CHEM 12 Polarity and Intermolecular Forces

Polarity and Intermolecular Forces Answer Key Vocabulary: dipole, dipole-dipole force, dipole-induced dipole force, electronegativity, intermolecular force, ionic bond, London dispersion force, molecule, nonpolar, nonpolar covalent bond, partial charges, polar, polar covalent bond, valence electron Prior Knowledge Questions (Do these BEFORE using the Gizmo.) [Note: The purpose of these questions is to activate prior knowledge and get students thinking. Students are not expected to know the answers to the Prior Knowledge Questions.] 1. A big bully is having a tug-of-war with a small child. There is a ball attached to the middle of the rope. Toward whom will the ball move? The big bully. 2. Two equally strong kids are having a tug-of-war. What do you expect to happen to the ball in this situation? The ball will stay in the middle, halfway between them. Gizmo Warm-up Just like in a tug-of-war, atoms that are bonded to one another pull on the electrons they share. In the Polarity and Intermolecular Forces Gizmo, you will explore how these opposing forces relate to bond types and the forces between molecules. To begin, drag the Na (sodium) and Cl (chlorine) atoms into the simulation area. Turn on Show valence electrons. A valence electron is found in the outermost energy level of the atom. 1. Click Play ( ). What do you notice? The Na atom shrinks and the Cl atom expands. The orange valence electron moves from the Na atom to the Cl atom. 2. Which atom seems to be pulling more on the sodium’s one valence electron? The Cl atom. How do you know? The chlorine atom wins the “tug of war” for the valence electron. 3. What happens to the colors of the simulated atoms, and what does this indicate? The Na atom turns blue, indicating a positive charge. The Cl atom turns red, indicating a negative charge. Please Do Not Share torondsb1617 / Activity A: Bond polarity Get the Gizmo ready: • On the BOND POLARITY tab, click Reset ( ). • Drag the atoms out of the simulation area. Introduction: A neutral atom has the same number of protons as electrons. Atoms that gain electrons become negatively charged, while those that lose electrons become positive. A polar bond forms when shared electrons are pulled closer to one atom than another, causing the bonded atoms to become partially charged. In a nonpolar bond, electrons are shared equally. Question: What causes bonds to be polar or nonpolar? 1. Observe: Select the Show polar molecule inset checkbox. The animation shows the probable location of electrons (orange dots) in a polar molecule. A. What do you notice about the distribution of the electrons? There are more electrons on the left side than on the right. B. How does this electron distribution affect the charges of the bonded atoms? The left atom has more electrons and a negative charge. The right atom is positive. 2. Observe: Turn on the Show nonpolar molecule inset. A. How are the electrons in this molecule distributed? The electrons are evenly distributed between the two atoms. B. Why do the bonded atoms remain neutral? Overall, neither atom has gained or lost electrons. 3. Experiment: Turn off Show polar molecule inset and Show nonpolar molecule inset. Check that Show valence electrons is turned on. Drag the Na and Cl atoms into the simulation area. Click Play. Note the colors. Red indicates a negative charge, while blue indicates a positive charge. A. Does a polar or nonpolar bond form? A polar bond. B. Try several other metal/nonmetal combinations. What do you notice about the bonds that form? Electrons are transferred from the metal to the nonmetal atom. Ionic bonds are polar bonds that form between metal and nonmetal atoms. In this bond, valence electrons are transferred from a metal to a nonmetal. Drag each of these metal/nonmetal combinations into the Ionic bond bin on the upper right. (Activity A continued on next page) Please Do Not Share torondsb1617 / Activity A (continued from previous page) 4. Experiment: Now try forming bonds between different combinations of nonmetals. A. What do you notice? Sample answer: One or more pairs of electrons are shared between the two atoms. In some cases, the shared electrons are closer to one atom than to another. B. Are all of these bonds polar? Explain. Some of the bonds are polar, and some are nonpolar. For the nonpolar bonds, the shared electrons are halfway between the bonded atoms. C. Which nonmetal appears to attract electrons the most? Oxygen. 5. Experiment: Turn on Show electronegativity. Electronegativity (EN) describes how strongly an atom attracts a pair of shared electrons. The higher the EN value, the greater the tendency of an atom to hold onto electrons in a bond and become negatively charged. Electronegativity difference is found by subtracting the EN value of one atom from another. Choose two nonmetals with a small (or no) EN difference between them. Click Play. What happens to the shared electrons in this bond? In this bond, the shared electrons are halfway between the two bonding atoms. When the shared electrons experience the same attraction from each atom, the result is a nonpolar covalent bond. Drag the bonded atoms to the Nonpolar covalent bond bin. 6. Experiment: Choose two nonmetals with a large electronegativity difference. Click Play. What happens to the valence electrons in this bond? In this bond, the shared electrons are pulled closer to the more electronegative atom. A bond in which the electrons are shared unequally is a polar covalent bond. Notice the small δ+ and δ– symbols, which indicate partial charges. Drag the resulting combination to the Polar covalent bond bin. 7. Classify: Use the Gizmo to categorize the remaining element combinations as forming either ionic, polar covalent, or nonpolar covalent bonds. List your results below. Ionic (EN difference > 1.67) CaO, Na2O, CaCl2, MgCl2, KCl, K2O, NaCl, MgO Polar covalent (EN diff. < 1.67 and > 0.40) CO2, NH3, CCl4, H2O, OCl2, CO Nonpolar covalent (EN difference < 0.40) NCl3, O2, CH4, N2, Cl2, H2 Please Do Not Share torondsb1617 / Activity B: Molecular polarity Get the Gizmo ready: • Select the MOLECULAR POLARITY tab. Introduction: Ionic bonds, like those found in NaCl, form crystalline solids. Covalent bonds, on the other hand, usually form discrete molecules. A polar molecule, while neutral overall, has a slight positive charge on one end and a slight negative charge on the other. Question: What determines the polarity of molecules?