CHEM 1412 Activity 2 - Electronegativity, Bond Polarity, Molecular Polarity, and Intermolecular Forces PDF

CHEM 1412 Online v.0912 Page 1 Activity 2. Electronegativity, Bond Polarity, Molecular Polarity, and Intermolecular Forces Why? Molecular interactions are those that produce many observable behaviors such as the states...

CHEM 1412 Online v.0912 Page 1 Activity 2. Electronegativity, Bond Polarity, Molecular Polarity, and Intermolecular Forces Why? Molecular interactions are those that produce many observable behaviors such as the states of compounds, boiling point, melting point, solution formation, and colligative properties. Understanding these interactions and how to predict them is critical to understanding and predicting the behaviors of molecules and ions. Learning Objectives To reinforce your knowledge of intermolecular forces. To reinforce your understanding of the origins of intermolecular forces. To reinforce your predictive abilities regarding molecular interactions based on intermolecular forces. Successfully predict the dominant and secondary intermolecular forces of a molecule. Resources Read section(s) in your textbook about liquids, solids and phase changes. Planning Ahead Read section(s) in your textbook about gases, gas laws, and gas behaviors. Model - Bonding Bonding between atoms is best described as a continuum. Bond types are easily determined by taking the absolute electronegativity difference of the bonded atoms. 1. At one end of this continuum are nonpolar, covalent bonds. True nonpolar bonds only occur between identical atoms, however, bonded atoms with an electronegativity difference of less than 0.5 are generally considered nonpolar. 2. At the other end of the continuum are ionic bonds between atoms having the greatest difference in electronegativity (e.g., CsF). Bonded atoms with electronegativity differences of greater than 2.0 are generally considered to share an ionic bond, where one atom has completely removed one or more electrons from the other. 3. Everything in between shows some degree of covalent behavior (atoms sharing electrons) and some degree of ionic behavior (some level of negative charge on one atom and some level of positive charge on the other atom). This hybrid type of bonding is called: polar covalent bonding. CHEM 1412 Online v.0912 Page 2 Key Question 1. What are the electronegativity differences for F2, HF, and CsF? Would you characterize HF as more covalent or more ionic, based on the electronegativity difference? Problem 2. For the following binary compounds determine the electronegativity differences, state whether the bonding is nonpolar covalent, polar covalent, or ionic. If it is polar covalent also state if it is more covalent or more ionic (type dominance). Model - Molecular Polarity The bond between different elements will be polar to some extent producing a dipole of a given size and direction (i.e. dipoles are vectors). The dipole vectors are additive and produce a result that is the overall molecular dipole. Recall that a polar bond has one end which is partially negative (more electronegative) and one end that is partially positive (less electronegative): This can lead to a molecular dipole that is represented by the red (light gray in print) arrow. The arrow head representing the negative end and a cross in the tail representing the positive end. Key Question 2. Why is there no arrow showing a molecular dipole for CO2? Model - Molecular Dipole Determination Nonpolar Polar Nonpolar Polar CHEM 1412 Online v.0912 Page 3 Key Question 3. Why does each of molecules in the table just above have the polarity shown? Problem 2. For the other molecules in the table below draw in the bond dipoles and determine the overall molecular dipole. If the bond is nonpolar write NPB on top of it, if the molecule is nonpolar write NPM next to the formula. BF3 NPM CH2O CCl4 NPM CH2Cl2 O2 NPM H2O H2S NPM NCl3 NPM Information - Intermolecular Forces Intermolecular interactions hold molecules together in in the liquid and solid state. As you are aware the input of heat energy is required cause a solid to liquefy (ΔHfusion) and to cause a liquid to evaporate (ΔHvaporization). The intermolecular forces are disrupted by the increase in kinetic energy of molecular motion as temperature increases. When these forces are overcome, a change of state can occur. The intermolecular forces include dipole-dipole interactions, dipole-ion interactions, dipole-induced dipole interactions, dispersion forces and hydrogen bonding. Key Questions 5. If heat is required to liquefy a solid, is ΔHfusion endothermic or exothermic for this process? What is the sign of this process? 6. If heat must be lost to condense a gas into a liquid, is ΔHvaporization endothermic or exothermic for this process? What is the sign of this process? Information - Permanent Dipoles For polar molecules, the dipole is said to be permanent, meaning it has a constant size. One of principle intermolecular forces is dipole-dipole interaction in which the molecular dipoles of polar molecules align with one another. In a solid molecular dipoles of polar molecules will be highly ordered and unable to move. In liquids, molecular dipoles are free to move but will have some order to them. CHEM 1412 Online v.0912 Page 4 Problem 6. In the following two images which represents the liquid state and which the solid state of SF4? What is your justification for those assignments? Depiction of State for SF4 State Justification Key Question 6. What extent of ordering is expected for molecules in the gas phase? What is your reasoning? Information - Dipole Moment The polarity of a molecular dipole can be calculated but more conveniently it can be measured. The measured polarity of a molecular dipole is called the dipole moment and the values are measured in units of debye (D). The boiling point and dipole moments of a series of molecules of similar mass and shape are given in the table: Compound CH3CH2CH3 CH3OCH3 CH3CHO CH3CN Structure Molecular Mass, 44 46 44 41 g/mol Dipole Moment, D 0.1 (nonpolar) 1.3 2.7 3.9 Boiling Point, K 231 248 294 355 CHEM 1412 Online v.0912 Page 5 Key Question 7. Considering the compounds in the table above are of similar size and mass, what does the increase in the boiling point temperature suggest? Model - London Dispersion Forces London dispersion forces occur in all types of atoms, molecules and ions because all atoms, molecules and ions have electrons orbiting nuclei. Since the electrons are in constant motion, they can sometimes have a greater density on one side of the atom, molecule, or ion. The graphic below is a depiction of the how electron density can move about a nucleus, creating short-lived, instantaneous dipoles within an atom like He. The formation of a instantaneous dipole can also cause other nearby atoms to form complimentary instantaneous dipoles, this is the dispersion force. However as quickly as the instantaneous dipoles form, they collapse, only to arise again, probably in a different direction. Formation and propagation of instantaneous dipoles permit nonpolar atoms and molecules to be condensed into a liquid or frozen into a solid. The size of the instantaneous dipole formed depends on the polarizability of the atom or molecule - meaning how much the electron cloud can distort away from the nucleus. CHEM 1412 Online v.0912 Page 6 Key Questions 8. Which atoms in the periodic table should be most polarizable? Why? 9. The boiling point temperature for the noble gases increases down the period, indicating an increase in the strength of the intermolecular interactions between the atoms of each gas. The boiling point data for the group 18 elements is shown in the table below. CHEM 1412 Online v.0912 Page 7 Element He Ne Ar Kr Xe Rn Boiling Point, K 4.22 27.07 87.30 119.93 165.03 211.3 Based on the information above, what best explains why there is an observed increase in these boiling points? Model - Molecular London Dispersion Forces All molecules exhibit London dispersion force interactions because the electrons of the atoms forming a molecule are in continuous motion. These interactions are most important for nonpolar molecules because London dispersion forces are their only means of intermolecular interaction. For a series of nonpolar molecules such as the linear alkanes the effect of increasing size is demonstrated in the table below: Compound CH4 CH3CH3 CH3CH2CH3 CH3CH2CH2CH3 CH3CH2CH2CH2CH3 Structure Boiling Point, K 113 185 231 273 309 Key Question 10. There are two reasons why London dispersion forces increase in strength in the molecules above, what are those reasons? Model - Molecular Shape Affect on Polarizability Increasing the number of electrons is not the only factor that affects the polarizability and thus the ultimate London dispersion force strength of a molecule. Isomers are molecules that share the same number of each type of atom but have different atomic arrangements and in most cases isomers have different physical properties. The following boiling point temperatures have been determined for the following isomers of pentane (C5H12): Compound CH3CH2CH2CH2CH3 CH3CH2CH(CH3)2 C(CH3)4 Structure Boiling Point, K 309 301 283 Key Questions 11. Why would the boiling point temperatures be different for these isomers of pentane? CHEM 1412 Online v.0912 Page 8 12. For the following series of nonpolar diatomic molecules the melting point temperatures are: Compound N2 O2 F2 Molecular Mass, g/mol 28.01 32.00 38.00 Melting Point, K 63.15 54.36 53.53 Why does the melting point temperature decline for this series as the mass of the molecules increases? Information - Combination of Forces in Molecules London dispersion forces also contribute to the interactions of molecules with a molecule dipole and to those molecules that hydrogen bond. The effect is demonstrated for a series of polar ether molecules in the table below: Compound CH3OCH3 CH3OCH2CH3 CH3OCH2CH2CH3 CH3OCH2CH2CH2CH3 Structure Dipole Moment, D 1.30 1.23 1.24 1.25 Boiling Point, K 248 284 312 344 Key Question 13. Why are increased London dispersion forces the only reasonable explanation for the increasing boiling points as the series of ether molecules increases in size? Information - Correlation vs. Causation Hopefully it has become apparent that while there is a correlation of molecular mass to boiling point temperatures for molecules having significant London dispersion force interactions but there is no such correlation regarding the extent dipole-dipole interactions. In both London dispersion force and dipole-dipole interactions, the molecular mass is not the cause of the strength of the interactions. Key Question 14. If mass is not the cause of London dispersion force and dipole-dipole interactions, what is(are) the cause(s)? Information - Dipole-Induced Dipole Interactions This type of intermolecular interaction involves the interaction of molecules possessing a permanent dipoles inducing nonpolar molecules to form instantaneous dipoles. The induced dipole in the nonpolar species can CHEM 1412 Online v.0912 Page 9 then interact to some extend with the polar molecules. The molecules in the following table can all dissolve the nonpolar molecule benzene (C6H6) to the extent indicated. Compound C6H6 CH3OCH3 CH3CH2OH H2 O CH3C(O)CH3 Structure Dipole Moment, D 0 1.3 1.69 1.85 2.91 Benzene Solubility completely completely completely slightly (1.8 g/L) completely Key Questions 15. Why is the molecule benzene completely soluble in a liquid composed entirely of other benzene molecules? 16. Why would the solvents dimethyl ether (CH3OCH3), ethanol (CH3CH2OH), and acetone (CH3C(O)CH3) be capable of completely dissolving benzene, while water can only slightly dissolve benzene? Information - Dipole-Ion Interactions This type of intermoleculare interaction involves the interaction of molecules possessing a permanent dipoles with ions. Because ions either have a positive charge (cations) or a negative charge (anions) they can be considered monopoles. This type of interaction is the basis for dissolving salts in water and other dipolar solvents. Key Question 17. Which end of a water molecule should interact with a cation? Which end of a water molecule should interact with an anion? Model - Hydrogen Bonding There is a special type of intermolecular force which only occurs between a hydrogen atom covalently boned to either N, O, or F and a nonbonding electron pair on another nearby N, O, or F atom. For example, in water, hydrogen bonding can occur between water molecules or water molecules and other hydrogen bonding molecules: Key Question 18. What are the restrictions on which molecules can hydrogen bond? CHEM 1412 Online v.0912 Page 10 Problem 19. Identify and justify which of the following pairs of molecules can hydrogen bond and which cannot. (If in doubt draw the Lewis structures and consider the requirements listed above.) Pair Can they H-bond? Justification H2O2, NH3 HCl, H2S HF, PH3 HCl, NH3 H2S, NH3 CH3C(O)CH3, HF CH3C(O)CH3, CH4 H2O2, HBr Summary - Intermolecular Forces There are five main intermolecular forces and some important behaviors associated with them: 1. London dispersion forces: resulting from the formation of instantaneous dipoles due to the motion of electrons in their orbitals. This force is present in all atoms, ions, and molecules but is only significant for neutral species that lack a permanent dipole and/or cannot hydrogen bond. 2a. Dipole-dipole forces: occur between all molecules possessing a permanent dipole. Related to this molecular dipoles can induce neutral atoms and molecules to form instantaneous dipoles. In a given molecule this force is stronger than the molecule's London dispersion forces and if the molecule can hydrogen bond as well, it will be weaker than the hydrogen bonding interactions. 2b. Dipole-ion forces: molecules possessing a dipole will also interact strongly with ions (monopoles). 2c. Dipole-induced dipole forces: molecules which possess a perment dipole can induce nonpolar molecules to form an instantaneous dipole which will then interact with the permanent dipole of the first molecule. 3. Hydrogen bonding interactions: these are limited to a hydrogen atom (covalently bound to one of the atoms N, O, F) being weakly attracted to another N, O, F atoms, which itself has one or more H atoms covalently bonded to it. For a molecule, hydrogen bonding will be stronger than its dipole-dipole interactions or London dispersion force interactions, if another hydrogen bonding molecule is available for interaction. Problem 20. What kind of intermolecular forces are at work in each of the following pairs? Justify your team's answer. F H F O H O O O hydrogen bonding London dispersion forces CHEM 1412 Online v.0912 Page 11 O O H S S - O Cl O O H ion-dipole dipole-dipole Key Questions 21. Why do these two molecules, having the same formula (C3H8O) and mass, have such different boiling points? H H H H H H C O H C H C H C O C H C H H H H H H ethyl methyl ether, 284 K propanol, 370 K Because their masses are the same the extent of London forces should be similar, and their permanent dipoles should be close to the same size but propanol can participate in hydrogen bonding that ethyl methyl ether cannot producing stronger intermolecular interactions in its liquid state. 22. What are the intermolecular forces common to both species in the following pairs? Justify your team's choices. Strongest Pair Compared Intermolecular Force in Justification Common Xe, CH3OH CH3OH, CH2Cl2 NH3, CH3OH 23. Why does replacing an -H with a -CH3 in the related pairs of molecules below cause the boiling points to decrease? Justify your answer. CHEM 1412 Online v.0912 Page 12 Structure Name ethanol ethyl methyl ether ethylene glycol ethylene glycol dimethyl ether Formula CH3CH2OH CH3CH2OCH3 HOCH2CH2OH CH3OCH2CH2OCH3 TBP, K 351 284 471 351 Information - Hydrogen Bonding in Water Hydrogen bonding in water results some if its unique properties such as its boiling temperature being 373 K (100 ºC). The boiling point for water is not consist with the dominant trend for the boiling point temperatures of the hydrides of Groups 14-17 in Table 1. A plot of the data of the data in Table 1 is provided below it. Table 1. Boiling Point vs. Mass of the Hydrides of Groups 14-17 Group Mass, TBP, K Group Mass, TBP, K Group Mass, TBP, K Group Mass, TBP, K 14 amu 15 amu 16 amu 17 amu CH4 16.04 109 NH3 17.03 240 H2O 18.02 373 HF 20.01 293 SiH4 32.11 161 PH3 34.00 188 H2S 34.08 212 HCl 36.46 186 GeH4 76.67 184 AsH3 77.95 213 H2Se 80.98 232 HBr 80.91 206 SnH4 122.7 223 SbH3 124.8 248 H2Te 129.62 271 HI 127.91 238 400 350 300 Group 14 250 Group 15 Boiling Point Temperature, K Group 16 200 Group 17 150 100 0 20 40 60 80 100 120 140 Mass, amu A partial explanation for the trends observed above can be deduced by examining the boiling point temperatures versus the dipole moments of the hydrides of groups 14-17 in Table 2 below. CHEM 1412 Online v.0912 Page 13 Table 2. Boiling Point vs. Dipole Moment of the Hydrides of Groups 14-17 Group Dipole TBP, K Group Dipole TBP, K Group Dipole TBP, K Group Dipole TBP, K 14 Moment, 15 Moment, 16 Moment, 17 Moment, D D D D CH4 0 109 NH3 1.47 240 H2O 1.85 373 HF 1.82 293 SiH4 0 161 PH3 0.58 188 H2S 0.97 212 HCl 1.11 186 GeH4 0 184 AsH3 0.20 213 H2Se * 232 HBr 0.83 206 SnH4 0 223 SbH3 0.12 248 H2Te * 271 HI 0.45 238 *Measured values unknown but are reasonably presumed to be small (