Lavelle mentioned something about hydrogen bonds breaking and forming again? Can someone explain what exactly is happening during the phase change from liquid to vapor? Why does water require so much heat to vaporize? Re: Boiling water Post by Thomas Gimeno » Mon Feb 01, am liquid water is held together by hydrogen bonds and they are the reason water can stay liquid at relatively high temperatures. Similar molecules that cannot form hydrogen bonds tend to have boiling points much lower than that of water.
Overall, when vaporizing water, you have to break those hydrogen bonds because gas particles are significantly father away from each other than liquid particles.
This process takes a lot of energy. A weak force of attraction forms between the partially negative oxygen of one water molecular, with the partially positive hydrogen of another molecule, forming a slightly stronger bond called a hydrogen bond.
This weak intermolecular force is what keeps solid ice held together, and keeps liquid from vaporizing. When enough heat is applied, these bonds are broken, forming water vapor.
Hope this helps! Re: Boiling water Post by Brennan McGurrr 3C » Mon Feb 01, pm Water can form a hydrogen bond using every atom in the molecule, making water have a large number of hydrogen bonds compared to other molecules. In order for water to be in the vapor state, the molecules need to be moving fast enough so the attractive forces of the hydrogen bonds cannot condense the water into a liquid. This requires a lot of energy to do.
Given the large difference in the strengths of intra- and intermolecular forces, changes between the solid, liquid, and gaseous states almost invariably occur for molecular substances without breaking covalent bonds. Intermolecular forces determine bulk properties, such as the melting points of solids and the boiling points of liquids. Liquids boil when the molecules have enough thermal energy to overcome the intermolecular attractive forces that hold them together, thereby forming bubbles of vapor within the liquid.
Similarly, solids melt when the molecules acquire enough thermal energy to overcome the intermolecular forces that lock them into place in the solid. Intermolecular forces are electrostatic in nature; that is, they arise from the interaction between positively and negatively charged species.
Like covalent and ionic bonds, intermolecular interactions are the sum of both attractive and repulsive components. Because electrostatic interactions fall off rapidly with increasing distance between molecules, intermolecular interactions are most important for solids and liquids, where the molecules are close together.
These interactions become important for gases only at very high pressures, where they are responsible for the observed deviations from the ideal gas law at high pressures. In this section, we explicitly consider three kinds of intermolecular interactions. There are two additional types of electrostatic interaction that you are already familiar with: the ion—ion interactions that are responsible for ionic bonding, and the ion—dipole interactions that occur when ionic substances dissolve in a polar substance such as water.
The first two are often described collectively as van der Waals forces. Polar covalent bonds behave as if the bonded atoms have localized fractional charges that are equal but opposite i.
If the structure of a molecule is such that the individual bond dipoles do not cancel one another, then the molecule has a net dipole moment. On average, however, the attractive interactions dominate.
In addition, the attractive interaction between dipoles falls off much more rapidly with increasing distance than do the ion—ion interactions. Video Discussing Dipole Intermolecular Forces. Their structures are as follows:. Asked for: order of increasing boiling points.
Compare the molar masses and the polarities of the compounds. Compounds with higher molar masses and that are polar will have the highest boiling points.
The first compound, 2-methylpropane, contains only C—H bonds, which are not very polar because C and H have similar electronegativities. It should therefore have a very small but nonzero dipole moment and a very low boiling point. As a result, the C—O bond dipoles partially reinforce one another and generate a significant dipole moment that should give a moderately high boiling point. The C—O bond dipole therefore corresponds to the molecular dipole, which should result in both a rather large dipole moment and a high boiling point.
Thus far, we have considered only interactions between polar molecules. Other factors must be considered to explain why many nonpolar molecules, such as bromine, benzene, and hexane, are liquids at room temperature; why others, such as iodine and naphthalene, are solids. What kind of attractive forces can exist between nonpolar molecules or atoms? This question was answered by Fritz London — , a German physicist who later worked in the United States.
In , London proposed that temporary fluctuations in the electron distributions within atoms and nonpolar molecules could result in the formation of short-lived instantaneous dipole moments , which produce attractive forces called London dispersion forces between otherwise nonpolar substances.
Consider a pair of adjacent He atoms, for example. On average, the two electrons in each He atom are uniformly distributed around the nucleus. Because the electrons are in constant motion, however, their distribution in one atom is likely to be asymmetrical at any given instant, resulting in an instantaneous dipole moment. The higher normal boiling point of HCl K compared to F 2 85 K is a reflection of the greater strength of dipole—dipole attractions between HCl molecules, compared to the attractions between nonpolar F 2 molecules.
A special type of dipole—dipole force—hydrogen bonds—have a pronounced effect on the properties of condensed phases liquids and solids. On progressing down the groups, the polarities of the molecules decrease slightly, whereas the sizes of the molecules increase substantially. The effect of increasingly stronger dispersion forces dominates that of increasingly weaker dipole—dipole attractions, and the boiling points are observed to increase steadily.
Liquids that can be homogeneously mixed in any proportion are said to be miscible. Miscible liquids have similar polarities. On mixing, methanol and water will interact through intermolecular hydrogen bonds and mix; thus, they are miscible. Likewise, nonpolar liquids like hexane C 6 H 14 and bromine Br 2 are miscible with each other through dispersion forces.
Two liquids that do not mix to an appreciable extent are called immiscible. For example, nonpolar hexane is immiscible in polar water. Relatively weak attractive forces between the hexane and water do not adequately overcome the stronger hydrogen bonding forces between water molecules. This text is adapted from Openstax, Chemistry 2e, Section To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove.
Your access has now expired. Provide feedback to your librarian. If you have any questions, please do not hesitate to reach out to our customer success team. Login processing Chapter Liquids, Solids, and Intermolecular Forces. Chapter 1: Introduction: Matter and Measurement.
Chapter 2: Atoms and Elements. Chapter 3: Molecules, Compounds, and Chemical Equations. Chapter 4: Chemical Quantities and Aqueous Reactions. Chapter 5: Gases. Chapter 6: Thermochemistry. Chapter 7: Electronic Structure of Atoms. Chapter 8: Periodic Properties of the Elements.
Chapter 9: Chemical Bonding: Basic Concepts. In order to do this, the intermolecular forces present in the liquid state must be overcome. Stronger intermolecular forces will require more energy to be overcome. A higher boiling point means more energy is required to overcome the intermolecular forces present in the liquid state. As a substance melts, some of the intermolecular forces present in the solid state are overcome.
More energy is required to overcome stronger intermolecular forces. A higher melting point means more energy is required to overcome some of intermolecular forces present in the solid state.
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