pentane and hexane intermolecular forces pentane and hexane intermolecular forces

We can first eliminate hexane and pentane as our answers, as neither are branched . The attraction between partially positive and partially negative regions of a polar molecule that makes up dipole-dipole forces is the same type of attraction that occurs between cations and anions in an ionic compound. Pentane is a non-polar molecule. Which substance(s) can form a hydrogen bond to another molecule of itself? Bodies of water would freeze from the bottom up, which would be lethal for most aquatic creatures. So let me write that down here. Consequently, N2O should have a higher boiling point. Pentane has five carbons, one, two, three, four, five, so five carbons for pentane. C5 H12 is the molecular So let me use, let me Direct link to Tombentom's post - Since H20 molecules hav, Posted 7 years ago. Legal. But that I can imagine best if the structure is rigid. Similarly, solids melt when the molecules acquire enough thermal energy to overcome the intermolecular forces that lock them into place in the solid. You'll get a detailed solution from a subject matter expert that helps you learn core concepts. This is the expected trend in nonpolar molecules, for which London dispersion forces are the exclusive intermolecular forces. force is, of course, the London dispersion forces. So C5 H12. In 1930, 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. This pageis shared under aCC BY-NC-SA 4.0licenseand was authored, remixed, and/or curated by Lance S. Lund (Anoka-Ramsey Community College) and Vicki MacMurdo(Anoka-Ramsey Community College). Whereas, if you look at pentane, pentane has a boiling Source: Dispersion Intermolecular Force, YouTube(opens in new window) [youtu.be]. In the structure of ice, each oxygen atom is surrounded by a distorted tetrahedron of hydrogen atoms that form bridges to the oxygen atoms of adjacent water molecules. Asked for: order of increasing boiling points. And so hydrogen bonding is possible. As a result, neopentane is a gas at room temperature, whereas n -pentane is a volatile liquid. In contrast to intramolecular forces, such as the covalent bonds that hold atoms together in molecules and polyatomic ions, intermolecular forces hold molecules together in a liquid or solid. Part 1Comparing Pentane and Octane This provides a simple opportunity for students to get used to some of the logistics such as choosing a liquid, using the ruler appropriately, and determining the point in the video they will measure the stretch of the liquid. Accessibility StatementFor more information contact us atinfo@libretexts.org. The molecules have enough energy already to break free of each other. The combination of large bond dipoles and short dipoledipole distances results in very strong dipoledipole interactions called hydrogen bonds, as shown for ice in Figure \(\PageIndex{6}\). The compound with the highest vapor pressure will have the weakest intermolecular forces. Transitions between the solid and liquid, or the liquid and gas phases, are due to changes in intermolecular interactions, but do not affect intramolecular interactions. And so neopentane is a gas at Obviously, there must be some other attractive force present in NH3, HF, and H2O to account for the higher boiling points in these molecules. Because a hydrogen atom is so small, these dipoles can also approach one another more closely than most other dipoles. What would be the effect on the melting and boiling points by changing the position of the functional group in a aldehyde/ketone and an alcohol? The compound with the highest vapor pressure will have the weakest intermolecular forces. And if you think about the surface area, all right, for an attraction The effect is most dramatic for water: if we extend the straight line connecting the points for H2Te and H2Se to the line for period 2, we obtain an estimated boiling point of 130C for water! So I'll just write "London" here. Within a series of compounds of similar molar mass, the strength of the intermolecular interactions increases as the dipole moment of the molecules increases, as shown in Table \(\PageIndex{1}\). Direct link to tyersome's post The wobbliness doesn't ad. In general, however, dipoledipole interactions in small polar molecules are significantly stronger than London dispersion forces, so the former predominate. London dispersion forces are due to the formation of instantaneous dipole moments in polar or nonpolar molecules as a result of short-lived fluctuations of electron charge distribution, which in turn cause the temporary formation of an induced dipole in adjacent molecules; their energy falls off as 1/r6. whereas pentane doesn't. Let's compare two molecules, And that's reflected in In . National Library of Medicine. And we know the only Direct link to Masud Smr's post Why branching of carbon c, Posted 8 years ago. Identify the most significant intermolecular force in each substance. of pentane, right? As a result, it is relatively easy to temporarily deform the electron distribution to generate an instantaneous or induced dipole. And therefore, the two What about neopentane on the right? So the same molecular formula, C5 H12. In addition, the attractive interaction between dipoles falls off much more rapidly with increasing distance than do the ionion interactions. In 1930, 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, or simply Londonforces or dispersion forces, between otherwise nonpolar substances. pretty close to 25 degrees C, think about the state If a substance is both a hydrogen donor and a hydrogen bond acceptor, draw a structure showing the hydrogen bonding. As a result, 2,2-dimethylpropane is a gas at room temperature, whereas pentane is a volatile liquid. There are two additional types of electrostatic interaction that you are already familiar with: the ionion interactions that are responsible for ionic bonding, and the iondipole interactions that occur when ionic substances dissolve in a polar substance such as water. Hexan-3-one by itself has no hydrogen bonding. And let's think about the }); dipole-dipole interaction. PageIndex: ["{12.1. In this section, we explicitly consider three kinds of intermolecular interactions, the first two of which are often described collectively as van der Waals forces. These dispersion forces are expected to become stronger as the molar mass of the compound increases. Compounds with higher molar masses and that are polar will have the highest boiling points. would take more energy for these molecules to Direct link to Mayla Singh's post What would be the effect , Posted 7 years ago. National Center for Biotechnology Information. b. nHexane contains more carbon atoms than 2,2dimethylbutane. The net effect is that the first atom causes the temporary formation of a dipole, called an induced dipole, in the second. Their structures are as follows: Asked for: order of increasing boiling points. And so this is a dipole, right? B. equationNumbers: { the higher boiling point for 3-hexanol, right? Why is this so? figure out boiling points, think about the intermolecular forces that are present between two molecules. #1}",1] We can kind of stack these Consequently, even though their molecular masses are similar to that of water, their boiling points are significantly lower than the boiling point of water, which forms four hydrogen bonds at a time. increased boiling point. Hydrogen bonds are an unusually strong version ofdipoledipole forces in which hydrogen atoms are bonded to highly electronegative atoms such asN, O,and F. In addition, the N, O, or F will typically have lone pair electrons on the atom in the Lewis structure. So neopentane has branching, Direct link to Erika Jensen's post Straight-chain alkanes ar, Posted 8 years ago. number of carbons, right? Bolling Points of Three Classes of Organic Compounds Alkane BP (*) Aldehyde MW BP (C) Corboxylic Acid BP (C) (o/mol) (o/mol) (o/mol) butane . In small atoms such as He, its two electrons are held close to the nucleus in a very small volume, and electron-electron repulsions are strong enough to prevent significant asymmetry in their distribution. Argon and N2O have very similar molar masses (40 and 44 g/mol, respectively), but N2O is polar while Ar is not. The three compounds have essentially the same molar mass (5860 g/mol), so we must look at differences in polarity to predict the strength of the intermolecular dipoledipole interactions and thus the boiling points of the compounds. Let's look at these three molecules. But dipole-dipole is a The polarizability of a substance also determines how it interacts with ions and species that possess permanent dipoles. The same setup over here on this other molecule of 3-hexanol. I found that the above relations holds good for them too but alkanes with even number of carbon atoms have higher melting point than successive alkanes with odd number of carbon atoms. higher boiling point, of 69 degrees C. Let's draw in another molecule two molecules of pentane on top of each other and This molecule cannot form hydrogen bonds to another molecule of itself sincethere are no H atoms directly bonded to N, O, or F. Themolecule is nonpolar, meaning that the only intermolecular forces present are dispersion forces. 3-Methylpentane is more symmetric than 2-methylpentane and so would form a more spherical structure than iso-hexane. And that means that there's We can still see that the boiling point increases with molar mass due to increases in the strength of the dispersion forces as we move from period 3 to period 5. 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. In addition, because the atoms involved are so small, these molecules can also approach one another more closely than most other dipoles. Macros: { Instead, each hydrogen atom is 101 pm from one oxygen and 174 pm from the other. Intermolecular forces are electrostatic in nature; that is, they arise from the interaction between positively and negatively charged species. G.Dimethyl ether has ionic intramolecular attractions. Dipoledipole interactions arise from the electrostatic interactions of the positive and negative ends of molecules with permanent dipole moments; their strength is proportional to the magnitude of the dipole moment and to 1/r3, where r is the distance between dipoles. In general, however, dipoledipoleforcesin small polar molecules are significantly stronger thandispersion forces, so the dipoledipole forces predominate. We already know there are five carbons. Pentane has the straight structure of course. Let's apply what we have learned to the boiling points ofthe covalent hydrides of elements in Groups 14-17, as shown in Figure \(\PageIndex{4}\) below. See Answer For example, it requires 927 kJ to overcome the intramolecular forces and break both OH bonds in 1 mol of water, but it takes only about 41 kJ to overcome the intermolecular attractions and convert 1 mol of liquid water to water vapor at 100C. Thus, London dispersion forces are responsible for the general trend toward higher boiling points with increased molecular mass and greater surface area in a homologous series of compounds, such as the alkanes (part (a) in Figure \(\PageIndex{4}\)). And so therefore, it Thus, the hydrogen bond attraction will be specifically between the lone pair electrons on the N, O, or F atom and the H of a neighboring molecule. If I draw in another molecule I get that hexane is longer and due to Londer dipsersion has more change to stick to eachother. And so therefore, it The n-pentane has the weaker attractions. get increased surface area and increased attractive forces. Arrange GeH4, SiCl4, SiH4, CH4, and GeCl4 in order of decreasing boiling points. Hydrogen bonding is much stronger than London dispersion forces. free of the attractions that exist between those molecules. *The dipole moment is a measure of molecular polarity. More energy means an Hydrogen bonds are the predominant intermolecular force. two molecules of pentane. ( 4 votes) Ken Kutcel 7 years ago At 9:50 between these two molecules, it's a much smaller surface area than for the two molecules [CDATA[*/ As a result, the CO bond dipoles partially reinforce one another and generate a significant dipole moment that should give a moderately high boiling point. Let's compare, let's So I'm showing the brief, the (b) Linear pentane molecules have a larger surface area and stronger intermolecular forces than spherical neopentane molecules. So we're talk about a dipole-dipole interaction. Draw the hydrogen-bonded structures. Science Chemistry Chemistry questions and answers Which intermolecular force (s) do the following pairs of molecules experience? The substance with the weakest forces will have the lowest boiling point. Why branching of carbon compounds have higher melting point than straight carbon compounds?? So it's just an approximation, but if you could imagine . Likewise, pentane (C5H12), which has nonpolar molecules, is miscible with hexane, which also has nonpolar molecules. compare a straight chain to a branched hydrocarbon. The overall order is thus as follows, with actual boiling points in parentheses: propane (42.1C) < 2-methylpropane (11.7C) < n-butane (0.5C) < n-pentane (36.1C). boiling point of your compound. And finally, we have 3-hexanol Direct link to maxime.edon's post The boiling point of ethe, Posted 8 years ago. So I can show even more attraction between these two molecules of hexane. Methane and its heavier congeners in group 14 form a series whose boiling points increase smoothly with increasing molar mass. Arrange n-butane, propane, 2-methylpropane [isobutene, (CH3)2CHCH3], and n-pentane in order of increasing boiling points. over here on the right, which also has six carbons. The n-hexane has the larger molecules and the resulting stronger dispersion forces. 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. This is because the large partial negative charge on the oxygenatom (or on a N or F atom) is concentrated in the lone pair electrons. Hydrogen Bonding. How come the hydrogen bond is the weakest of all chemical bonds but at the same time water for example has high boiling point? For example, Figure \(\PageIndex{3}\)(b) shows 2,2-dimethylpropane and pentane, both of which have the empirical formula C5H12. means it takes more energy for those molecules to strongest intermolecular force. The four compounds are alkanes and nonpolar, so London dispersion forces are the only important intermolecular forces. The intermolecular forces are also increased with pentane due to the structure. Consequently, HN, HO, and HF bonds will have very large bond dipoles, allowing the H atoms to interact strongly with thelone pairs of N, O, or F atoms on neighboring molecules. Direct link to Jaap Cramer's post I was surprised to learn , Posted 4 years ago. So once again, we've talked To log in and use all the features of Khan Academy, please enable JavaScript in your browser. This carbon here, this So on the left down here, once again we have pentane, all right, with a boiling and so does 3-hexanone. The n-hexane has the stronger attractions between its molecules. London dispersion forces. As previously described, polar moleculeshave one end that is partially positive (+)and another end thatis partiallynegative (). Example Thus a substance such as \(\ce{HCl}\), which is partially held together by dipoledipole interactions, is a gas at room temperature and 1 atm pressure. So hydrogen bonding is our Apperantly the latter is stronger, but do I make an error in my thinking? In contrast, each oxygen atom is bonded to two H atoms at the shorter distance and two at the longer distance, corresponding to two OH covalent bonds and two OH hydrogen bonds from adjacent water molecules, respectively. So I could represent the London dispersion forces like this. Let's think about electronegativity, and we'll compare this oxygen to this carbon right here. Direct link to Srk's post Basically, Polar function, Posted 6 years ago. The answer lies in the highly polar nature of the bonds between hydrogen and very electronegative elements such as O, N, and F. The large difference in electronegativity results in a large partial positive charge on hydrogen and a correspondingly large partial negative charge on the O, N, or F atom. In small atoms such as He, the two 1s electrons are held close to the nucleus in a very small volume, and electronelectron repulsions are strong enough to prevent significant asymmetry in their distribution. So we haven't reached the Doubling the distance (r 2r) decreases the attractive energy by one-half. Direct link to Ken Kutcel's post At 9:50 in the video, 3-h, Posted 6 years ago. The resulting open, cage-like structure of ice means that the solid is actually slightly less dense than the liquid, which explains why ice floats on water, rather than sinks. Each water molecule accepts two hydrogen bonds from two other water molecules and donates two hydrogen atoms to form hydrogen bonds with two more water molecules, producing an open, cagelike structure. And let me draw another Molecules with hydrogen atoms bonded to electronegative atoms such as O, N, and F (and to a much lesser extent, Cl and S) tend to exhibit unusually strong intermolecular interactions. Dispersion forces are the only intermolecular forces present. Like covalent and ionic bonds, intermolecular interactions are the sum of both attractive and repulsive components. Compare the molar masses and the polarities of the compounds. Interactions between these temporary dipoles cause atoms to be attracted to one another. A hydrogen bond is usually indicated by a dotted line between the hydrogen atom attached to O, N, or F (the hydrogen bond donor) and the atom that has the lone pair of electrons (the hydrogen bond acceptor). Pentane will have the weakest attractive forces, followed by heptane, and nonane will have the . Intermolecular forces are generally much weaker than covalent bonds. Electrostatic interactions are strongest for an ionic compound, so we expect NaCl to have the highest boiling point. Therefore, their arrangement in order of decreasing boiling point is: Which intermolecular forces are present in each substance? The presence of the stronger dipole-dipole force causes the boiling points of molecules in Groups 15-17 to be greater than the boiling point of the molecules in Group 14 in the same period. Pentane, 1-butanol and 2-butanone share an intermolecular force that is approximately the same strength for all three compounds. So partially negative oxygen, partially positive hydrogen. I was surprised to learn why it costs more energy for hexane, compared to pentane, to break free en become gas. increased attractive force holding these two molecules The properties of liquids are intermediate between those of gases and solids, but are more similar to solids. MW Question 17 (1 point) Using the table, what intermolecular force is responsible for the difference in boiling point between pentane and hexane? Thus,dispersion forces are responsible for the general trend toward higher boiling points with increased molecular mass and greater surface area in a homologous series of compounds, such as the alkanes in Figure \(\PageIndex{3}\)(a)below. Posted 8 years ago. boiling point than hexane. The boiling point of ethers is generally low, the most common ether, diethyl ether (C2H5-O-C2H5), having a bp of 35C. Because each end of a dipole possesses only a fraction of the charge of an electron, dipoledipole interactions are substantially weaker than the interactions between two ions, each of which has a charge of at least 1, or between a dipole and an ion, in which one of the species has at least a full positive or negative charge. Larger atoms tend to be more polarizable than smaller ones, because their outer electrons are less tightly bound and are therefore more easily perturbed. The effect is most dramatic for water: if we extend the straight line connecting the points for H2Te and H2Se to the line for period 2, we obtain an estimated boiling point of 70C for water! In contrast, the energy of the interaction of two dipoles is proportional to 1/r3, so doubling the distance between the dipoles decreases the strength of the interaction by 23, or 8-fold. Conversely, NaCl, which is held together by interionic interactions, is a high-melting-point solid. Conversely, \(\ce{NaCl}\), which is held together by interionic interactions, is a high-melting-point solid. The bridging hydrogen atoms are not equidistant from the two oxygen atoms they connect, however. And we know that hydrogen bonding, we know the hydrogen bonding is really just a stronger dipole-dipole interaction. So now we're talking Of the compounds that can act as hydrogen bond donors, identify those that also contain lone pairs of electrons, which allow them to be hydrogen bond acceptors. remember hydrogen bonding is simply a stronger type of dipole- dipole interaction. Legal. In larger atoms such as Xe, there are many more electrons and energy shells. Dispersion forces and dipole-dipole forces are present. Considering the structuresin Example \(\PageIndex{1}\) from left to right, the condensed structuralformulas and molar masses are: Since they all have about the same molar mass, their boiling points should decrease in the order of the strongest to weakestpredominant intermolecular force. So pentane is a liquid. As a result, the boiling point of neopentane (9.5C) is more than 25C lower than the boiling point of n-pentane (36.1C). If ice were denser than the liquid, the ice formed at the surface in cold weather would sink as fast as it formed. Source: Hydrogen Bonding Intermolecular Force, YouTube(opens in new window) [youtu.be]. } Hydrogen bonds are especially strong dipoledipole interactions between molecules that have hydrogen bonded to a highly electronegative atom, such as O, N, or F. The resulting partially positively charged H atom on one molecule (the hydrogen bond donor) can interact strongly with a lone pair of electrons of a partially negatively charged O, N, or F atom on adjacent molecules (the hydrogen bond acceptor). The resulting open, cagelike structure of ice means that the solid is actually slightly less dense than the liquid, which explains why ice floats on water, rather than sinks. 2,2-dimethylpropane is almost spherical, with a small surface area for intermolecular interactions, whereas pentane has an extended conformation that enables it to come into close contact with other pentane molecules. On average, however, the attractive interactions dominate. Hydrogen bond formation requires both a hydrogen bond donor and a hydrogen bond acceptor. This means that dispersion forcesarealso the predominant intermolecular force. So we can say for our trend here, as you increase the branching, right? Straight-chain alkanes are able to pack and layer each other better than their branched counterparts. about hydrogen bonding. )%2F12%253A_Intermolecular_Forces%253A_Liquids_And_Solids%2F12.1%253A_Intermolecular_Forces, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\). Despite having equal molecular weights, the boiling point of nhexane is higher than that of 2,2dimethylbutane. If there is more than one, identify the predominant intermolecular force in each substance. Thus far, we have considered only interactions between polar molecules. In this section, we explicitly consider three kinds of intermolecular interactions. For example, it requires 927 kJ to overcome the intramolecular forces and break both O-H bonds in 1 mol of water, but it takes only about 41 kJ to overcome the intermolecular attractions and convert 1 mol of liquid water to water vapor at 100C. Direct link to Ernest Zinck's post Dipole-dipole forces are , Posted 4 years ago. Select the reason for this. This effect, illustrated for two H2 molecules in part (b) in Figure \(\PageIndex{3}\), tends to become more pronounced as atomic and molecular masses increase (Table \(\PageIndex{2}\)). Thus a substance such as HCl, which is partially held together by dipoledipole interactions, is a gas at room temperature and 1 atm pressure. Because molecules in a liquid move freely and continuously, molecules experience both attractiveand repulsive forces while interacting with each other. Intermolecular forces determine bulk properties, such as the melting points of solids and the boiling points of liquids. Direct link to Ryan W's post Youve confused concepts , Posted 7 years ago. I agree there must be some polarization between the oxygen and the carbon in the alcohol, but I don't think it would be as strong as in the ketone. autoNumber: "all", This molecule can form hydrogen bonds to another molecule of itself since there is an H atomdirectly bonded to O in the hydroxyl group (OH). molecules here of 3-hexanone are attracted to each other more than the two molecules of hexane. The most significant intermolecular force for this substance would be dispersion forces. KBr (1435C) > 2,4-dimethylheptane (132.9C) > CS2 (46.6C) > Cl2 (34.6C) > Ne (246C). Neopentane is almost spherical, with a small surface area for intermolecular interactions, whereas n-pentane has an extended conformation that enables it to come into close contact with other n-pentane molecules. point of 36 degrees C, which is higher than room temperature. intermolecular force that exists between two non-polar molecules, that would of course be the All right? Video Discussing Dipole Intermolecular Forces. between the molecules are called the intermolecular forces. Well, there's one, two, three, four, five carbons, so five carbons, and one, two, three, four, five, six, seven, eight, nine, 10, 11 and 12 hydrogens. Intermolecular forces hold multiple molecules together and determine many of a substance's properties. Instantaneous dipoleinduced dipole interactions between nonpolar molecules can produce intermolecular attractions just as they produce interatomic attractions in monatomic substances like Xe. Video Discussing London/Dispersion Intermolecular Forces. Oxygen is more And that will allow you to figure out which compound has the And that's because dipole-dipole Dipole-dipole forces are between molecules that always have a positive end and a negative end. partially positive carbon. The reason for this is that the straight chain is less compact than the branching and increases the surface area. When comparing the structural isomers of pentane (pentane, isopentane, and neopentane), they all have the same molecular formula C 5 H 12. Same number of carbons, Direct link to Ernest Zinck's post Hexan-3-one by itself has, Posted 8 years ago. Molecules in liquids are held to other molecules by intermolecular interactions, which are weaker than the intramolecular interactions that hold the atoms together within molecules and polyatomic ions. Given the large difference in the strengths of intramolecularand intermolecular forces, changes between the solid, liquid, and gaseous states almost invariably occur for molecular substances without breaking covalent bonds. And pentane has a boiling Right? The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Hexane has six carbons, one, two, three, four, five, and six. Next, let's look at 3-hexanone, right? Direct link to jeej91's post How come the hydrogen bon, Posted 5 years ago. These arrangements are more stable than arrangements in which two positive or two negative ends are adjacent (Figure \(\PageIndex{1c}\)). So the two molecules of hexane attract each other more than the two molecules of pentane. Compounds such as HF can form only two hydrogen bonds at a time as can, on average, pure liquid NH3. This allows greater intermolecular forces, which raises the melting point since it will take more energy to disperse the molecules into a liquid. To describe the intermolecular forces in molecules.