The following quote about ammonia \(\left(\mathrm{NH}_{3}\right)\) is from a textbook of inorganic chemistry: "It is estimated that \(26 \%\) of the hydrogen bonding in \(\mathrm{NH}_{3}\) breaks down on melting, \(7 \%\) on warming from the melting to the boiling point, and the final \(67 \%\) on transfer to the gas phase at the boiling point." From the standpoint of the kinetic energy of the molecules, explain (a) why there is a decrease of hydrogen-bonding energy on melting and (b) why most of the loss in hydrogen bonding occurs in the transition from the liquid to the vapor state.

Short Answer

Expert verified
In short, the decrease in hydrogen bonding during phase transitions in ammonia is due to the increase in kinetic energy of the molecules. (a) During melting, increased molecular motion reduces the effectiveness of hydrogen bonding, leading to a 26% breakdown. (b) During the liquid-to-gas transition, the drastic increase in kinetic energy causes molecules to move more rapidly and spread out, overcoming the attractive hydrogen bonding interactions, resulting in a 67% loss of hydrogen bonding.

Step by step solution

01

Understanding hydrogen bonding in NH3

Ammonia (NH3) has strong hydrogen bonding due to the electronegativity of the nitrogen atom, which creates a partial positive charge on the hydrogen atoms and a partial negative charge on the nitrogen atom. These charges attract each other, leading to the formation of hydrogen bonds between adjacent NH3 molecules.
02

Change in hydrogen bonding during melting

(a) As the temperature increases and NH3 reaches its melting point, the kinetic energy of the molecules also increases. This increased molecular motion reduces the effectiveness of the hydrogen bonding forces between the molecules because they have less time to "stick" together due to hydrogen bonding. As a result, 26% of hydrogen bonding in ammonia breaks down during melting.
03

Change in hydrogen bonding during boiling

(b) When NH3 is heated further from melting to boiling point, the kinetic energy of the molecules continues to increase. This slight increase in heat and molecular motion only cause a 7% reduction in hydrogen bonding. However, when it reaches the boiling point and transitions from liquid to gas, the majority (67%) of the hydrogen bonding is lost.
04

Role of kinetic energy

The reason this dramatic loss of hydrogen bonding occurs during the liquid to vapor transition is because of the drastic increase in kinetic energy. The high kinetic energy causes the molecules to move rapidly and spread out, overcoming the attractive hydrogen bonding interactions. In the gas phase, molecules have enough kinetic energy to break free from the hydrogen bonding forces that held them together in the liquid phase. In conclusion, the decrease in hydrogen bonding during phase transitions in ammonia can be explained by the increase in the kinetic energy of the molecules. As their kinetic energy increases, the molecules move more rapidly, making hydrogen bonding less effective and eventually leading to the majority of the hydrogen bonds breaking during the liquid-to-gas phase transition.

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Most popular questions from this chapter

The following data present the temperatures at which certain vapor pressures are achieved for dichloromethane \(\left(\mathrm{CH}_{2} \mathrm{Cl}_{2}\right)\) and methyl iodide \(\left(\mathrm{CH}_{3} \mathrm{I}\right)\) : $$ \begin{array}{lllll} \text { Vapor Pressure } & & & & \\ \text { (torr): } & 10.0 & 40.0 & 100.0 & 400.0 \\ \hline T \text { for } \mathrm{CH}_{2} \mathrm{Cl}_{2}\left({ }^{\circ} \mathrm{C}\right): & -43.3 & -22.3 & -6.3 & 24.1 \\ T \text { for } \mathrm{CH}_{3} \mathrm{I}\left({ }^{\circ} \mathrm{C}\right): & -45.8 & -24.2 & -7.0 & 25.3 \end{array} $$ (a) Which of the two substances is expected to have the greater dipole-dipole forces? Which is expected to have the greater dispersion forces? Based on your answers, explain why it is difficult to predict which compound would be more volatile. (b) Which compound would you expect to have the higher boiling point? Check your answer in a reference book such as the CRC Handbook of Chemistry and Physics. (c) The order of volatility of these two substances changes as the temperature is increased. What quantity must be different for the two substances in order for this phenomenon to occur? (d) Substantiate your answer for part (c) by drawing an appropriate graph.

The fluorocarbon compound \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{~F}_{3}\) has a normal boiling point of \(47.6^{\circ} \mathrm{C}\). The specific heats of \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{~F}_{3}(l)\) and \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{~F}_{3}(g)\) are \(0.91 \mathrm{~J} / \mathrm{g}-\mathrm{K}\) and \(0.67 \mathrm{~J} / \mathrm{g}-\mathrm{K},\) respectively. The heat of vaporization for the compound is \(27.49 \mathrm{~kJ} / \mathrm{mol} .\) Calculate the heat required to convert \(35.0 \mathrm{~g}\) of \(\mathrm{C}_{2} \mathrm{Cl}_{3} \mathrm{~F}_{3}\) from a liquid at \(10.00^{\circ} \mathrm{C}\) to a gas at \(105.00^{\circ} \mathrm{C}\).

Butane and 2 -methylpropane, whose space-filling models are shown at the top of the next column, are both nonpolar and have the same molecular formula, \(\mathrm{C}_{4} \mathrm{H}_{10}\), yet butane has the higher boiling point \(\left(-0.5^{\circ} \mathrm{C}\right.\) compared to \(\left.-11.7^{\circ} \mathrm{C}\right)\). Explain.

As the intermolecular attractive forces between molecules increase in magnitude, do you expect each of the following to increase or decrease in magnitude? (a) vapor pressure, (b) heat of vaporization, (c) boiling point, (d) freezing point, (e) viscosity, (f) surface tension, (g) critical temperature.

Liquid butane, \(\mathrm{C}_{4} \mathrm{H}_{10}\), is stored in cylinders to be used as a fuel. The normal boiling point of butane is listed as \(-0.5^{\circ} \mathrm{C}\). (a) Suppose the tank is standing in the sun and reaches a temperature of \(35^{\circ} \mathrm{C}\). Would you expect the pressure in the tank to be greater or less than atmospheric pressure? How does the pressure within the tank depend on how much liquid butane is in it? (b) Suppose the valve to the tank is opened and a few liters of butane are allowed to escape rapidly. What do you expect would happen to the temperature of the remaining liquid butane in the tank? Explain. (c) How much heat must be added to vaporize \(250 \mathrm{~g}\) of butane if its heat of vaporization is \(21.3 \mathrm{~kJ} / \mathrm{mol} ?\) What volume does this much butane occupy at 755 torr and \(35^{\circ} \mathrm{C} ?\)

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