Intermolecular Forces

Size: px

Start display at page:

Download "Intermolecular Forces"

Branden Lyons
5 years ago
Views:

1 AP * Chemistry Intermolecular Forces René McCormick *AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this product by René McCormick. All rights reserved.

2 Net Ionic Equation Practice Site»

Solids, Liquids and IMF s These terms may give a clue you should consider solids/liquids/imfs in your

solids; metallic solids; sea of electrons; delocalized electrons; triple point, critical

)( q ) Coulomb s Law: F = justifies property differences in ionic compounds 2 d Key Concepts All

The difference is what occupies the points in the lattice and the strength of the forces holding them in

properties Metal cations (Fe 2+, Cu +, Al 3+, etc.

3 Solids, Liquids and IMF s These terms may give a clue you should consider solids/liquids/imfs in your answer: intermolecular forces; boiling points; vapor pressure; melting points; network solid; crystalline solids; metallic solids; sea of electrons; delocalized electrons; triple point, critical point/temp/press; sublimation; deposition; condensation; boiling; melting; freezing; Key Formulas + ( q )( q ) Coulomb s Law: F = justifies property differences in ionic compounds 2 d Key Concepts All crystalline solids have a lattice framework. The difference is what occupies the points in the lattice and the strength of the forces holding them in that lattice: Neutral molecules (CO 2, H 2 O, sucrose, etc) IMFs hold molecular solid Charged ions (Na + + Cl, Ca 2+ + SO 4 2, etc.) ion-ion attraction (ionic bond) ionic solid/ a salt use Coulomb s law to compare 2 ionic solids properties Metal cations (Fe 2+, Cu +, Al 3+, etc.) delocalized electrons or sea of electrons hold metallic solid C or Si atoms (diamond, graphite, silicon) covalent bonds hold network solid AP is a registered trademark of the College Board. The College Board was not involved in the production of and does not endorse this product by René McCormick. All rights reserved. Page 1

Solids, Liquids and IMF s Intermolecular Forces (IMF s) are frequently where the elaboration point is earned. Think of IMF s as the stickyness factor.

But only between molecules containing highly E n atoms such as N, O, and F. [Always a good answer choice if question involves water.] Dipole-dipole also strong. Both molecules must be polar.

4 Solids, Liquids and IMF s Intermolecular Forces (IMF s) are frequently where the elaboration point is earned. Think of IMF s as the stickyness factor. Always discuss energy and relate it to the IMF involved in your discussion. Hydrogen bonding is strongest IMF. It s a special case of dipole-dipole. But only between molecules containing highly E n atoms such as N, O, and F. [Always a good answer choice if question involves water.] Dipole-dipole also strong. Both molecules must be polar. Dipole-induced dipole next strongest. A polar molecule induces a temporary dipole in a nonpolar molecule. London Dispersion Forces (induced dipoleinduced dipole) can vary from weak to strong depending on number of electrons in the molecule. The key phrase here is the more electrons in the molecule, the more polarizable its electron cloud, and the stronger the LDF forces Melting points, vapor pressure, and boiling points are all very closely tied to the strength of the inter-particle forces. Any of these processes will require that particles overcome whatever stickiness force is holding them together. Comparison of two molecular compounds will be a function of their IMFs. [See above] Comparison of two ionic salts will be a function of Coulomb s law and will depend on the charge (q = 1+, 2+, etc) of the ions and the radius of the ions (d). F lattice + ( q )( q ) = 2 d 2 Page 2

5 Need a cheat sheet? Page 3

Solids, Liquids and IMF s Figures and Graphs

along each line and all three at triple point.

s/l line this is because ice is less dense than

Know all the changing of state terms and how to

This shows WHY increased temperature increases

The Clausius-Clapeyron equation makes a line of

The slope is H vap - R Heating AND cooling curve

slanted lines q=mδh vap or fus on the flat lines.

6 Solids, Liquids and IMF s Figures and Graphs Phase diagrams remember equilibrium of processes along each line and all three at triple point. Water is a freak and has a negative slope to the s/l line this is because ice is less dense than water. Know all the changing of state terms and how to find the normal BP and FP. This shows WHY increased temperature increases the vapor pressure. The Clausius-Clapeyron equation makes a line of T and VP data by plotting lnvp vs. 1/T. The slope is H vap - R Heating AND cooling curve for water. To calculate energy used: q=mc p ΔT on the slanted lines q=mδh vap or fus on the flat lines. Notice temperature remains constant during the phase change process. Energy is being used to overcome IMF s not increase particle motion. 3 Page 4

7 Solids, Liquids and IMF s Concept Map Solids, Liquids, and IMF s 4 Page 5

8 AP Central - Ending Misconceptions About Inter- and Intramolecular Forces Page 1 of 2 Close Here is the article in a print-friendly format. Click the button above to print this page. Ending Misconceptions About Inter- and Intramolecular Forces by George Miller University of California Irvine, California Understanding Physical and Chemical Changes There is often confusion on several issues relating to inter- and intramolecular forces. Physical change can involve changes in intramolecular forces (chemical bonds). The strength of the chemical bonds depends only on their type (ionic or covalent). When asked to explain differences between various substances as they are heated, students can confuse physical and chemical changes in the substances. Further, students, given the opportunity, will incorrectly assign relative boiling points to substances. For example, they may assert that methane has a high boiling point because it contains strong covalent bonds. Alternatively, they may overgeneralize, such as arguing that diamond has a lower melting point than salt because ionic bonds are stronger than covalent bonds. They may incorrectly use mass arguments, such as that water has a higher boiling point than ammonia because water molecules (mass 18) have a higher mass than ammonia molecules (mass 17). While mass may be part of the story in terms of correlation (e.g., bromine is more massive than fluorine), the important factors always involve attractive forces. Why do these misconceptions exist? This appears to be founded in a lack of understanding that more than one kind of "force," or interaction, can be occurring in one substance at the same time. Furthermore, there is a lack of appreciation of the relative magnitude of the forces within substances and modest levels of temperature increase as a result of heating. Some students appear to have insufficient practice in thinking of "two variables at once" and in calibrating their thinking with regard to energy and forces. Unfortunately, lack of familiarity may be responsible for students reporting that water splits into hydrogen and oxygen (something they may have seen once in an electrolysis demonstration) when heated to its boiling point. Linguistic issues do not help, causing some students to reverse the terms. One dictionary says inter- can mean "between, or among" but also "within," whereas intra- means "within" or "during"! The word internal seems to relate to inter- when the appropriate term is intra-, and "intermolecular" forces are actually "external" to the molecules! Inter- in the latter case stands for "between," not "internal." What is the correct picture? Attractive forces hold molecules together internally and with one another. In all substances - - except the noble gases -- strong, stable chemical bonds hold atoms together in assemblies called molecules. These require significant amounts of energy as well as suitable pathways, such as catalysts, to "break." Energy often has to come from formation of stronger bonds, as when hydrogen gas burns in air to form water, or a surface catalyst such as nickel metal permits hydrogen molecules to break apart and hydrogenate an olefin. Modest amounts of energy added to a solid substance will cause melting and eventually boiling of that substance. In these transformations, intermolecular forces arising from generally dynamic interactions between the moving molecules are given energy so that further motion and separation of the molecular units from one another is possible. The amounts of energy involved in the physical change are an order of magnitude smaller than those involved in breaking bonds (e.g., 40.7 kj to vaporize 1 mole of water versus 934 kj to break 1 mole of O-H bonds). The various types of forces, both intermolecular and intramolecular, vary significantly in strength both within their category and between one another. It is thus a serious oversimplification to make general conclusions about material substances without detailed consideration of all the various force types that are present. Many texts treat this fairly well but often rather briefly. Even charts constructed to help students identify the types of forces are accompanied only by a statement of the general trend in strength, and covalent bonding may not be included in the list. The additional complication to be mastered is the formation by many substances of network solids involving multiple bonds of either ionic or covalent type. Rarely is a single covalent bond directly compared to a single ionic attraction in an ion pair, the only true comparison of "strength." This has to be done to clarify the diamond or sand comparison to salt, for example. Page 6 6/26/2008

9 AP Central - Ending Misconceptions About Inter- and Intramolecular Forces Page 2 of 2 Texts usually list the forces in order of increasing strength as London dispersion (induced dipoles), dipole-dipole, hydrogen bonding, ion-dipole, ionic, and covalent. While this is a general trend, many exceptions exist, and only continued discussion will bring familiarity with when the general trend is sufficient as explanation and when more detailed consideration is needed. Instructional Recommendations Standard textbooks contain adequate and careful explanations of the correct picture. Students need to be made more familiar with energy transfer and substances in which it is frequently observed to cause physical changes. In addition, the possible existence of multiple forces within an assemblage of molecules must always be considered. The latter can usefully be emphasized with the use of molecular models. However, models are usually used to detail "intra-" chemical bonding (ball and stick) rather than intermolecular interactions. Ideally the atoms in model kits should be equipped with weak Velcro to allow then to stick together weakly to illustrate the weaker forces. One possibility is to use Lego bricks firmly pegged together to form the molecules and then hold assemblies of the molecules together with loose elastic bands. The pegs represent the intramolecular covalent bonds, and the elastic bands represent the weaker intermolecular forces holding together a model of a molecular solid that will have low melting and boiling points compared to a fully interlocked covalent network solid. Emphasize that just pushing such models around (e.g., shaking them up in a Ziploc bag) is modeling adding heat, and it may "break" or loosen the weaker elastic bands but will not break the tight peg "bonds." Students need practice in "explaining" these issues and considerable dialogue with teachers and one another about what is a satisfactory explanation. They have to be encouraged to take the explanation to the core of the issue rather than stay on the surface. One of my favorite examples is to ask students to explain why baseball team X won the game against team Y and to test their explanation against that of a baseball fanatic. The explanation that team X won because they scored more runs than team Y is perfectly accurate, but it is a seriously insufficient explanation for the baseball fanatic, who would want to know about each pitch and its result. They should assume their explanations for chemistry are to be read by an equivalent chemistry fanatic. The AP Chemistry Examination often simplifies the question by presenting closely similar molecules for comparison of physical properties (such as NH 3 and NF 3 in 2005, F 2 and I 2 in 2004, and NH 3 and CH 4 in 2001). However, this may not always be the case, as in 2001, when students were asked to explain why Si melts at a much higher temperature (1,410 C) than Cl 2 (-101 C). While the scoring guidelines may be simple, to "get the points," the best students should be able to answer this somewhat as follows: Si is a network solid with a diamondlike structure (though weaker than diamond), with each silicon atom bonded to four neighboring silicon atoms. When heated, a large number of moderately strong bonds must break for the atoms to be free to move and the solid to melt. In contrast, each chlorine atom forms one strong covalent bond to a second chlorine atom to form a Cl 2 molecule. The diatomic molecule formed is symmetrical and thus nonpolar. Weak induced-dipole forces (London dispersion forces) resulting from temporary distortions of the electron clouds on the molecules are the only attractive forces between one Cl 2 molecule and the next. At low temperature, with very little heat added, these molecules are thus free to move and behave as a liquid. Hence the low melting point of Cl 2. Note that it is important to fully discuss both the intermolecular and intramolecular forces in both species that are being compared. Dr. George E. Miller is senior lecturer SOE emeritus in the Department of Chemistry at the University of California, Irvine, where he is also the principal scientist and the reactor supervisor of UCI's nuclear reactor and faculty director for science education programs at UCI's Center for Educational Partnership, including Faculty Outreach Collaborations Uniting Scientists, Students and Schools (FOCUS). He was a member and chair of the AP Chemistry Development Committee and is currently a member of the College Board Subject Test in Chemistry Development Committee. He has been a Reader, Table Leader, and Question Leader for AP Chemistry. Copyright 2008 collegeboard.com, Inc. 6/26/2008 Page 7

10 INFORMATION IN THE TABLE BELOW AND IN THE TABLES ON PAGES 3-5 MAY BE USEFUL IN ANSWERING THE QUESTIONS IN THIS SECTION OF THE EXAMINATION. DO NOT DETACH FROM BOOK. 1 H Li Na K Rb Cs Fr (223) 4 Be Mg Ca PERIODIC TABLE OF THE ELEMENTS Sr Ba Ra Sc Y *La Ac Ti Zr Hf V Nb Ta Cr Mo W Mn Tc (98) 75 Re Fe Ru Os Co Rh Ir Rf Db Sg Bh Hs Mt (261) (262) (263) (262) (265) (266) 28 Ni Pd Pt (269) 29 Cu Ag Au (272) 30 Zn Cd Hg (277) 5 B Al Ga In Tl C Si Ge Sn Pb N P As Sb Not yet named Bi O S Se Te Po (209) 9 F Cl Br I At (210) *Lanthanide Series Actinide Series Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (145) Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr (244) (243) (247) (247) (251) (252) (257) (258) (259) (260) 2 He Ne Ar Kr Xe Rn (222) 2 GO ON TO THE NEXT PAGE. Page 8

11 2001 AP CHEMISTRY FREE-RESPONSE QUESTIONS 8. Account for each of the following observations about pairs of substances. In your answers, use appropriate principles of chemical bonding and/or intermolecular forces. In each part, your answer must include references to both substances. (a) Even though NH 3 and CH 4 have similar molecular masses, NH 3 has a much higher normal boiling point (-33 C) than CH 4 (-164 C). (b) At 25 C and 1.0 atm, ethane (C 2 H 6 ) is a gas and hexane (C 6 H 14 ) is a liquid. (c) Si melts at a much higher temperature (1,410 C) than Cl 2 (-101 C). (d) MgO melts at a much higher temperature (2,852 C) than NaF (993 C). END OF EXAMINATION Copyright 2001 by College Entrance Examination Board. All rights reserved. Advanced Placement Program and AP are registered trademarks of the College Entrance Examination Board. 13 Page 9

12 2004 AP CHEMISTRY FREE-RESPONSE QUESTIONS Answer EITHER Question 7 below OR Question 8 printed on page 13. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent. 7. Use appropriate chemical principles to account for each of the following observations. In each part, your response must include specific information about both substances. (a) At 25 C and 1 atm, F 2 is a gas, whereas I 2 is a solid. (b) The melting point of NaF is 993 C, whereas the melting point of CsCl is 645 C. (c) The shape of the ICl 4 ion is square planar, whereas the shape of the BF 4 ion is tetrahedral. (d) Ammonia, NH 3, is very soluble in water, whereas phosphine, PH 3, is only moderately soluble in water. Copyright 2004 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 12 GO ON TO THE NEXT PAGE. Page 10

13 2005 AP CHEMISTRY FREE-RESPONSE QUESTIONS Answer EITHER Question 7 below OR Question 8 printed on page 14. Only one of these two questions will be graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score weighting for the question you choose is 15 percent. 7. Use principles of atomic structure, bonding, and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. (a) At a pressure of 1 atm, the boiling point of NH 3 (l ) is 240 K, whereas the boiling point of NF 3 (l ) is 144 K. (i) Identify the intermolecular force(s) in each substance. (ii) Account for the difference in the boiling points of the substances. (b) The melting point of KCl(s) is 776 C, whereas the melting point of NaCl(s) is 801 C. (i) Identify the type of bonding in each substance. (ii) Account for the difference in the melting points of the substances. (c) As shown in the table below, the first ionization energies of Si, P, and Cl show a trend. Element First Ionization Energy (kj mol 1 ) Si 786 P 1,012 Cl 1,251 (i) For each of the three elements, identify the quantum level (e.g., n = 1, n = 2, etc.) of the valence electrons in the atom. (ii) Explain the reasons for the trend in first ionization energies. (d) A certain element has two stable isotopes. The mass of one of the isotopes is amu and the mass of the other isotope is amu. (i) Identify the element. Justify your answer. (ii) Which isotope is more abundant? Justify your answer. Copyright 2005 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 13 Page 11 GO ON TO THE NEXT PAGE.

14 2005 AP CHEMISTRY FREE-RESPONSE QUESTIONS (Form B) 8. Use principles of atomic structure, bonding, and intermolecular forces to answer the following questions. Your responses must include specific information about all substances referred to in each part. (a) Draw a complete Lewis electron-dot structure for the CS 2 molecule. Include all valence electrons in your structure. (b) The carbon-to-sulfur bond length in CS 2 is 160 picometers. Is the carbon-to-selenium bond length in CSe 2 expected to be greater than, less than, or equal to this value? Justify your answer. (c) The bond energy of the carbon-to-sulfur bond in CS 2 is 577 kj mol 1. Is the bond energy of the carbon-toselenium bond in CSe 2 expected to be greater than, less than, or equal to this value? Justify your answer. (d) The complete structural formulas of propane, C 3 H 8, and methanoic acid, HCOOH, are shown above. In the table below, write the type(s) of intermolecular attractive force(s) that occur in each substance. Substance Boiling Point Intermolecular Attractive Force(s) Propane Methanoic acid 229 K 374 K (e) Use principles of intermolecular attractive forces to explain why methanoic acid has a higher boiling point than propane. END OF EXAM Copyright 2005 by College Entrance Examination Board. All rights reserved. Visit apcentral.collegeboard.com (for AP professionals) and (for AP students and parents). 14 Page 12

Ch. 9 NOTES ~ Chemical Bonding NOTE: Vocabulary terms are in boldface and underlined. Supporting details are in italics.

Ch. 9 NOTES ~ Chemical Bonding NOTE: Vocabulary terms are in boldface and underlined. Supporting details are in italics. I. Review: Comparison of ionic and molecular compounds Molecular compounds Ionic