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14.1 Molecular Substances Intermolecular Forces

14.1 Molecular Substances Intermolecular Forces. Forces between molecules determine the melting and boiling point of substances. Ionic compounds have high melting and boiling points due to their electrostatic attraction. Molecular substances are held together by one of three forces. Dispersion

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14.1 Molecular Substances Intermolecular Forces

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  1. 14.1 Molecular SubstancesIntermolecular Forces • Forces between molecules determine the melting and boiling point of substances. • Ionic compounds have high melting and boiling points due to their electrostatic attraction. • Molecular substances are held together by one of three forces. • Dispersion • Dipole • Hydrogen Bond

  2. Dispersion Forces • Dispersion forces, also called London Forces, are a result of the temporary location arrangement of the electrons. • Electrons have a probability of being located on one side of the atom. • This side of the atom becomes temporarily charged negative while the other side becomes positive. • This is a temporary situation • This is the weakest of the forces

  3. Temporary Dipole

  4. Dipole Forces • Polar molecules have a permanent asymmetrical distribution of electrons. • Although the total molecule is neutral, it has a positive and negative side. • The opposite attraction between +/- sides of the molecule is permanent and thus will cause polar substances to have a higher melting and boiling point than non-polar substances. • Dipole forces are the next strongest.

  5. Forces Between Polar Molecules

  6. Hydrogen Bonding • The strongest intermolecular force is a subset of dipole forces. • If hydrogen is bonded to oxygen, nitrogen, or fluorine the molecule has the strongest of polarities. • This is called a hydrogen bond. • The hydrogen is highly positive and the negative side is similarly negative. • These substances have the highest melting and boiling points.

  7. Hydrogen Bonding in Water

  8. Unique Properties of Water • Survey • Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth. • Water freezes at 32o Fahrenheit (F) and boils at 212o F. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice floats. • Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans. • Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.

  9. Homework • Terms Chapter 14 • Come tomorrow with a unique water fact • Pg 514 [1-3, 7,8,10-13]

  10. Heating Curves • A heating Curve is a graphical representation of the phase change of a substance.

  11. Heating Curves • A to B is heating the solid • B to C is melting the solid • C to D is heating the liquid • D to E is vaporizing the liquid • E to F is heating the gas

  12. Heating Curves • The amount of energy from B to C and from D to E is dependent on the intermolecular forces between atoms or molecules. • The slope of AB or CD or EF depends on the specific heat of the substance.

  13. Evaporation & Vapor Pressure • Evaporation is a natural process in which a liquid converts to a gas in an endothermic process. • H2O(l) → H2O(g) ∆H = + • Evaporation is a cooling process • The energy is supplied from the environment. • Fun Fact! Lake Tahoe evaporates 335 million of water daily

  14. If we can contain evaporation by enclosing our liquid source (think of placing a lid on a pot) a pressure would begin to build within the air space. • Eventually the pressure will cause condensation at the same rate as evaporation. • The pressure this happens at is called the vapor pressure of water. • At vapor pressure, Vp, the system is at dynamic equilibrium. • This pressure is above atmospheric pressure • Total Pressure = Vp + 1 atm.

  15. Boiling Point • The first bubbles to form are the air molecules (N2, O2) coming out of the solution. • Boiling occurs when the pressure formed within an air bubble is great enough to sustain the inward pressure of the water. • This occurs when the vapor pressure of the bubble is equal to atmospheric pressure.

  16. Boiling Points • B.P. decreases with altitude because atm pressure decreases. • You have to cook food different at altitude!

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