Intermolecular Forces Lesson (PDF)

Intermolecular Forces This Photo by Unknown Author is licensed under CC BY-SA WHY CAN A PAPERCLIP FLOAT ON WATER? Surface tension exists in water because water molecules stick to each other. This tension is so strong that when you gently lay a paperclip onto the water, it sits on top of th...

Intermolecular Forces This Photo by Unknown Author is licensed under CC BY-SA WHY CAN A PAPERCLIP FLOAT ON WATER? Surface tension exists in water because water molecules stick to each other. This tension is so strong that when you gently lay a paperclip onto the water, it sits on top of the water instead of sinking into it. It is the high surface tension of water that allows the paperclip, with much higher density, to float on water.  This Photo by Unkno wn Author is licensed under CC BY This Photo by Unkno wn Author is licensed under CC BY-SA-NC The surface tension of water also helps propels water- striding insects on the surface of ponds. Intermolecular Forces (IMF) This force holds the molecules together which can be attractive or repulsive. Intermolecular forces are responsible for most of the physical and chemical properties of matter.  Intermolecular Force and Boiling Point By comparing the boiling points of different substances, we can compare the strengths of their intermolecular forces. This is because the heat absorbed by the substance at its boiling point is used to break these intermolecular forces and to convert the liquid into vapour. This Photo by Unkno wn Author is licensed under CC BY-NC-ND  Types of Intermolecular Forces 1. Ion-ion Interactions 2. Dipole-dipole interaction 3. Hydrogen Bonding 4. Dispersion Forces or London Forces 1. Ion-ion Interactions Exists between oppositely charged ions. It occurs between ionic compounds. Most ion-ion interaction is strong compounds which have high melting and boiling points. This Photo by Unkno wn Author is licensed under CC BY-SA The compound orients itself in such a way as to minimize repulsion. Ion-ion interaction is the strongest intermolecular forces. This Photo by Unknown Author is licensed under CC BY-SA-NC  Ion-ion Interaction These interactions are most significant in the solid state. When dissolved in water, ions are shielded from one another by water molecules, making ion- ion interactions less prevalent. Because each ion has a full positive or negative charge, the forces holding two ions together are relatively strong. 2. Dipole-dipole interaction Occurs between polar molecules. This is due to the partial positive pole and the partial negative pole of the molecule. Average dipole-dipole interaction is relatively weak. This interaction is effective over a very short range. Dipole-dipole interactions occur in HCl molecules Chlorine is comparatively more electronegative than hydrogen and it, therefore, acquires a partial negative charge (whereas hydrogen acquires a partial positive charge). The dipole-dipole interaction then takes place between the HCl molecules. Chlorine EN=3.0 Hydrogen=2.1 This Photo by Unknown Author is licensed under CC BY-SA Electronegativity Both attraction and repulsion occur simultaneously. This Photo by Unknown Author is licensed under CC BY-SA-NC  3. Hydrogen Bonding Hydrogen bond is a very strong dipole-dipole interaction. Hydrogen bond occurs in polar molecules containing H and any one of the highly electronegative elements, in particular F, O, and N. Hydrogen tends to be strongly positive due to the strong tendencies of F, O, or N to attract the electron towards itself. The highly electronegative elements make hydrogen strongly positive. This Photo by Unkno wn Author is licensed under CC BY-NC Hydrogen bonding is responsible for the unusually high boiling point and melting point of water as compared to compounds of similar molecular weight and geometry. Water/Boiling point- 100 °C Compound Formula Boiling Point ethanol CH3CH2OH 78ºC propanol CH3(CH2)2OH 98ºC diethyl ether (CH3CH2)2O 34ºC propyl amine CH3(CH2)2NH2 48ºC Water as a Universal Solvent The ability of water to form bonds relates to its ability to be a universal solvent. Due to its polarity, it is able to dissolve (or interact) with ionic compounds and polar molecules. Evaporation of water Hydrogen bonding is also very important to life in general as the H- bonding prevents water from quickly evaporating into the atmosphere. This Photo by Unknown Author is licensed under CC BY-SA-NC In freezing temperature The H-bonding causes the water molecules to form a crystal lattice thereby increasing its volume. This is why ice floats in water. This prevent the water from cooling down further as the ice sheet acts as a protective layer.  4. Dispersion Forces or London Forces Dispersion force is present in all molecules. It is the only force present in nonpolar molecules. It is very weak and acts in very small distances. It is formed due to the attraction between the positively charged nucleus of an atom with the negatively charged electron cloud of nearby atom. This Photo by Unkno wn Author is licensed under CC BY-SA-NC Without dispersion forces substances would not be able to condense to liquid and solid phase. Molecules without dipole can condense to form liquids or solids as a result of London dispersion forces. Dispersion force slows down the motion of the atoms and locks the atoms into a place to produce solid, at very low temperatures. Without dispersion forces, noble gases wouldn't be able to become liquids, since there are no other intermolecular (between molecules/atoms) forces acting on them. As the molecular weight of molecule increases (which also corresponds to an increased number of electrons), the polarization increases due to dispersion forces. Sometimes, dispersion forces can be stronger than dipole-dipole interaction or even H- bonding. This Photo by Unknown Author is licensed under CC BY-SA-NC  Properties of Matter in Relation to Intermolecular Forces Surface Tension A phenomenon caused by cohesive forces (intermolecular forces) between molecules allowing liquids to create a thin film on its surface. This causes liquids to acquire a certain shape when put on container or dropped on surfaces. Basilisk Lizard or Jesus Lizard being able to walk in water by virtue of surface tension.  Surface tension This Photo by Unknown Author is licensed under CC BY This Photo by Unknown Author is licensed under CC BY-NC Water Strider Fish Spider Formation of Meniscus Concave Meniscus Occurs when there is stronger adhesive force between the container and the liquid than the liquid’s molecule. The adhesive force overcomes the cohesive force of the liquid. This causes the liquid to climb up the sides of the container. Adhesion-interaction between different materials touching each other. Ex. Tape and paper, Skin and glass Cohesion-interaction between same materials. Formation of Meniscus Convex Meniscus Occurs when there is stronger cohesive force between the liquid’s molecules than the adhesive force between the liquid and the container. This causes the liquid to create a dome shape on its surface. Adhesion-interaction between different materials touching each other. Ex. Tape and paper, Skin and glass Cohesion-interaction between same materials. This Photo by Unkno wn Author is licensed under CC BY-SA Water and Mercury  Viscosity Viscosity is the resistance of a liquid to flow. The more viscous a liquid, it has a thicker consistency. In layman’s term, it is the measure of the thickness of a liquid. The stronger intermolecular forces leads to higher viscosity.  Capillary Action A phenomenon wherein a liquid is able to rise up on a narrow tube. Adhesive forces between the tube and the liquid allow the liquid to exceed its height. The narrower the tube, the This Photo by Unkno wn Author is licensed under CC BY-SA-NC higher the liquid will reach.  1st Performance Task for 2nd quarter (by group) Fact sheet or Poster-Slogan 1. Water and the Earth. How is water stored? (polar ice, underground, sea water, atmosphere). How much water does the Earth have? How does water shape land? 2. Water and the human body. How does a human being’s water content change throughout his life? (fetal stage, at birth, adulthood). How much water do we consume in our lifetime? 3. The triple point of water 4. Water and agriculture 5. Why is water called the universal solvent? 6. Water and electricity production 7. Water and religion/ myths 8. The different shapes of water (solid, liquid, ice, snow) 9. Water and its high specific heat capacity 10. Water and its availability to all Criteria for Fact Sheet CRITERIA PERCENTAGE Title 20% Appropriateness to the given topic Relevance to the given topic 40% Exact information related to the topic given Content 20% Clear, Concise, and easy to understand Organization 20% Information is arranged accordingly TOTAL 100% Criteria for Poster-Slogan CRITERIA PERCENTAGE Relevance to the given topic 40% All graphics are related to the topic and make it easier to understand. Attractiveness 20% Exceptionally attractive in terms of design, layout, and neatness. Creativity 20% Several graphics used on the poster reflect an exceptional degree of student’s creativity. Slogan 20% Has required number of words, it is original and catchy. TOTAL 100% Submission/Presentation: November 21, 2024 Reference: Whitten, K. W., Davis, R. E., Peck, M. L., & Stanley, G. G., (2005). General Chemistry 7th ed. Singapore: Thomson/ Brooks/ Cole.