. When the temperature reaches 0oC, the melting point of ice, further addition of heat does not change the temperature. At this phase transition temperature, the added energy goes to changing the Potential Energy of the system. This is the energy associated with the IMF, which are holding the H2O molecules in the solid state. It is coulombic in nature, arising from the attraction of charged species. In the case of H2O, it is the attraction between the partial positive charges on the H and the partial negative charges on the O. As we discussed earlier in the semester, these are hydrogen bonds, holding the water molecules in the crystalline structure of ice. At the phase transition temperature, 0oC, all of the ice will be converted to liquid water. As soon as the phase change is complete (s ® l), addition of heat will then lead to an increase in temperature of the liquid water. The increase in temperature is, again, an increase in the KE of the system. The movement of the water molecules will increase in the liquid phase. There is still some degree of hydrogen bonding between molecules, but they are no longer in fixed positions in a crystal lattice.
The IMF govern the motion of molecules as well. In the gaseous phase, molecules are in random and constant motion. Each gas molecule moves independently of the others. In liquids, the molecules slide past each other freely. In solids, the molecules vibrate about fixed positions.
Heating Curves
The transitions between the phases, phase changes, can be viewed in terms of a Heating Curve, like the one shown below, for water. It is a plot of time versus temperature. The time axis represents the addition of heat as a function of time. The longer the time span, the more heat has been added to the system. In this Heating Curve, we are starting with ice at -20oC.

As we add heat, we raise the temperature of the ice. The heat that we are adding is increasing the Kinetic Energy of the system (KE = ½ mu2) which is proportional to the absolute temperature (K) of the system. In the solid phase, the allowed motions are in vibrational movements within the molecules. In the case of water, the O-H bonds are stretching and bending. The bond lengths and angles are oscillating around the predicted values.

The amount of heat required to raise the temperature of the ice is determined by the heat capacity of ice, the heat required to change the temperature of 1 gram of ice by 1oC. The heat capacity of each phase of each substance is unique, and depends on the chemical nature of the substance. When the temperature reaches 0oC, the melting point of ice, further addition of heat does not change the temperature. At this phase transition temperature, the added energy goes to changing the Potential Energy of the system. This is the energy associated with the IMF, which are holding the H2O molecules in the solid state. It is coulombic in nature, arising from the attraction of charged species. In the case of H2O, it is the attraction between the partial positive charges on the H and the partial negative charges on the O. As we discussed earlier in the semester, these are hydrogen bonds, holding the water molecules in the crystalline structure of ice. At the phase transition temperature, 0oC, all of the ice will be converted to liquid water. As soon as the phase change is complete (s ® l), addition of heat will then lead to an increase in temperature of the liquid water. The increase in temperature is, again, an increase in the KE of the system. The movement of the water molecules will increase in the liquid phase. There is still some degree of hydrogen bonding between molecules, but they are no longer in fixed positions in a crystal lattice.
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SPELLING IS WRONG. ITS INTERMOLECULAR .
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