General Chemistry 2 PDF - Intermolecular Forces

Lesson 1.2 Intermolecular Forces of Attraction Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It!...

Lesson 1.2 Intermolecular Forces of Attraction Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 4 Intermolecular Forces of Attraction 4 Ion-Ion Interactions 4 Ion-Dipole Interactions 5 Dipole-Dipole Interactions 6 Hydrogen Bonding 7 London Dispersion Forces 8 Induced Dipoles 10 Predicting Intermolecular Forces of Attraction for Molecules 10 Ionic Compounds 11 Covalent Compounds 12 Polar Covalent Compounds 14 Nonpolar Covalent Compounds 15 Key Points 17 Check Your Understanding 19 Challenge Yourself 20 Bibliography 20 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Lesson 1.2 Intermolecular Forces of Attraction Introduction Have you ever wondered why some substances boil easier than others? For example, liquid nitrogen, when exposed to room temperature, immediately turns into vapor. On the other hand, water needs to be heated first to be converted to steam. In the previous lesson, you were able to learn about the kinetic molecular theory. This theory states that matter is composed of tiny particles that carry energy, interact with one another, and are in constant random motion. The interaction between particles and their strength determines certain properties for that matter. The particles in liquid nitrogen have a different intermolecular force of attraction than those present in liquid water. This difference affects their boiling points. In this lesson, you will discuss the different types of intermolecular forces of attraction and learn how to predict the intermolecular forces between specific molecules. 1.2. Intermolecular Forces of Attraction 1 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Learning Objectives DepEd Competencies Describe and differentiate the In this lesson, you should be able to do the types of intermolecular forces following: (STEM_GC11IMF-IIIa-c-100). Differentiate the different types of Predict the intermolecular forces intermolecular forces of attraction. possible for a molecule (STEM_GC11IMF-IIIa-c-101). Predict the intermolecular forces that may exist for a molecule. Warm Up Together, Forever? 20 minutes One of the postulates of the kinetic molecular theory is that particles have interaction with one another. These interactions affect the properties of the substance. This activity demonstrates the interaction between particles. Materials magnets marbles paper clips box ruler Procedure 1. Place two marbles side-by-side. 2. Try pulling the marbles 5 mm apart in 5 increments. 3. Observe whether there is an attractive force between the two particles. 4. Repeat the previous steps using the following setups: a. magnet and marble b. two magnets 1.2. Intermolecular Forces of Attraction 2 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids c. magnet and paper clip d. two paper clips e. A paper clip (other end attached to a magnet) and another paper clip. 5. Record your observations. 6. Answer the guide questions that follow. Observation Table Table 1.2.1. Observations when the objects are placed side-by-side Materials Observation marble + marble magnet + marble magnet + magnet magnet + paper clip paper clip + paper clip paper clip (other end attached to a magnet) + paper clip Guide Questions 1. Which of the following setups have no interaction with one another? 2. Which of the following setups have interactions between the particles? 3. Why did the setups in question 2 have interaction between the particles? 4. When the magnet was attached to one paper clip, did it change the interaction of the two paper clips? Why? 1.2. Intermolecular Forces of Attraction 3 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Learn about It! Intermolecular Forces of Attraction Intermolecular forces are attractive forces present in between molecules. Although there are various attractive forces, four main types of intermolecular forces of attraction (IMFA) are most commonly observed. These are London dispersion forces, dipole-dipole forces, ion-dipole forces, and hydrogen bonding forces. The first two are collectively known as van der Waals forces of attraction, named after the Dutch scientist Johannes van der Waals. What are the different types of intermolecular forces of attraction? Ion-Ion Interactions Ion-ion interaction is the interaction between two oppositely charged particles. In chemistry, charged particles are called ions. Cations are positively charged particles, while anions are negatively charged particles. Recall that ion formation is a result of atoms gaining or losing electrons. Cations are formed when an atom or molecule loses electrons. Anions are formed when an atom or molecule gains electrons. Ion-ion interaction is also known as ionic bonds. This type of bond holds together the particles in an ionic compound. Fig. 1.2.1. The positively charged sodium ion (Na+) interacts with the negatively charged chloride (Cl-) ion. 1.2. Intermolecular Forces of Attraction 4 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Remember Ion-ion interactions are between electrically charged particles. Ion-Dipole Interactions Ion-dipole interaction results from the electrostatic attraction of a molecule containing a dipole and an ion. This type of interaction is responsible for the dissolution of most ionic solids in polar solvents. The strength of this kind of IMFA increases as the charge of the ion increases. It is often observed in solutions such as brine (NaCl in water). When NaCl dissolves in water, it exists as Na+ and Cl-. The cation Na+ is attracted to the partially negative O atom of water while the anion Cl- is attracted to the partially positive H atom of water. Fig. 1.2.2. The cation is attracted to the partial negative end of the molecule while the anion is attracted to the partially positive end of the molecule. Remember The partially positive end of the polar molecule interacts with the anion, whereas the partially negative end of the polar molecule interacts with the cation. 1.2. Intermolecular Forces of Attraction 5 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids The strength of ion-dipole interaction depends on the charge density of the ion. Charge density is defined as the actual charge distributed over the total volume of the ion. For example, cations with higher charge magnitudes have high charge density because they are significantly smaller compared to cations with smaller charge magnitudes. The interaction between Mg2+ and water is stronger than the interaction of Na+ with water. Dipole-Dipole Interactions Dipole-dipole interactions are attractive forces that are a moderately strong type of IMFA and are present in between polar molecules. Dipole-dipole forces are the result of the electrical interactions among dipoles on neighboring molecules. This means that the partially positive end of one molecule interacts with the partially negative end of a neighboring molecule. Partial charges are symbolized by the lowercase delta (ẟ) followed by a plus (+) sign for partial positive, or a minus (-) sign for partial negative. Fig. 1.2.3. HCl molecule with partially positive and partially negative ends. For example, HCl is a polar molecule. It has partially positive and partially negative ends. The dipole-dipole force exists between the partially positive end of one HCl molecule and the partially negative end of another HCl molecule. 1.2. Intermolecular Forces of Attraction 6 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Fig. 1.2.4. Molecules with partially positive ends attracted to the partially negative ends of other molecules. Remember All polar molecules exhibit dipole-dipole interactions. Hydrogen Bonding Hydrogen bonding is a special kind of dipole-dipole force and one of the strongest types of IMFA. It is an attractive force that exists when hydrogen is bonded to the most electronegative atoms, namely F, O, or N. In such cases, the partially positive hydrogen of one molecule interacts with the partially negative F, O, or N atoms in another molecule. This relatively strong attraction explains why molecules with this type of IMFA tend to have high boiling and melting points. Many unusual properties of water are attributed to hydrogen bonding. Consider the water molecule, H2O. The hydrogen of one molecule is attracted to the oxygen atom of another molecule. 1.2. Intermolecular Forces of Attraction 7 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Some molecules may also interact with water even though they cannot form hydrogen bonds themselves. Let us look at the interaction between formaldehyde and water. Formaldehyde does not exhibit hydrogen bonding on its own. However, in the presence of water, the O atom of formaldehyde can form hydrogen bonds with the H atoms of water. Formaldehyde becomes a hydrogen bond acceptor, while water is the hydrogen bond donor. A hydrogen bond donor is a molecule that provides the hydrogen atom participating in a hydrogen bond, while a hydrogen bond acceptor is a molecule that contains the lone pair-bearing electronegative atom. Fig. 1.2.6. Hydrogen bond formation between water and formaldehyde. Remember Hydrogen bonding can only be exhibited when one molecule has a hydrogen atom is directly bonded to fluorine, oxygen, or nitrogen atom. London Dispersion Forces London dispersion forces (LDFs) are the weakest type of IMFA and are present in between all electrically neutral molecules ― polar and nonpolar molecules. This IMFA was named after the German-American physicist Fritz London who initially proposed this intermolecular force of attractions. 1.2. Intermolecular Forces of Attraction 8 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids LDFs are caused by fluctuations in the electron distribution within atoms or molecules. This happens when an atom, which is usually nonpolar, becomes polar due to the continual motion of its electrons, resulting in a temporary dipole. In this case, one end of the molecule can temporarily have a partial negative charge while another end can temporarily have a partial positive charge. This temporary dipole can cause a neighboring atom to be distorted and make its nucleus attracted to the negative end of the first atom. (a) (b) (c) Fig. 1.2.7. A temporary dipole (b) produced from a nonpolar molecule (a) induces instantaneous dipoles to neighbor molecules (c). Another example can be observed in nonpolar molecules such as O2, where there are no positive or negative ends. Because the electrons of these molecules are constantly moving, there are times when electrons move to one end, making such end partially negative while the other end becomes partially positive. Hence, the molecule can have an instantaneous dipole. The temporary dipole of a molecule induces instantaneous dipoles to neighbor molecules. Fig. 1.2.8. The instantaneous dipole of O2. Remember All electrically neutral molecules exhibit London dispersion forces (LDF). 1.2. Intermolecular Forces of Attraction 9 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Induced Dipoles There are two types of induced dipole forces—ion-induced and dipole-induced. The main difference between the two is the kind of inducing particle present. This occurs when a nonpolar atom becomes polar due to the presence of an ion or a dipole. This is similar to the paper clip to paper clip interaction in the presence of a magnet. Initially, the two paper clips will not have any force of attraction between them since they are not magnetic themselves. However, when one end of the paper clip is attached to the magnet, then the paper clip becomes magnetic by the induction effect. Fig. 1.2.9. Representation of ion-induced dipole and dipole-induced dipole. How are induced dipoles created? Predicting Intermolecular Forces of Attraction for Molecules Recall that compounds can be classified as ionic or covalent based on the types of bonds present. 1.2. Intermolecular Forces of Attraction 10 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids How can one determine the intermolecular force present for a molecule? Ionic Compounds Recall that an ionic compound is composed of atoms bonded by ionic bonds. Ionic bonds involve the transfer of an electron from a metal to a nonmetal. The cations and anions in an ionic compound interact via ion-ion interactions. The strength of the ion-ion interaction is governed by Coulomb's law. Equation 2.1 where F is coulombic force, q1 and q2 are the charges of the particles, and r is the distance between the particles. The equation shows that the coulombic force is directly proportional to the product of the charges of the particles and inversely proportional to the distance between the particles. Table 1.2.2. Melting points of some ionic compounds Compound Melting Compound Melting Compound Melting Point ( C)O Point ( C) O Point (OC) NaF 993 CaF2 1423 MgO 2800 NaCl 801 Na2S 1180 CaO 2580 NaBr 747 K2S 840 BaO 1923 Let’s look at NaF and NaCl. The cation for both compounds is sodium ion (Na+) that has a charge of +1. The anions are fluoride (F-) and chloride (Cl-), respectively. Both anions have a charge of -1. Since the charges are essentially the same for the ions in NaF and NaCl, then the difference between their melting points can be attributed to the distance, r, between the particles. Chloride ion is larger than fluoride ion; therefore, the distance between the ions in 1.2. Intermolecular Forces of Attraction 11 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids NaCl is larger than in NaF. Since the Coulombic force is inversely proportional to the distance between the ions, then NaCl has a weaker Coulombic force. This results in a lower melting point. Covalent Compounds Covalent bonds, on the other hand, involve the sharing of electrons between two nonmetal atoms. Covalent compounds can be further classified based on polarity as polar or nonpolar covalent molecules. Recall that the polarity of the molecule can be determined by identifying the polarity of the bonds and the molecular geometry for the compound. In order to determine the polarity of the molecule, the following steps may be used. Step 1: Draw the correct Lewis structure and determine the molecular geometry of the molecule. Step 2: Identify the polarity of each bond present in the molecule. Step 3: Draw the dipole moment vector for each polar bond. Step 4: Determine the sum of the dipole moment vectors. Let us take SO2 as an example. Step 1: Draw the correct Lewis structure and determine the molecular geometry of the molecule. The correct Lewis structure is shown below. Fig. 1.2.11. Lewis structure for SO2 1.2. Intermolecular Forces of Attraction 12 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Based on the Lewis structure, there are two bonding domains and one nonbonding domain around the central atom, sulfur. The electron group geometry is trigonal planar, and the molecular geometry is bent. Step 2: Identify the polarity of each bond present in the molecule. Oxygen (χ = 3.44) is more electronegative than sulfur (χ = 2.58). Therefore, the S—O bonds are polar. Step 3: Draw the dipole moment vector for each polar bond. The dipole moment is towards the direction of the more electronegative atom. Since oxygen is more electronegative than sulfur, then the dipole moment vector moves from sulfur to oxygen, as shown below. Fig. 1.2.12. Dipole moment vectors represented in SO2 Step 4: Determine the sum of the dipole moment vectors. As seen in the figure above, the dipole moments in SO2 do not cancel out since the molecule is bent-shaped. As you add the two, there is a resultant dipole vector moving downwards. Therefore, SO2 is a polar molecule. Remember It is necessary to determine both molecular geometry and bond polarity in order to predict whether a molecule is polar or nonpolar. Some molecules have polar bonds but are nonpolar as a whole. This is due to the cancellation of the dipole moment due to molecular geometry. 1.2. Intermolecular Forces of Attraction 13 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Polar Covalent Compounds Polar covalent compounds are molecules with a net dipole moment. This means that the electrons are not equally shared between the atoms. This causes the molecule to have a partial positive (δ+) and a partial negative (δ-) charges, which are also known as a dipole. Polar covalent compounds can either have dipole-dipole interactions or hydrogen bonding, depending on the presence of H and its connectivity to other atoms in the compound and London dispersion forces. Fig. 1.2.13. Boiling points for the different hydrides for group 5A, 6A, and 7A. Notice that H2O, HF, and NH3 have higher boiling points than the rest of their groups. This is because, unlike the other members in their group, those three compounds can form hydrogen bonding. 1.2. Intermolecular Forces of Attraction 14 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Nonpolar Covalent Compounds Nonpolar covalent compounds are molecules with zero dipole moment. This means that the electrons in the bonds are shared equally between the atoms. The only intermolecular force present in these compounds is the London dispersion forces. Table 1.2.3. Polarizability, molar mass, and boiling point of selected compounds Polarizability, Molar Mass, Boiling Point, Compound 10–25 cm3 amu K H2 7.9 2.02 20.35 O2 16.0 32.00 90.19 N2 17.6 28.01 77.35 CH4 26.0 16.04 109.15 C2H6 44.7 30.07 184.55 Cl2 46.1 70.91 238.25 C3H8 62.9 44.11 231.05 CCl4 105.0 153.81 349.95 Notice how the boiling point increases as the molecule gets larger. This is due to the larger size of the molecule and the presence of more electrons. When more electrons are present in a molecule, the stronger the LDFs are. This is because larger molecules are more polarizable. Polarizability is the measure of how easy it is to distort the electron distribution of a molecule. In large molecules, the electrons are less tightly held by the attraction with the nucleus so they can form temporary dipoles easier. Polarizability can be related to how easy one can squeeze a balloon: the larger the balloon is, the more squeezable it is, and the stronger is its LDF. This also explains why nonpolar substances such as halogens and noble gases freeze into solids and condense into liquids at a sufficiently lowered temperature. LDF also explains why, generally, liquids made up of molecules with no permanent dipole attraction have 1.2. Intermolecular Forces of Attraction 15 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids lower boiling points. For example, bromine, Br2, bears more electrons than chlorine, Cl2, which means that Br2 has stronger London dispersion forces than Cl2. The strength of LDF affects their boiling points. Thus, Br2 has a boiling point of 59 °C compared to Cl2, which has a lower boiling point of -35 °C. Fig. 1.2.15. Structure for neopentane (left) and pentane (right). In terms of polarizability, the larger the surface area, the stronger the LDF will be. For example, between neopentane and pentane, the latter will have a stronger LDF due to the larger surface area. Tips In order to predict the intermolecular forces between two molecules, you must first determine the type of compound present. If it is ionic, then you have ion-ion interactions. If it is covalent, it depends on whether the molecule is polar or nonpolar. Take note that whether the molecule is polar or nonpolar, it will always be capable of interacting through London dispersion forces. For nonpolar molecules, it is the only IMFA present. For polar 1.2. Intermolecular Forces of Attraction 16 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids molecules, dipole-dipole interactions are present, on top of London dispersion forces. If you have a hydrogen atom directly bonded to a fluorine, oxygen, or nitrogen atom, then hydrogen bonding is present. Fig. 1.2.15. Comparison of IMFAs in molecules with approximately the same molecular weight For molecules with roughly the same molecular weight, the strength of IMFA depends on the polarity of molecules. Fig. 1.2.15 shows that the strength of IMFA increases as polarity increases, reflective of their boiling points. Key Points ___________________________________________________________________________________________ Intermolecular forces of attraction are attractive forces present in between molecules. ○ Ion-ion interaction is the interaction between two oppositely charged particles. 1.2. Intermolecular Forces of Attraction 17 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids ○ Ion-dipole interaction results from the electrostatic attraction of a molecule containing a dipole and an ion. ○ Dipole-dipole interactions are attractive forces that are a moderately strong type of IMFA and are present in between polar molecules. ○ Hydrogen bonding is a special kind of dipole-dipole force that exists when hydrogen is bonded to the most electronegative atoms, namely F, O, or N. ○ London dispersion forces (LDFs) are the weakest type of IMFA and are present in between all electrically neutral molecules―polar and nonpolar molecules. ○ Induced dipoles occur when a nonpolar atom becomes polar due to the presence of an ion or a dipole. The strength of ion-ion interactions is dependent on the Coulombic force between the particles. It is directly proportional to the product of the charges and inversely proportional to the distance between the particles. The strength of ion-dipole interactions depends on the charge of the ion present. The strength of LDFs depends on the polarizability of the molecule. Polarizability refers to the ease at which the electron cloud can be distorted. ___________________________________________________________________________________________ 1.2. Intermolecular Forces of Attraction 18 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Check Your Understanding A. Identify the terms described in each of the following items. ___________________________ 1. It refers to the interaction between polar molecules. ___________________________ 2. It refers to the ease at which an electron cloud is distorted. ___________________________ 3. It is the IMFA exhibited by compounds with hydrogen atoms directly bonded to fluorine, oxygen, or nitrogen. ___________________________ 4. It refers to the IMFA caused by instantaneous dipoles. ___________________________ 5. It is the type of interaction between a nonpolar molecule and an ion or a polar molecule. ___________________________ 6. It refers to the interaction between two charged particles. ___________________________ 7. This interaction is responsible for the dissolution of most ionic solids in polar solvents. ___________________________ 8. It is the molecule that provides the hydrogen atom participating in a hydrogen bond. ___________________________ 9. It is the molecule that provides the lone-pair containing atoms participating in a hydrogen bond. ___________________________ 10. These are attractive forces present in between molecules. B. Write T if the following statement is true. Otherwise, write false. _______ 1. The intermolecular forces of attraction present in molecules affect the properties of the substance. 1.2. Intermolecular Forces of Attraction 19 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids _______ 2. The weakest intermolecular force of attraction is ion-ion interaction. _______ 3. Polar molecules only have dipole-dipole interactions. _______ 4. Hydrogen bonding is present in HI molecules. _______ 5. As polarizability increases, the strength of the London dispersion force also increases. C. Determine all the IMFAs present in the following molecules. 1. CH3OH __________________________________________________________________ 2. H2S __________________________________________________________________ 3. (CH3)2CO (acetone) __________________________________________________________________ 4. C6H6 __________________________________________________________________ 5. KCl __________________________________________________________________ Challenge Yourself Answer the following. 1. Compare and contrast dipole-dipole interactions with hydrogen bonding 2. Relate polarizability with the strength of London dispersion forces. 3. Arrange the following in increasing IMFA strength: ethanol, ethylene glycol, ethane. Answer the following. 4. Construct a schematic diagram on how to determine IMFA present in molecules. 5. Explain how to use the diagram you have constructed in the previous item. Bibliography Brown T.L. et al. 2012. Chemistry: The Central Science. Pearson Prentice Hall.Brown. Chemistry: The Central Science. Prentice-Hall, 2005. 1.2. Intermolecular Forces of Attraction 20 Unit 1: Intermolecular Forces of Attraction and Solids and Liquids Bettelheim, Frederick A., et al. 2015. Introduction to General, Organic and Biochemistry. Boston: Cengage Learning. Ebbing, Darrell and Steven Gammon. 2016. General Chemistry. Boston: Cengage Learning. Moore, John W, and Conrad L. Stanitski. 2015. Chemistry: The Molecular Science, 5th ed. USA: Cengage Learning. Petrucci, Ralph H. General Chemistry: Principles and Modern Applications. Toronto, Ont.: Pearson Canada, 2011. Print. Reger, Daniel L., et al. 2009. Chemistry: Principles and Practice. Boston: Cengage Learning. Silberberg, Martin S. 2007. Principles of General Chemistry. McGraw-Hill Company. 2007 Spencer, James N., et al. 2010. Chemistry: Structure and Dynamics. New Jersey: John Wiley & Sons. 1.2. Intermolecular Forces of Attraction 21