A. In liquid water, water molecules are continually forming and breaking hydroge

Question

A. In liquid water, water molecules are continually forming and breaking hydrogen bonds. In water, a hydrogen bond has a bond energy of about 0.2 eV. Two water molecules are shown forming a hydrogen bond (dashed line) in the figure at the right. Is the formation of the hydrogen bond exothermic (releases energy, reducing the internal energy of the molecules) or endothermic (absorbs energy, increasing the internal energy of the molecule)?
---Select---

Exothermic;

Endothermic;

Neither ;

There is not enough information to decide Explain briefly.

B. Let's see whether hydrogen bonds can account for the energy needed to boil water. Assume the model that when water boils, the energy that has to be put in is the energy needed to break the hydrogen bonds. It takes about 2.3 x 106 J to boil 1 kg of water.

C. First, figure out how many water molecules there are in 1 kg of water. (Hint: The atomic mass of water is 18D and Avogadro's number is about 6 x 1023 molecules/mole.)
Number of molecules =

D. Do you expect that there would be more hydrogen bonds than water molecules or fewer? Explain your reasoning.

E. Next, figure out in our model of boiling, that is, if boiling is breaking hydrogen bonds, how many hydrogen bonds are broken in order to boil 1 kg of water?
Number of bonds =  

Explain your reasoning.

F. Does your calculation support our simple model or not? Why?

A. In liquid water, water molecules are continually forming and breaking hydrogen bonds. In water, a hydrogen bond has a bond energy of about 0.2 eV. Two water molecules are shown forming a hydrogen bond (dashed line) in the figure at the right. Is the formation of the hydrogen bond exothermic (releases energy, reducing the internal energy of the molecules) or endothermic (absorbs energy, increasing the internal energy of the molecule)?
---Select---

Exothermic;

Endothermic;

Neither ;

There is not enough information to decide Explain briefly.

B. Let's see whether hydrogen bonds can account for the energy needed to boil water. Assume the model that when water boils, the energy that has to be put in is the energy needed to break the hydrogen bonds. It takes about 2.3 x 106 J to boil 1 kg of water.

C. First, figure out how many water molecules there are in 1 kg of water. (Hint: The atomic mass of water is 18D and Avogadro's number is about 6 x 1023 molecules/mole.)
Number of molecules =

Explanation / Answer

A) Exothermic. When a bond forms, energy has to be put in to break it, therefore the energy of the bonded system is lower than the energy of the separated system. When the bond is formed, energy would be released in order to conserve total energy. This released energy could be thermal (kinetic energy of molecules), or a photon could be emitted.

B) It has no question. The following questions are based on this.

C) Since 1 kg of water has 1000 g, that corresponds to 1000/18 moles. That would have (10^3/18) x (6 x 10^23) = 3 x 10^25 molecules of water.

D) You can't really tell, but since water is pretty cohesive, I expect each molecule has at least one hydrogen bond to another water molecule. Since two is the maximum each can have, we would expect somewhere between 1 and 2 bonds per molecule.

E) To boil 1 kg of water we need 2.3 x 10^6J. Each hydrogen bond requires an input of 0.2 eV to break it. Converting that to Joules we get (0.2 eV)x (1.6 x 10^-19J / 1 eV) = 0.32 x 10^-19J per hydrogen bond. So if all our energy of boiling is breaking hydrogen bonds, we must be breaking

N = (2.3 x 10^6J) / (0.32 x 10^-19J/bond) ~ 7 x 10^25bonds.

F) Its not 100% convincing, but it is satisfying that the number we get is pretty close to 2 bonds per molecule. This suggests that the model is not totally unreasonable.

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