4 super tough chemistry questions:
1.) Water has a considerably higher boiling point and lower vapor pressure than other molecules of similar or larger molar mass. For example, dinitrogen (N2), methane (CH4), etc. all have lower boiling points and higher vapor pressures than water at the same temperature. How might this be explained?
2.) If the intermolecular forces between molecules of a substance were very small, what effect would you expect this to have on the boiling point and vapor pressure of the substance? What properties would you look for in a molecule in order to have a low boiling point and high vapor pressure? Should the atoms of the molecules be large or small? Why might this matter? Give two or three examples of molecules where you might expect the intermolecular forces to be very small.
3.) Why would contact with steam at 100 C produce a more severe burn than contact with liquid water at the same temperature?
4.) Orange growers often spray water on their trees to protect the fruit in freezing weather. Explain how the energy of the water --> ice phase transition could provide protection from freezing weather.
We will be happy to critique your thinking.
1, 2, and 3 I have no answers. I've been staring at my monitor for 47 minutes now and I'm not coming up with any answer.
As for no. 4, I think it's because water protects/shields the fruit.
Actually, I think my answer for no. 3 is because steam has heat which is why it will produce a more severe burn than just plain water.
#4. The "protection" afforded by the ice is not so much from a shielding effect (I think a common misconception) as it is from the heat. Look at it this way,
ice + heat ==> liquid water.
So melting ice is an endothermic reaction; that is, you put heat in to melt the ice. So the reverse (turn the arrow around) must be an exothermic reaction; that is, heat must be given off.
The steam question. Burns by steam (at 100 C) are worse than burns by water (at 100 C) because, in addition to the burns of the water, there is an excess of 2260 joules for every grams of steam that turns into water when it condenses. So that much more energy is emitted.
1.) The higher boiling point and lower vapor pressure of water compared to other molecules of similar or larger molar mass can be explained by the presence of hydrogen bonding in water. Water molecules can form hydrogen bonds due to the partial positive charge on hydrogen atoms and the partial negative charge on oxygen atoms. These hydrogen bonds are stronger than the intermolecular forces in other molecules, such as London dispersion forces or dipole-dipole interactions, which are responsible for the lower boiling points and higher vapor pressures in molecules like dinitrogen and methane. The presence of hydrogen bonding in water requires more energy to break these bonds and convert water from a liquid to a gas, resulting in a higher boiling point and lower vapor pressure.
2.) If the intermolecular forces between molecules of a substance are very small, it would lead to a low boiling point and high vapor pressure. Intermolecular forces play a crucial role in determining the stability of the substance and its tendency to evaporate. To have a low boiling point and high vapor pressure, you would look for molecules with weak intermolecular forces like London dispersion forces or dipole-dipole interactions. These forces are weaker when the molecules involved are small, allowing for easier separation and vaporization. Examples of molecules with weak intermolecular forces where you might expect low boiling points and high vapor pressures are helium (He), methane (CH4), and ethane (C2H6).
3.) Steam at 100°C would produce a more severe burn than liquid water at the same temperature because steam contains significantly more amount of heat energy due to the latent heat of vaporization. When liquid water turns into steam, it undergoes a phase transition where additional heat energy is absorbed without increasing the temperature. This energy is stored as the potential energy of the vapor molecules, making them hotter than liquid water at the same temperature. When this hot steam comes into contact with the skin, it transfers a larger amount of heat energy to the skin, causing more severe burns compared to liquid water at the same temperature.
4.) The energy involved in the water → ice phase transition can provide protection from freezing weather for orange growers. When water freezes, it releases a considerable amount of heat energy known as the heat of fusion or latent heat of fusion. This energy is released as water molecules transition from a liquid to a solid state. By spraying water on the trees, the released heat of fusion helps to keep the temperature around the oranges above the freezing point. The energy released during freezing acts as a source of heat, preventing the oranges from reaching freezing temperatures and potentially protecting them from damage associated with freezing weather.