Chem 150 Unit 3 - Physical Properties & Intermolecular Interactions

Chem 150Unit 3 - Physical Properties & Intermolecular Interactions • When discussing the physical properties of molecules the discussion often focuses on topics such as melting points, boiling points and solubilities in various solvents. These are considered physical properties because they do not change the composition or identity of the molecules involved. At the heart of these discussions is what causes molecules to be attracted to and repelled by one another. This discussion is critical to our understanding biological systems.

States of Matter • For molecular substances there are basically three states or phases of matter. • Solids (s): Molecules are held in place by intermolecular interactions. • Liquids (l): Molecules are held next to one another by noncovalent, intermolecular interactions, however, these interactions are not strong enough to prevent the molecules from flowing past one another. • Gases (s): The intermolecular interactions are too weak to hold the molecules next to one another, so the molecules wander off on their own.

States of Matter Water vapor, H2O(g) Ice, H2O(s) Liquid water, H2O(l)

States of Matter • The strength and numbers of the noncovalent intermolecular interactions determine which state a molecular substance is in. • The predominant noncovalent interaction between water molecules is the hydrogen bond:

Questions (Clickers) • If you were interested in disrupting the noncovalent interactions between water molecules in ice you could: • Shine light on the ice. • Stick the ice in a freezer. • Hit the ice with a hammer. • Add heat to the ice.

States of Matter • These interactions can be disrupted by adding heat. • Adding heat increases the kinetic energy of the molecules • This is most readily observed with gases by looking at the Ideal Gas Law equation: • As the temperature of a gas increases, so does its kinetic energy.

States of Matter • Ideal Gas Law Simulation

States of Matter • A as heat is added to a molecular substance, it warms until reaching one of the phase transition temperature. • At that point the heat (kinetic energy) that is added to the substance is used to break the noncovalent, intermolecular interactions.

States of Matter • For a more detailed description of phase transitions, along with an animation of the process,see the Chem 150Elaboration - States of Matter

Questions (Clickers) • When heat is added to liquid propane, (CH3CH2CH3), it warms until reaching the boiling point, and then changes into a gas. Which of the following statements most accurately describes what is going on: • The added kinetic energy is causing the carbon and hydrogen atoms in propane to separate from one another. • The added kinetic energy is causing the disruption of both hydrogen bonds and London dispersion forces between the propane molecules. • The added kinetic energy is causing the disruption of the London dispersion forces between the propane molecules. • The added kinetic energy is causing the propane to change into methane gas.

Questions (Clickers) • When heat is added to liquid ethanol, (CH3CH2OH), it warms until reaching the boiling point, and then changes into a gas. Which of the following statements accurately describes what is going on: • The added kinetic energy is causing the carbon and hydrogen atoms to separate from one another. • The added kinetic energy is causing the disruption of both hydrogen bonds and London dispersion forces between the ethanol molecules. • The added kinetic energy is causing the disruptions of the London dispersion forces between the ethanol molecules. • The added kinetic energy is causing the ethanol to change into methane gas and water vapor.

Questions (Clickers) • Both propane (M = 44 g/mol) and ethanol (M = 46 g/mol) have comparable molecular weights. Which do you predict has the higher boiling point? Explain why. • They should have similar boiling points. • The ethanol should have the higher boiling point. • The propane should have the higher boiling point. • It is hard to tell without doing the experiment.

Enthalpy, Entropy and Free Energy • Energy is defined as the ability to do work. • The heat energy we have been talking about is also called Enthalpy (H) • It was used to do the work of breaking the noncovalent, intermolecular interactions present in solids and liquids. • When Enthalpy is put into an object, such as an ice cube, the change in Enthalpy for the ice cube increases. • (ΔH > 0). the Δ symbol means “change in”. • Changes in nature can be either spontaneous (favorable), or nonspontaneous (unfavorable).

Questions (Clickers) • When the ice melts from the surface of a pond on a warm spring day, this change is • spontaneous • nonspontaneous

Questions (Clickers) • When the ice forms on the surface of a pond on a cold winter day, this change is • spontaneous • nonspontaneous

Enthalpy, Entropy and Free Energy • Why are some changes spontaneous while others are nonspontaneous? • Why, like the ice on a pond, are changes spontaneous some of the time and nonspontaneous at other times? • Asking some questions about the energy changes that take place can to help answer theses questions

Enthalpy, Entropy and Free Energy • Changes occur spontaneously in nature when energy is released. • The case of the rolling stone.

Enthalpy, Entropy and Free Energy • Although Enthalpy is a form of energy, it alone cannot be used to answer these questions. • The melting of the ice from a pond on a warm spring day is spontaneous • However, the ice is absorbing heat (ΔΗ > 0) (endothermic) • A second factor called Entropy (S), needs to also be considered to determine if a change is spontaneous or nonspontaneous.

Enthalpy, Entropy and Free Energy • Entropy is a measure of disorder. • When ΔS > 0, things become more disorder. • Nature prefers things to be disordered:

Enthalpy, Entropy and Free Energy • Enthalpy and Entropy can be combined to calculate another type of energy called Free Energy (G). • ΔG = ΔΗ - ΤΔS • The change in Free Energy can be used to predict whether a change is spontaneous or nonspontaneous. • When ΔG < 0, the change is spontaneous (favorable) • When ΔG > 0, the change is nonspontaneous (unfavorable)

Enthalpy, Entropy and Free Energy • When Ice melts, ΔH > 0 and ΔS > 0 • It gains heat and becomes more disordered • Above the freezing temperature, TΔS > ΔH and ΔG is negative ( ΔG < 0) • Ice melts spontaneously. • Below the freezing Temperature, TΔS < ΔH and ΔG is positive ( ΔG > 0) • Ice does not melt spontaneously.

Enthalpy, Entropy and Free Energy • When Ice freezes, ΔH < 0 and ΔS < 0 • It loses heat and becomes more ordered • ΔH makes a negative contribution to ΔG • ΔS makes a positive contribution to ΔG • Below the freezing Temperature, the magnitude of TΔS < ΔH and ΔG is NEGATIVE (ΔG < 0) • Ice freezes spontaneously. • Above the freezing temperature, the magnitude of TΔS > ΔH and ΔG is positive (ΔG > 0) • Ice freezes nonspontaneously.

Enthalpy, Entropy and Free Energy • For a more detailed description using the enthalpy, entropy an free energy changes to predict if a process is spontaneous or not,see the Chem 150Elaboration - Ethalpy, Entropy & Free Energy

Enthalpy, Entropy and Free Energy • Figure 5.6, Raymond ΔS < 0 (molecules are more ordered) ΔS > 0 (molecules are more disordered)

Questions (Clickers) • Before going on a picnic on a hot sumer day, you stopby the store and pick up a block of dry ice, CO2(s). • In terms of ΔH alone, is the sublimation of dry ice • Spontaneous • Nonspontaneous

Questions (Clickers) • Before going on a picnic on a hot sumer day, you stopby the store and pick up a block of dry ice, CO2(s). • In terms of ΔS alone, is the sublimation of dry ice • Spontaneous • Nonspontaneous

Questions (Clickers) • Before going on a picnic on a hot sumer day, you stopby the store and pick up a block of dry ice, CO2(s). • At 50°C (This is one hot day!!) the ΔG for the sublimation of dry ice as a • negative value. • positive value.

Liquids • Liquids have various physical properties that reflect the strength of the intermolecular interactions that hold the liquid together • Boiling point temperature • Viscosity • Resistance to flow • Vapor pressure

Liquids • Viscosity • Resistance to flow

1 atm = 760 Torr Liquids • Vapor Pressure and Boiling Points are related • The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure.

Questions (Clickers) • You planning to do some surgery on your kitchen table and know that you need to sterilize your instruments by heating them to 120°C. You rummage around in the kitchen cupboards and find a pressure cooker that can heat water to a pressure of 1.4 atm. Will this be sufficient for sterilizing your instruments? (You may use Table 5.6 in your book to answer this question; see the previous slide.) • Yes • No • Explain you answer.

1 atm = 760 Torr Questions (Answer) • 1.4 atm (760Torr/atm) = 1064 Torr • This is less than the pressure required to reach 110°C (1075 Torr), therefore it is an insufficient pressure to reach 120°C. • (120-100)/(125-100)*(1741-760)+760=~1544 Torr (interpolation)

Solutions • Biological systems are mixtures of substances • Pure substances contain only one type of element or compound • They contain only one type of atom or molecule: • H2 • Hg • O2 • H2O • sucrose (C12H22O11) • Mixtures contain more than one type of pure substance • Heterogeneous mixture - components are not evenly mixed at the molecular level. • Homogeneous mixture - components are evenly mixed at the molecular level.

Solutions • A solution is another name for homogeneous mixture. • Solvent - the major component in a solution • Solute - the minor component in a solution.

Solutions • A solution is another name for homogeneous mixture. • Liquid solutions should be clear (transparent). • Liquid solution’s solutes should not settle with time • This distinguishes solutions from suspensions and colloids.

Solutions • In order form a solution to form • the solute molecules have to be able to form similar noncovalent interactions with the solute molecules as • the solute molecules form with themselves • the solvent molecules form with themselves.

States of Matter • Simulation of Glycerol and PropaneDissolving in Water

Solutions • Solubility is a measure of how much solute will dissolve in a solvent. • Solubility depends on temperature. • The solubility of gases decrease with increasing temperature • The solubility of solids and liquids usually increase with increasing temperature.

Solutions • When a solution is saturated, the solute dissolves and precipitates at the same rate.

Solutions • In order form a solution to form • the solute molecules have to be able to form similar noncovalents with the solute molecules as • the solute molecules form with themselves • the solute molecules form with themselves.

Solubility of Gases in Water • Henry’s Law - The solubility of a gas in a liquid is proportional to the pressure of the gas over the liquid. • The fizzing of soda when the cap is removed is an example of the lowered solubility of CO2 in water when it’s pressure above the soda is descrease. • The solubility of CO2 in water is very high, because it can react with water to produce and even more soluble product, H2CO3 (carbonic acid): • We will see that this is a very important reaction in biochemistry

Organic Compounds • Nonpolar, organic solutes will dissolve readily in nonpolar, organic solvents. • “Like dissolves Like”

Organic Compounds • The solubility is determined by the balance between the polar and nonpolar portions of the molecule.

Questions (Clicker) • Pentanoic acid and 1-pentanol have the same number of carbon atoms. Which one is expected to have the higher solubility in water? Explain. • Pentanoic acid • 1 Pentanol • Neither

Biochemical Compounds &Their Interactions with Water • Biological molecules are grouped into three categories. • Hydrophilic (water loving) molecules. • Polar molecules that can interact favorably with water • Hydrophobic (water fearing) molecules. • Nonpolar molecules that cannot interact favorably with water • Amphipathic molecules, which are conflicted about their feelings towards water. • Molecules containing both very polar and very nonpolar parts.

Biochemical Compounds &Their Interactions with Water • Hydrophilic (water loving) molecules. • Polar molecules that can interact favorably with water • Carbohydrates (sugars) have lots of polar hydroxyl groups

Biochemical Compounds &Their Interactions with Water • Hydrophilic (water loving) molecules. • Polar molecules that can interact favorably with water • Amino acids have both an amino and a carboxylic acid group, which are polar.

Biochemical Compounds &Their Interactions with Water • Hydrophobic (water fearing) molecules. • Nonpolar molecules that cannot interact favorably with water • The carboxylic acid groups, though polar, are dominated by the long hydrocarbon portions

Biochemical Compounds &Their Interactions with Water • Hydrophobic (water fearing) molecules. • A nonpolar solute "organizes" water • The H-bond network of water reorganizes to accommodate the nonpolar solute • This is an increase in "order" of water-This is a decrease in ENTROPY

Chem 150 Unit 3 - Physical Properties & Intermolecular Interactions