Wed 4-06 and Thurs 407 IMFs and Dont Flip Your Lid Lab Mrs. Wilson Objectives Relate the structure and polarity of water to its properties and role in biological and chemical systems. Describe and distinguish between intramolecular forces and intermolecular forces.
Describe and distinguish between three different types of intermolecular forces. Relate structure and polarity of different substances to their properties based on intermolecular forces. Homework: Lesson 6.1 Homework on pg 29 of your new packet. Daily Quiz next class. Engage: Which of These Molecules Is/Are POLAR? How? How do you think polarity affects the physical properties of these three substances?
Explore (1): Lets Talk About Water Read the reading called A. Properties of Water beginning on pg. 2 and continuing on pg. 3. Answer the questions in the Review. Check your answers and discuss them with two partners. 104.5 bond
angle Bent molecular shape (NOT LINEAR) The bent molecular shape (caused by 2 lone e pairs on O) and highly electronegative O-H bonds make water polar. Partial + and charges on Hs and Os cause hydrogen bonding which is an unusually strong force, allows water molecules to attract, and needs more energy (in J/mole) to disrupt than other types of forces.
Waters polarity allows it to dissolve nutrients and waste (which are also at least somewhat polar) Waters polarity allows it to be cohesive and adhesive (crucial for plants water transport up stems by capillary action) and have surface tension. The hydrogen bonds in water require more energy (in kJ/mole) to disrupt (crucial for evaporation and boiling) so water has a high specific heat water can retain heat for long periods of time. This helps regulate the temperature of the earth. Waters polarity allows it to dissolve many ionic and polar substances. Water
dissolves them by surrounding the particles and pulling them away from each other. Ice floats due to waters bent structure forming a hexagonal lattice, increasing its V and decreasing its density. Introduction to Intramolecular vs. Intermolecular Forces Intramolecular Forces: bonds that hold an individual particle (molecules or formula units) together. Intermolecular Forces: forces that hold neighboring molecules or formula units together
Explore (1) Introduction to the Lab The properties of a substances are directly related to the strength and type of forces existing between its molecules or formula units. This is described by Coulombs law: the force of attraction or repulsion between two particles is directly proportional to the magnitude of the charges and inversely related to the distance between them. Answer these questions with a partner: 1. Within an atom, list the forces that exist between charged particles and categorize them as
attractive or repulsive forces. 2. Using Coulombs law, explain why energy is required to move two positively charged particles closer together. 3. In your own words, describe what is happening on a particulate level during the melting of a solid. Explore (2): Do the Lab The purpose of the lab is for you to relate intermolecular forces (type, strength, etc.) to physical properties melting point, in this case. SAFETY ALERT: You must wear goggles the whole time. Do
not inhale the iodine. You MUST clean up your bin and unplug the hot plate. The next class must have a cold hotplate to start with. Complete the procedure, hypothesis, and data/observations. Do not move on. Explain: B. Intermolecular Forces (pg. 4) Intermolecular forces (type, strength, molecular polarity, etc.) determine physical properties. Intramolecular forces
(covalent bonds, ionic bonds) do not. NONPOLAR MOLECULES (ex. CO2, CH4) Neighboring molecules collide and distort the electron clouds surrounding them. The amount of distortion = polarizability. More # electrons = more distortion possible. Nonpolar Molecules (continued)
The collisions induce temporary dipoles to form. Temporary partial + and charges formed on neighboring molecules attract and form London dispersion forces. LDFs require the least amount of energy to disrupt (so theyre the weakest). Their Coulombic attractions are the weakest per mole. ALL molecules have London dispersion forces. It may not be the most important IMF, but its always present even if the molecule is polar. Consequences: Molecules with primarily LDFs are soft, waxy, have lowest boiling/melting points, cannot dissolve in water, cannot conduct electricity, and are usually gases/liquids are room temperature.
Do you get it? CO2 and CS2 are both nonpolar. Which of these molecules is more polarizable and thus has stronger London dispersion forces? Forming London Dispersion Forces is like a coffee commercial Each person is an electron Polar Molecules Polar molecules have permanent dipoles (partial + and charges).
Attraction between multiple permanent dipoles is called dipole-dipole force. More energy per mole is required to disrupt dipole-dipole force than London dispersion force. Coulombic attractions are stronger. Consequences: Relative to molecules with primarily LDFs, molecules with dipole-dipole force have higher melting/boiling points, dissolve more readily in water (since they can now form attractive IMFs with polar water molecules), and conduct electricity when dissolved.
Polar Molecules When H is bonded to a highly EN atom like N, O, or F, the electron cloud around H is highly distorted. Molecules with H-N, H-O and H-F bonds inside have very strong permanent partial charges (dipoles). The H of one molecule can now attract the
N, O, or F inside another molecule if its bonded with a second H. This type of IMF has its own name = hydrogen bonding. Polar Molecules Consequences: Molecules with primarily hydrogen bonding as their main IMF have the highest melting/boiling points, are the most soluble in water, and conduct the most electricity when dissolved. Coulombic attractions are the strongest here
because the (partial) charges are the strongest. Be careful! The effects of these forces can override and interfere with each other! Do you get it? Iodine, I2, has a melting point of 114. Ice melts at 0. Use your . Ice melts at 0. Ice melts at 0. Use your . Use your knowledge of intermolecular forces to explain why. Which will have a higher boiling point, methanol (CH 3OH) or ethanol (C2H5OH)? Explain, using your knowledge of IMFs. Now
Go back to your lab and complete the Analysis and Conclusions Questions. Check your answers with several partners. Feel free to ask Mrs. Wilson any questions ;) Lets Talk Vapor Pressure (pg. 6) Vapor pressure = the pressure exerted by vapor molecule above its liquid in a closed
container Molecules with primarily LDFs = highest vapor pressure (these molecules easily vaporize) Higher temp = higher KE = molecules vaporize more easily Ionic Compounds One formula unit consists of an ionic bond between positive cations and negative anions in a defined
ratio so that the net charge on the formula unit = 0. They do NOT have dipoles; they have charges. Ex. Na2CO3 - one formula unit contains two Na+ ions and one CO32- ion This electrostatic attraction between formula units in the same crystal is VERY strong and requires the MOST energy to disrupt. Consequences: Ionic compounds are hard and brittle (force causes repulsion between adjacent, identical charges), dissolve easily in water (the ions form attractive forces with
water), conduct electricity when melted or dissolved (ions are now free to move) and have very very high melting and boiling points. Lets Talk Vapor Pressure (pg. 6) Molecules with primarily LDFs = highest vapor pressure (these molecules easily vaporize) Since liquids w/ high VPs already have a lot of vapor molecules
present it doesnt take a lot of energy to cause boiling and interrupt the IMFs. High VP = Low BP London dispersion forces are present in all four of the compounds from question 1. It is just the most important force in compound c). Hydrogen
bonding Dipoledipole London dispersion force Ionic bonding HF should have the highest boiling point. Neighboring molecules show hydrogen bonding (the H of one molecule
attracts with the F of a second molecule) and these permanent dipoles exert the greatest amount of Coulombic attraction. Thus they require the greatest amount of energy to cause boiling in order to interrupt the attractions. HF has the lowest vapor pressure as well. The stronger the IMF, the greater the heat of vaporization or heat of fusion. Stronger IMFs (= greater Coulombic attractions) require the absorption of more energy to interrupt the attraction between the molecules. This means substances with predominantly hydrogen bonding have the highest heats of vaporization or fusion. Answers to 6.1 Homework
All three liquids contain London dispersion forces. However, its most important in CH4. CH4 is a nonpolar molecule and its London dispersion forces are the weakest of all three intermolecular force types. These forces are formed from the temporary formation of dipoles between neighboring molecules, due to moving electrons causing distortion in the cloud. H2S is a polar molecule and contains dipole dipole forces. The S atom in H2S is a permanent negative dipole and the H atoms are permanent positive dipoles. The Coulombic attractions between the Hs of one molecule and the S of another will therefore be stronger than the attractions between two CH4 molecules. More energy is needed to interrupt the attraction between two H2S molecules. NH3 is a polar molecule, but two NH3 molecules will have hydrogen bonding between them. The
permanent negative dipole (N) in one NH3 molecule attracts the permanent positive dipole of another NH3 molecule (one of the Hs). Those Hs are in turn bonded to an N. Hydrogen bonded molecules have the strongest Coulombic attractions because these dipoles are the strongest, and need the most energy to cause boiling. Very similar to #1. H2 and C3H8 are both nonpolar molecules and London dispersion forces are the most important intermolecular force in both substances. Temporary induced dipoles exhibit the weakest Coulombic attractions between molecules. C3H8 is more polarizable because one molecule of it has more electrons than H2 does (more electrons = more electron cloud distortion = slightly stronger dipoles), so C3H8 has slightly stronger LDFs than H2 does. More energy is needed to melt C3H8 because its
LDF forces are a bit stronger. HF has hydrogen bonding (see previous answers). Permanent dipoles means stronger forces because the Coulombic attraction between the positive end of one HF (the H) and the negative end of another HF (the F) and more energy required to cause HF molecules to start moving, and cause melting. CsI is an ionic compound. It does not have dipoles it has full charges. The Coulombic attractions are the strongest in CsI by far. More/stronger attractions = more energy required to make the CsI formula units move past each other. Alcohol vaporizes more easily than water does (weaker Coulombic attractions between molecules; weaker intermolecular forces than water). The vapor pressure of the alcohol molecules over the surface of the alcohol/water becomes higher than if the liquid was pure water. Since quite a few molecules are
already vapor, just a little energy should be added to cause the rest of the mixture to boil. Pure water has a lower vapor pressure than the alcohol/water mix because water has hydrogen bonds, which are evidence of stronger Coulombic attraction between the water molecules and more energy should be absorbed by the water to cause boiling to occur. Salt (NaCl) has full charges, not temporary or permanent dipoles. Salt reduces the vapor pressure of the water its mixed with because its very attracted to the water molecules, and exerts more Coulombic attraction to the water molecules. Thus you need the salt/water mixture to absorb more energy than normal to increase the vapor pressure enough that it will match atmospheric pressure, interrupt the intermolecular forces present, and start to boil.
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