lOMoARcPSD|8751011 CHM2046 12.2-12.7 Textbook Notes Chemistry Fundamentals II (University of Central Florida) StuDocu is not sponsored or endorsed by any college or university Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 12.2 - Solids, Liquids, and Gases: A Molecular Comparison ● General properties of the states of matter: ● Properties of water: ○ ● ● Water as a solid is less dense than as a liquid, which is atypical ■ Due to crystal structure of ice Solids can be either amorphous, where atoms/molecules have no long-range order, or crystalline, where they are well-ordered in a 3D array Pressure changes can be used in addition to temperature can be used to change the state of a substance, with higher pressure generally favoring a denser state of matter Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 12.3 - Intermolecular Forces: The Forces That Hold Condensed States Together ● Intermolecular forces result from interactions among charges, partial charges, and temporary charges on molecules (or atoms and ions) ● Coulomb’s Law: ● ● ● ● E=potential energy, q1 and q2=opposite particle charges, r=separation of particles Dispersion force (London force) is found between all atoms and molecules, resulting from fluctuations in electron distribution ○ An instantaneous or temporary dipole occurs when electrons aren’t symmetrically situated around the nucleus, causing a slight negative or positive charge depending on the side of the atom ○ Dispersion force occurs when negative side of an atom and the positive side of another atom attract each other ○ A larger electron cloud results in a greater dispersion force, resulting in easier polarization ■ As molar mass (and electron cloud volume) increases, there is greater dispersion force, causing a higher boiling point for similar atoms and compounds Dipole-dipole force occurs between all polar molecules, resulting from uneven charge distribution ○ Polar molecules have electron-rich and electron-poor regions depending on electronegativity ○ A molecule with a permanent dipole always has a slightly negative charge on one end and a slight positive charge on the other ○ This force between polar molecules results in them having higher melting and boiling points than nonpolar molecules of similar molar mass ○ Miscibility is the ability for liquids to mix without separating into two phases ■ Depends on polarity, polar mixes with polar Hydrogen bonding occurs when polar molecules containing hydrogen bonded to small electronegative atoms (ie F, O, N) causes the hydrogen to have a positive charge, attracting it to nearby electronegative atoms of other molecules ○ Not nearly as strong as covalent bonds, but they are the strongest of the above intermolecular forces ○ Further increase boiling point Ion-dipole force occur when an ionic compound is mixed with a polar compound ○ Stronger than all of the above forces ○ Reason for the ability of ionic substances to form solutions in water Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 12.4 - Intermolecular Forces in Action: Surface Tension, Viscosity, and Capillary Action ● Surface tension is the the energy required to increase the surface area of a liquid by a unit amount ○ Liquids have a tendency to minimize their surface area ○ ie water has a surface tension of 72.8 mJ/m2 - takes 72.8 mJ to increase surface area by one square meter ○ Surface tension decreases as intermolecular forces decrease ● Viscosity is the measure of a liquid’s resistance to flowing ○ Viscosity increases as intermolecular forces increase ○ Also depends on molecular shape, increasing in longer molecules which can tangle ○ Depends on temperature, thermal energy partially overcomes intermolecular forces, allowing molecules to flow past each other ● Capillary Action is the ability of a liquid to flow against gravity up a narrow tube due to cohesive (between molecules in liquid) and adhesive (between molecules and tube) forces ○ If adhesive>cohesive, the liquid rises until gravity balances against it Causes concave meniscus ○ If adhesive<cohesive, the liquid does not rise Causes convex meniscus Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 12.5 - Vaporization and Vapor Pressure ● Vaporization is the phase transition from liquid to gas ○ Rate increases with increasing temperature, increasing surface, and/or decreasing strength of intermolecular forces ○ Volatile liquids tend to vaporize easily ○ Nonvolatile liquids do not vaporize easily ○ Endothermic process - it requires energy to vaporize liquid molecules (overcome intermolecular forces) ● Condensation is the phase transition from gas to liquid ○ Exothermic - heat is released when gas condenses to a liquid ● Heat (or enthalpy) of vaporization ( Δ Hvap ) is the amount of heat required to vaporize 1 ● ● mol of a liquid to a gas ○ Dependent on beginning temperature, as a liquid could start with varying amounts of thermal energy ○ Always positive, as process is endothermic Dynamic equilibrium is the point at which the reverse reaction or process equals the rate of the forward reaction or process ○ Liquid molecules evaporate in sealed container, so their partial pressure increases, causing some to start condensing into a liquid, with vaporization and condensation rates eventually balancing ○ The pressure of a gas in dynamic equilibrium with its liquid is its vapor pressure ■ Depends on strength of intermolecular forces as well as temperature ● Volatile substances (weak forces) have high vapor pressures at a given temp ● Nonvolatile substances (strong forces) have low vapor pressures at a given temp ○ A liquid in dynamic equilibrium with its vapor tends to return to equilibrium if disturbed ■ ie increase in volume causes more vaporization to return to equilibrium, decrease causes gas to condense to return to equilibrium ○ When the temperature of a liquid increases, its vapor pressure also increases Boiling point is the temperature where the vapor pressure of a liquid equals the external pressure ○ Normal boiling point is the temperature where a liquid’s vapor pressure equals 1 atm ■ Depends on atmospheric pressure ○ Once the boiling point has been reached, additional heating causes more rapid boiling, not raising the liquid’s temperature Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 ● The relationship between vapor pressure and temperature is exponential ○ The Clausius-Clapeyron equation expresses this ● ● ● ● Pvap is vapor pressure, β is a gas-dependent constant, Δ Hvap is the heat of vaporization, R is the gas constant (8.314 J/mol K), T is the temperature in K Gives linear relationship between natural log of vapor pressure and inverse of temperature Two-point form: The Critical Point ○ The critical temperature (Tc) is the temperature above which a liquid cannot exist, regardless of pressure ○ The critical pressure (Pc) is the pressure required to bring a transition to a liquid at the critical temperature Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 12.6 - Sublimation and Fusion ● Sublimation is the phase transition from solid to gas ● Deposition is the phase transition from gas to solid ● Melting (fusion) is the phase transition from solid to liquid ○ Once the melting point of a solid is reached, additional heating only results in more rapid melting, not raising a solid’s temperature over its melting point ● Freezing is the phase transition from liquid to solid ● Heat of fusion ( Δ Hfus) is the amount of heat required to melt 1 mol of a solid ○ ○ ● Since melting is endothermic, heat of fusion is positive The change of enthalpy for freezing has the same magnitude of the heat of fusion but the opposite sign ○ Generally significantly less than heat of vaporization, as solid and liquid states are closer together than gases are ■ Less energy to melt 1 mol of solid than to vaporize 1 mol of that liquid Heat (or enthalpy) of sublimation ( Δ Hsub ) is the amount of heat required to sublime 1 mol of solid into a gas ○ Sum of the heat of fusion and heat of vaporization 12.7 Heating Curve for Water Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com) lOMoARcPSD|8751011 ● Segment 1 is ice warming, 2 is ice melting into liquid water, 3 is liquid water warming, 4 is liquid water vaporizing into steam, 5 is steam warming ○ The temperature is constant in segments 2 and 4 because additional heat goes into phase transition, not temperature increase ■ q = n • Δ H, n=mol ○ The temperature increases linearly in segments 1, 3, and 5, according to a substance’s specific heat capacity ■ q = m • Cs • Δ T, m=grams Downloaded by Hassan Sobhie (hassan.sobhie@nahdaschools.com)
