Unit 3 Forces Test
AB2E2, BP=2, LP=2, 104.5deg
V-shaped or bent- Always polar unless ionic
AB2E, BP=2, LP=1, 117deg
V-shaped or bent- always polar unless ionic
Boiling Point and Vapor Pressure
Vapor Pressures of liquids can vary depending on how strong the intermolecular forces are. "Volatile" describes liquids with high vapor pressures (easy to evaporate)
Why dipole-dipole interactions between polar molecules not important in the vapor phase?
When molecules are in the vapor phase they are moving to quickly and to far away from each other for the slight charges to attract.
Boiling
When the vapor pressure equals the atmospheric pressure. Occurs throughout the liquid.
AB2, BP=2, LP=0, 180deg
linear- non polar if atoms are same otherwise polar unless ionic *** ALSO works as B2 which means the bond has no central atom like O=O.
Along BC line:
rate at which liquid boils to form a gas = rate at which gas condenses to form a liquid
Along BD line:
rate at which solid melts to form a liquid = rate at which liquid freezes to form a solid
Along AB line:
rate at which solid sublimes to form a gas = rate at which gas condenses to form a solid
Normal boiling point (segment D)
"Normal" means at Standard Pressure - 1 atm, 760 mm Hg, or 101.3 kPa. The temperature at which a substance boils at 1 atm of pressure. For water = 100°C.
Normal freezing point (segment B)
"Normal" means at Standard Pressure - 1 atm, 760 mm Hg, or 101.3 kPa. The temperature at which a substance freezes at 1 atm of pressure. For water = 0°C
Why is "D" longer than "B"?
At segment "D" ALL of the IMF (Hydrogen Bonding, Dipole-Dipole, and/or London Disperson Forces must be overcome to get a gas. At segment "B" only some of the IMF's need to be overcome so it takes less energy
Why does the boiling point of a liquid vary with altitude?
Boiling occurs when the vapor pressure equals the atmospheric pressure which allows the particles to escape freely into the gas state. At higher altitudes, the atmospheric pressure is lower which means the liquid will boil at a lower vapor pressure. Because of this, the temperature of the liquid does not need to be as high to reach the necessary vapor pressure resulting in a lower boiling point.
Lewis Dot Structure
Central atom is usually the atom with the lowest electronegativity, CAN'T BE HYDROGEN, usually not a halogen, Carbon and nitrogen are mostly. Electrons are represented by dashes (2e-) or dots (e-). Each atom except H & He need octet (8e-). Bonding pairs(BP) are the number of atoms attached to central atom. Lone pairs(LP) is the pair of dots on central atom. NEVER PUT LPS ON HYDROGEN When working with Ions anions need more electron and cations need less electrons, Put charge outside of [ ].
How do the strengths of dipole-dipole forces compare with the strengths of typical covalent bonds?
Covalent bonds are much stronger because they involve a sharing of electrons. Dipole-dipole forces only involve a short-range attraction between weaker opposite charges.
Determining Polarity
Determined by the electronegativity difference and molecular shape. Steps Find electronegativity for each elements on table Find difference of electronegativity (largest-smallest) Evaluate the bond polarity, If difference is... Greater than 1.8 = Ionic 1-8-0.4 = Polar covalent Less than .4 = Non-polar covalent
How is the strength of dipole-dipole interactions related to the distance between polar molecules? Are dipole-dipole forces short range or long range forces?
Dipole-Dipole forces are short-range. The molecules need to be close enough together so the slight opposite charges can attract.
Kinetic Energy:
Energy of Motion. Temperature is a measure of Average KE so if temperature increases so does Kinetic Energy
Potential Energy:
Energy of Position or Stored Energy!
Explain the difference between the process of evaporation and of boiling.
Evaporation involves only the escaping of particles at the surface of the liquid where boiling involves the escaping of particles throughout the liquid. Evaporation can occur at almost any temperature but boiling requires a certain temperature/pressure to occur.
What conditions are necessary for hydrogen bonding to exist in a substance or mixture? Provide a molecular level sketch and label the hydrogen bonding.
Hydrogen atoms must be directly bonded to oxygen, fluorine or nitrogen atoms. The extreme dipoles created result in hydrogen bonding
Although the noble gas elements are monatomic and could not give rise to dipole-dipole forces or hydrogen bonding, these elements can still be liquefied or solidified. Explain.
If you change the conditions to allow London dispersion forces to draw them together then they can become liquid or solids.
What type of force must be overcome to melt a solid or vaporize a liquid?
Intermolecular forces need to be overcome to vaporize a substance. (Physical change)
intramolecular forces vs. intermolecular forces
Intramolecular forces are the bonds (Colavent, Ionic, & Metallic) inside the compound, while, Intermolecular forces are the magnetic attractive forces(Hydrogen Bonding, Dipole-Dipole, London Dispersion) between different molecules or individual atoms. Intramolecular Forces are stronger than Intermolecular Forces.
Distinguish between ionic and covalent bonds. Give an example with Lewis structures to show how the electrons are used in bond formation.
Ionic Bond: Metal bonds with a non-metal. There is a transfer of electrons. Covalent Bond: Bond between two non-metals. Electrons are shared between atoms.
Ionic compounds vs. Covalent compounds (Lab)
Ionic compound: Crystalline solids (mad of ions), high melting/boiling points, soluble in water not non-polar solutions, conductors of electricity Covalent Bond: Gases, liquids or solids (made of molecules), Low melting/boiling points, Soluble in non-polar liquids but not water, Poor conductor of electricity.
Hydrogen bonding (Intermolecular)
It is the strongest and a special case of Dipole-Dipole, Occurs when Hydrogen bonds to Fluorine(F), Oxygen(O), Nitrogen(N), very polar and small, the slightly positive end of one molecule is attracted to the slightly negative end of another molecule
Phase Diagram for Carbon Dioxide
Liquid phase is less dense than the solid phase. Positive slope on the line A-C
ionic BOND (INTRAmolecular)
Metal & Non-metal, electrons are transferred between atoms--> Ions formed, gains electron--> anion, loses electrons--> cation, oppositely charged ions attract each other to form a networks of ions(crystal) called ionic compounds(not molecules)
metallic bond (Intramolecular)
Metal & metal, Can combine in any ratio, not set chemical formula, Alloy-when 2 or more metal are mixed together, "Free" electrons flow between the cations, delocalized electrons, great conductors of electricity
What are Phase Changes?
No chemical bonds are broken, Takes energy to melt ice and vaporize water due to intermolecular forces, During the phase change from liquid to gas, some water molecules are still present as liquid water and some are found as water vapor, Temperature will begin to rise again when all of the molecules are in the gas phase, During cooling, heat will continue to be lost as molecules of water vapor condense into liquid water but the temperature will remain the same.
covalent bond (Intramolecular)
Non-metal & Non-metal, Electrons are SHARED between 2 atoms, The sharing can be equal(non-polar) or unequal(polar), sometimes electrons spend more time with one of the atoms with more electronegativity, this are molecules.
Principle Characteristics of Evaporation
Occurs among surface level molecules only. Occurs as a result of increases in kinetic energy among specific molecules
dipole-dipole (Intermolecular)
Occurs between polar molecules, attraction between the positive end of one molecule and negative end of another molecule
Principle Characteristics of Boiling
Occurs throughout a liquid. Occurs as a result of the vapor pressure of the liquid being equal to the atmospheric pressure.
AB6, BP=6, LP=0, 90deg
Octahedral- non polar if atoms are same otherwise polar unless ionic
London dispersion forces (Intermolecular)
Present in all molecules, LDF are only found in Non polar molecules or in a neutral atoms, They are momentarily uneven electron distribution that creates an instantaneous of temporary dipole gets a slight attraction between atoms. Occurs most in very large, vary slow & very closely packed atoms
Heating / Cooling Curve
Shows the temperature change that occurs as a substance is heated or cooled. Along Segments A,C,E the KE is increasing since the temperature is increasing. Along Segments B & D, as heat is added, the energy is used to overcome IMF, so PE is increasing.
AB4E2, BP=4, LP=2, 90deg
Square planar- non polar if atoms are same otherwise polar unless ionic
Intermolecular force properties and strength
Strongest to Weakest: Hydrogen Bonding, Dipole-Dipole, London Dispersion The stronger the IMF the higher the melting point, boiling point, viscosity, surface tension, solubility in water (polar substance.) The stronger the IMF the lower the vapor pressure, evaporation rate,
Evaporation and Vapor Pressure
Surface molecules of liquids will evaporate (or escape) when enough energy is absorbed. Some vapor molecules may condense (or be "pulled") back in to the liquid if they get close enough. Equilibrium is reached when the number of molecules evaporating equals the number of molecules condensing. Vapor Pressure is the pressure of the vapor present at equilibrium with its liquid. In this case, the process of evaporation and the process of condensation balance, and the amount of liquid and the amount of gas are unchanged.
AB4, BP=4, LP=0, 109.5deg
Tetrahedral- non polar if atoms are same otherwise polar unless ionic
Why does a sample of boiling water remain at the same temperature until all the water has been boiled away?
The energy added is going towards overcoming the intermolecular forces and not towards increasing the temperature. Once all of the particles have changed state, the temperature will begin to rise again.
The normal boiling point of water is unusually high, compared to the boiling point of H2S or H2Se. Explain this observation in terms of hydrogen bonding that exists in water but that doesn't exist in the other compounds
The hydrogen bonds in water are stronger than the Dipole-Dipole attractions in the other substances. It takes more energy(heat) to release molecules from the hydrogen bonds than the dipole-dipole attractions.
Explain how the process of vaporization and condensation represent an equilibrium in a closed container.
The number of particles escaping into the gas phase (evaporating) is equal to the number of particles returning to the liquid phase (condensing).
Evaporation
The process of surface level molecules in a liquid absorbing energy and overcoming the strong intermolecular forces on the surface of the liquid to become a vapor (gas)
Why is the magnitude of a liquid's vapor pressure related to its intermolecular forces?
The weaker the intermolecular forces between the particles in a liquid, the higher the vapor pressure. This occurs because more particles are able to overcome the intermolecular forces and vaporize at a given temperature which results in the higher vapor pressure.
Vapor Pressure diagrams
To compare liquids on a diagram pick a pressure and find the temperature of each substance at that given vapor pressure. The substance with the strongest IMF will be the one with the highest temperature and the substance with the lowest IMF have the lowest temperature.
AB5, BP=5, LP=0, 120 & 90degs
Trigonal Bipyramid- non polar if atoms are same otherwise polar unless ionic
AB3, BP=3, LP=0, 120deg
Trigonal Planar- non polar if atoms are same otherwise polar unless ionic
AB3E, BP=3, LP=1, 107deg
Trigonal pyramid- Always polar unless ionic
Phase Diagram for Water
Triple Point: Temperature and pressure at which the solid, liquid and vapor phases of a substance can coexist at equilibrium. Critical Pressure: Lowest pressure at which the substance can exist as a liquid at the critical temperature. Critical Temperature: The temperature above which the substance can't exist in the liquid state. Critical Point: Point that indicates the critical temperature and critical pressure. Remember that the "normal" in normal boiling point and normal freezing point means at 1 atm pressure. Solid phase is less dense than the liquid phase. This is indicated by the negative slope of line A-C
Phase diagram
a graph showing the conditions at which a substance exists as a solid, liquid, or vapor S-->L = Melting L-->S = Freezing L-->G = Vaporization/boiling G-->L = Condensation S--> G = Sublimation G-->S = Deposition
