[Hadjiev]

Hey, I am posting to help clear up one of my chemistry assignment questions. Here is the question:

How do the intermolecular forces account for the different rates of cooling observed for the three liquids? - we did not record the rate of cooling in the lab, so she just wants to know why some cooled less or more than others.

The three liquids are: water, 2-propanol, and ethoxyethane.



My findings are:

Water --> Initial Temp: 24*        Final Temp: 14*                                            Thus cooled the least

2-propanol --> Initial Temp: 24*         Final Temp: 10*                                  Cooled more than water but less than                                                                                                                                  ethoxyethane

Ethoxyethane --> Initial Temp: 24*        Final Temp: -8*                                Cooled the most



Here is my take on the question:

The weaker the intermolecular forces, the less energy (heat energy) required to break the intermolecular bonds, and vice versa. This means a non-polar molecule, such as ethoxyethane, will require less heat energy to evaporate, than say a highly polar molecule, such as water. Molecules will have different cooling rates because each molecule differs in intermolecular bond strength. Some molecules require more heat energy than other to break the intermolecular forces. Logically, the less heat energy required, the cooler the liquid will be during evaporation, and vice versa. For example: between water, 2-propanol, and ethoxyethane, water has the strongest intermolecular forces (less LDF than 2-propanol), and thus requires the most heat energy to break the intermolecular bonds. This additional heat energy will cause water to cool the least while evaporating.

My take on the question doesn't explain why the temperature decreases instead of increases, since I say that the heat energy required to break the bonds will actually add to the overall temperature of the liquid. I assumed that since bonds were breaking, energy was released, which accounts for the loss of temp while evaporating. I am basing my answer on the notion that the heat energy required to break the bonds (to evaporate the liquids) will affect the cooling rate of the liquid.

This is the only logical reason that came to mind. If there is some other explanation I would be glad to hear it. However, keep in mind this is 12U chem (grade 12), so I cannot use a university explanation.

Thanks in advance:
Nick


PS: if molecules hydrogen bond, do they also have dipole-dipole bonds?

Thanks again!

[Mitch]

PS: if molecules hydrogen bond, do they also have dipole-dipole bonds?

Yes

[Donaldson Tan]

My take on the question doesn't explain why the temperature decreases instead of increases, since I say that the heat energy required to break the bonds will actually add to the overall temperature of the liquid. I assumed that since bonds were breaking, energy was released, which accounts for the loss of temp while evaporating. I am basing my answer on the notion that the heat energy required to break the bonds (to evaporate the liquids) will affect the cooling rate of the liquid.

these substances have different heat capacity. Assuming we are dealing with same mass of the 3 substances, water has the highest heat capacity, ie. in order for water to drop its temperature by 1 celcius, much more heat must be lost, compared to the cases for the organic compounds.

I can't see how the mechanism of evaporation explains the different cooling rate. It is along that line of arguement (on intermolecular bonding), but it is still not about evaporation. When we consider a substance cooling down, we understand that the internal energy (U) of the subtance decreases.

What constitutes the internal energy? The kinetic energy (rotational and translational) of each molecule, the vibrations and rotations of chemical bonds, and intermolecular bonding pretty much makes the internal energy.

How does the structure of each substance affect the various components of the internal energy? Heat capacity (at constant volume) is given as dU/dT, ie. the partial derrivative of the internal energy as a function of temperature.