Biochemistry Study Set 13

Biology

Quiz 1 :

Water

Quiz 1 :

Water

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Gas-phase water (i.e., water vapor) has properties similar to most other gases, yet condensed water (i.e., liquid and solid water) is unusual. a. Why are the properties the same in the gas phase? b. What are the unusual phase transitions of water? c. What accounts for the unusual properties of water in general?
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(a)The water molecules in the gaseous state show the same properties as that of other gases. This is because under standard temperature and pressure set up by IUPAC (International Union of Pure and Applied Chemists), all the gases tends to have same number of molecules.
It was observed by the IUPAC scientists that when gases are subjected to similar physical conditions of temperature and pressure, they tend to arrange freely with negligible force of attraction. When gases attain such state, all of them were found to have same number of molecules in the given volume. Thus, gas-phase of water has same properties as that of other gases.
(b)We have observed that any substance exists maximum only in two phases that can be; liquid and solid (like metals), liquid and gas (like acids) or solid and gas (like camphor). There is only one substance that is water which is versatile in its nature as it can exist in all the three forms.
The water is found in the form of gaseous state as vapours, in liquid state as liquid water and in solid state as ice. This is possible due to the structure of water molecule. The two atoms of hydrogen and one of oxygen arrange themselves in such a way in three dimensions that it can be found in all the three states of matter. Thus, the unusual phase transitions of water are solid, liquid and gas.
In the solid state water molecules are at very low temperature that tends to compress them close to each other forming ice cube. In liquid state, the water molecules are under standard temperature and pressure and are free to move but attached to each other by hydrogen bonds. In the gas phase, the water molecules are under high temperature that allows them to absorb energy and get free form forces of attraction and move freely.
(c)
We have already discussed above the unusual phase transitions of water. The reason for this unusual behavior is the physical conditions and the three dimensional structure of individual molecule. In the solid state water molecules are at very low temperature that tends to compress them close to each other forming ice cube.
In liquid state, the water molecules are under standard temperature and pressure and are free to move but attached to each other by hydrogen bonds. In the gas phase, the water molecules are under high temperature that allows them to absorb energy and get free form forces of attraction and move freely.

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In a solution of pure water, it is possible to calculate the concentration of water itself. What is its value?
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The water molecule consists of two hydrogen atom and one oxygen atom arranged as follows:
img In pure water, the water molecules tend to remain in equilibrium between its stable and protonated state. This can be understood as follows:
img Thus, it can be said that one water molecule at present carries proton and exists as hydronium ion or in the protonated (H ₃ O + ) state while others would be in the stable (H ₂ O) state. This proton is transported from one molecule of water to another at a very high rate as compared to diffusion.
Thus, it can be said that above 90 % of water concentration can be found as water (H ₂ O).

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Electronegativity values are assigned to individual elements, and yet the concept of electronegativity applies only to bonds. Explain this disparity.
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Electronegativity means the capacity of the atom to attract electrons and enter them into their valence shells. This property depends upon the size of atom. If the atom's size is small them its valence (outermost) shell will be close to the nucleus and thus, nucleus can easily pull the incoming electron/s.
This property is carried out by atoms to attain stable electronic configuration. If the atoms fall short of two electrons they will gain from other atoms which have two extra electrons. In this way, if some atom is there to collect electron/s then there must be the one who can donate.
Thus, the concept of electronegativity applies only to bonds as there are two atoms always. It is not possible that there is an atom who can release electrons but there is no one to take it up or vice-versa. Thus, an atom alone is not capable of taking up electrons instead there should also be the one to donate. So, it applies only to bonds.

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Oxygen and sulfur are both nucleophiles, they have the same valence, and both form dihydrides. Yet, H ₂ S does not form hydrogen bonds to other molecules of H ₂ S; indeed, sulfur does not participate in hydrogen bonding at all. Why is that?
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Oxygen and nitrogen appear to be very different. Whereas oxygen appears in acidic compounds and tends to assume a negative charge, nitrogen appears in basic compounds and tends to assume a positive charge. Both elements, however, participate in hydrogen bonding. Explain these facts.
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Why is it that a molecule like CO ₂ , which contains polar bonds but is not polar overall, cannot attract polar molecules?
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Ammonia is a polar molecule by virtue of its lone pair. When protonated to ammonium, however, it is symmetrical and would seem, by vector addition, to have no polarity. Yet, the ammonium ion readily dissolves in water. Why?
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The hydrophobic effect is due to attractive forces only and not repulsive ones. Rationalize this with the term hydrophobic.
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A rule of thumb in chemistry is "like dissolves like." Explain how this applies differently to polar and nonpolar substances.
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When enough NaCl is added to water, the salt no longer dissolves and solid NaCl falls out of solution. At this point, the solution is said to be saturated. Explain this phenomenon.
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Water is a temperature buffer, which means that it resists changes in temperature more than other liquids. Explain this based on the structure of water.
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Water ionizes to a very small extent, although this can be increased with increasing temperature. Provide a reasonable explanation.
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Protons produced by the self-ionization of water have an unusually rapid mobility in solution; they move faster than can be accounted for by diffusion. Provide a possible mechanism to account for this proton jumping.
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The definition of an acid as an entity that increases the proton concentration in water (known as the Arrhenius definition) has the advantage that we can call CO ₂ an acid. How could we handle this situation if instead we used the Brönsted (proton donor) definition?
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While useful, the pH scale can also be confusing because it is a double transform: both the negative of the number and the log of the number are taken. How does this affect your intuition about a change in proton concentration?
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