CHM1046: Chapter 10.2 Homework - Properties of Liquids

At 25 °C, how high will water rise in a glass capillary tube with an inner diameter of 0.63 mm? Refer to Example 10.4 for the required information.

=0.047m=4.7cm

It is often recommended that you let your car engine run idle to warm up before driving, especially on cold winter days. While the benefit of prolonged idling is dubious, it is certainly true that a warm engine is more fuel efficient than a cold one. Explain the reason for this.

A warm engine's oil is less viscous, and so it flows more freely and thus more effectively lubricates the engines internal parts (primarily its pistons). This permits the engine to run with less friction and, subsequently, more fuel efficiently.

As temperature increases, what happens to the surface tension of water? Explain why this occurs, in terms of molecular interactions and the effect of changing temperature. (Camera Roll pic)

As the water reaches higher temperatures, the increased kinetic energies of its molecules are more effective in overcoming hydrogen bonding, and so its surface tension decreases. Surface tension and intermolecular forces are directly related.

You may have heard someone use the figure of speech "slower than molasses in winter" to describe a process that occurs slowly. Explain why this is an apt idiom, using concepts of molecular size and shape, molecular interactions, and the effect of changing temperature.

Temperature has an effect on intermolecular forces: The higher the temperature, the greater the kinetic energies of the molecules and the greater the extent to which their intermolecular forces are overcome, and so the more fluid (less viscous) the liquid. The lower the temperature, the less the intermolecular forces are overcome, and so the less viscous the liquid.

Explain their differences in surface tension in terms of the size and shape of their molecules and their IMFs: (Camera Roll Pic)

The higher the surface tension, the stronger the intermolecular forces are. Ethylene glycol has a higher surface tension than ethanol, since it can hydrogen bond more with its two -OH groups per molecule. Surprisingly, the surface tension of ethanol and acetone are roughly the same. The acetone molecules are planar, and so are able to orient themselves in a planar manner so as to create a tight skin at the surface to match the hydrogen bonding of ethanol. Ether is nonplanar, and it displays a weaker dipole and so its surface tension is lowest.

Explain their differences in viscosity in terms of the size and shape of their molecules and their IMFs. (Camera roll pic)

The higher the viscosity, the stronger the intermolecular forces are. Ethylene glycol and ethanol both exhibit hydrogen bonding, but ethylene glycol molecules contain twice as many -OH groups as ethanol molecules, and so its viscosity is higher. Acetone has a lower viscosity than ethanol since its dipole-dipole force is weaker than hydrogen bonding. Ether has a lower viscosity than acetone, since it is less polar than acetone.

As temperature increases, what happens to the viscosity of water? Explain why this occurs, in terms of molecular interactions and the effect of changing temperature. (Camera roll pic)

The same trend in viscosity is seen as in surface tension, and for the same reason.

Although steel is denser than water, a steel needle or paper clip placed carefully lengthwise on the surface of still water can be made to float. Explain at a molecular level how this is possible.

The water molecules have strong intermolecular forces of hydrogen bonding. The water molecules are thus attracted strongly to one another and exhibit a relatively large surface tension, forming a type of "skin" at its surface. This skin can support a bug or paper clip if gently placed on the water.

Water rises in a glass capillary tube to a height of 17 cm. What is the diameter of the capillary tube?

r= 8.6 x10^-5 m d= 1.7x 10^-4 m