Chemistry Notes v Polarity Experiment Ø Things involved Polarity Solubility Dispersion Ø Polarity Shaving cream has soap steric acid Water is polar Food coloring is polar/ionic because dissolved Like dissolves like Paper Shaving cream soap has Non- polar tail Polar head Paper has some polarity Made of cellulose Ø Hydroxyl group O- H F- H N- H Can form hydrogen bonds Bonds really a force between atoms Intra- molecular force v Gas Ø Increase in temperature proportional to kinetic energy Ø Can be made into liquid Decrease temperature Increase pressure v Bonging Ø Elements neutral v Types of forces Ø Intra- molecular forces (bonds) Stronger Ionic Covalent (polar and non- polar) Metallic Ø Intermolecular forces IMFs Weaker Highest to lowest Ion- dipole Hydrogen bond Dipole- dipole London dispersion Non- polar only Abby Carroll 1
Definition: the attractive forces between molecules and atoms that allows them to condense into liquids and solidify into solids v Liquids and solids Ø Liquids and solids are distinctly different from gasses More dense than gasses (1-20 g cm 3 ) compared to gas densities (@ STP) (10-2 - 10 4 g cm 3 ) Ø Presence of significant attractive forces in liquids and solids Ø Lack of significant attractive forces in gases Ø Physical differences between the 3 states of matter are explained on the basis of IMFs v Ionic bonds Ø Metal and non- metal (also polyatomic ions) Ø Electrons are lost (by metal atoms) and gained (non- metal atoms) Ø Metal full positive charge(s) Ø Non- metal full negative charge(s) Ø Ionic bond complete transfer of one or more valence electrons Full charges on resulting ions v Covalent bonds Ø Two non- metal elements Sharing of electrons Equal sharing = non- polar Unequal sharing = polar Non- polar covalent bond bonding electrons shared equally between two atoms, no charges on atoms Electron density Polar covalent bond bonding electrons shared unequally between two atoms, partial charges on atoms v Intermolecular attractions Ø The strength of the attraction between the particles of a substance determines its state (gas, liquid, solid) Ø At room temperature moderate to strong attractive forces result in materials being solids or liquids Ø The stronger the attractive forces are, the higher will be the boiling point of the liquid and the melting point of the solid Other factors also influence melting point v Why molecules are attracted to each other Ø Intermolecular attractions are due to attractive forces between opposite charges Positive ion to negative ion Positive partial end of molecule and negative partial end of polar molecule Only for polar molecules Q1Q2/d One Q partial negative, the other partial negative Ø H- bonding particularly strong Abby Carroll 2
Even non- polar molecules will have temporary changes Larger charges = stronger attraction Longer distance = weaker attraction These attractive forces are small relative to the bonding forces between atoms Generally smaller charges Generally over much larger distances v Trends of the strength of intermolecular attraction Ø The stronger the attractions between the atoms or molecules the more energy will be take to separate them Ø Boiling a liquid requires that we add enough energy to overcome all the attraction between the particles Not breaking covalent bonds Ø The higher the normal boiling point of the liquid the stronger the intermolecular attractive forces v Kinds of attractive forces Ø Temporary polarity in the molecules due to unequal electron distribution leads to attractions called dispersion forces Ø Permanent polarity in the molecules due to their structure leads to attractive forces called dipole- dipole attractions Ø An especially strong dipole- dipole attraction results when H is attached to an extremely electronegative atom these are called hydrogen bonds v Dipole- dipole attractions Ø Polar molecules have a permanent dipole Bond polarity and shape Dipole moment The always resent included dipole Ø The permanent dipole adds to the attractive forces between the molecules raising the boiling and melting points relative to non- polar molecules of similar shape and size v Dispersion forces Ø Fluctuations in the electron distribution in the atoms and molecules result in a temporary dipole Region with excess electron density has partial negative charge Region with depleted electron density has partial positive charge Ø The attractive forces caused by these temporary dipoles are called dispersion forces Also known as London forces Ø All molecules and atoms have them Ø As a temporary dipole is established in one molecule it induce a dipole in all the surrounding molecules v Size of the induced dipole Ø The magnitude of the induced dipole depends on several factors Ø Polarity of the electrons Volume of the electron cloud Abby Carroll 3
Larger molar mass = more electrons = larger electron cloud = increased polarized ability = stronger attractions Ø Shape of the molecule More surface- to- surface contact = larger induced dipole = stronger attraction v Effect of molecular size on size of dispersion force Ø The noble gasses are all non- polar atomic elements Ø The stronger the attractive forces between the molecules the higher the boiling point will be Ø As the molar size increases the number of electrons increases therefore the strength of the dispersion force increases Ø The stronger the attractive forces between the molecules the higher the boiling point will be v Alkane boiling points Ø Branched chains have lower Boiling points than straight chains Ø The straight chain isomers have more surface- to- surface contact v Attractive forces and solubility Ø Solubility depends in part on the attractive forces of the solute and solvent molecules Like dissolves like Miscible liquids will always dissolve in each other Ø Polar substances dissolve in polar solvents Hydrophilic groups: OH CHO C=O COOH NH2 Cl Ø Non- polar molecules dissolve in nonpolar solvents Hydrophobic groups C- N C- C Ø Many molecules have both hydrophilic and hydrophobic parts stability in water becomes a competition between the attraction of the polar groups for the water and the attraction of the non- polar groups of their own kind v Immiscible liquids Ø Pentane C5H12 is a nonpolar molecule Ø Water is a polar molecule Ø The attractive forces between the water molecules is much stronger than their attractions for the pentane molecules the result is that the liquids are immiscible v Hydrogen bonding Ø When a very electronegative atom I bonded to hydrogen it strongly pulls the bonding electrons toward it O- H N- H F- H Ø Because hydrogen has no other electrons when its electron is pulled always the nucleus becomes de- shielded exposing the H proton Ø The exposed proton acts as a very strong center of positive charge attracting all the electron clouds from neighboring molecules Ø Hydrogen bonding is very strong intermolecular attractive forces Stronger than the dipole- dipole or dispersion forces Ø Substance that can hydrogen bond will have higher boiling melting points than similar substances that cannot Abby Carroll 4
Ø But hydrogen bonds are not nearly as strong as chemical bonds 2-5% the strength of covalent bonds v Boiling points of group 4A and group 6A compounds Ø HF H2O and NH3 have hydrogen bonds therefore they have higher boiling points than would be expected from the general trends Ø For non- polar molecules such as the hydrides of group 4 the intermolecular attractions are due to dispersion factors therefore, they increase down the column causing the boiling point to increase Ø Polar molecules such as the hydrides of group 5-7 have both dispersion forces and dipole- dipole attraction therefore they have higher boiling points than the corresponding group 4 molecules v Ion- dipole attraction Ø In a mixture ions from an ionic compound are attracted to the dipole of polar molecules Ø The strength of the ion- dipole attraction is one of the main factors that determines the solubility of ionic compounds in water v Summary Ø Dispersion forces are the weakest of the intermolecular attractions Ø Dispersion forces are present in all molecules and atoms Ø The magnitude of the dispersion forces increase with molar mass Ø Polar molecules also have dipole- dipole attractive forces Ø Hydrogen bonds are the strongest of the intermolecular attractive forces a pure substance can have Ø Hydrogen bonds will be present when a molecule has H directly bonded to either O N of F atoms The only example of H bonded to F is HF Ø Ion- dipole attractions are present in mixtures of ionic compounds with polar molecules Ø Ion- dipole attractions are the strongest intermolecular attraction Ø Ion- diploe attractions are especially important in aqueous solutions of ionic compounds v Re- cap Ø Dispersion (or London) forces Weak forces that are present between any two atoms or any two molecules Due to attraction of the electrons of one molecule to the nuclei of another May be the ONLY force present if no polarity exists. Ø Dipole-dipole forces Ordinary: any polar molecule would exhibit these Polarity requires knowledge of electronegativity Polarity requires knowledge of shape Hydrogen bonds: special molecules that have N-H, O-H, or F-H bonds see boiling points here. Ø Ionic forces Refers to the very strong interactions that exist in solids formed from metal and non-metal. v Bonding Abby Carroll 5
Ø Strongest to weakest Ion-dipole Ion Polar Hydrogen bonds F-H O-H N-H Polar Dipole-dipole Polar Permanent dipole Dispersion All Temporal or induced dipole v VSEPR Ø Gives us information about the 3-D shape of a molecule Ø It allows us to determine the overall polarity of a molecule v Dispersion Ø Distortion of electron cloud is called polarizability v Determining the polarity of a molecule Ø Write Louis dot structure (2-D) Are there any polar bonds Ø Use VSEPR theory to determine 3-D electrons geometry 3-D molecular geometry Polarity of molecule Not dipole moment v How can you tell its polar Ø Are there any polar bonds No non- polar Ø Are there any lone pairs Yes polar Ø Look at symmetry v Objectives Ø Predict properties of substances based on their chemical formula When the electronegativity is not equal, electrons are not shared equally and partial ionic charges develop. The greater the electronegativity difference, the more ionic the bond is. Bonds that are partly ionic are called polar covalent bonds. Nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativity of the two atoms are equal. A bond between 2 nonmetal atoms that have the same electronegativity and therefore have equal sharing of the bonding electron pair Polar covalent bond Abby Carroll 6
A bond between 2 nonmetal atoms that have different electronegativity and therefore have unequal sharing of the bonding electron pair The result is a bond where the electron pair is displaced toward the more electronegative atom. This atom then obtains a partialnegative charge while the less electronegative atom has a partialpositive charge. This separation of charge or bond dipole can be illustrated using an arrow with the arrowhead directed toward the more electronegative atom. Within a molecule each polar bond has a bond dipole A polar molecule always contains polar bonds, but some molecules with polar bonds are nonpolar. Polar molecule A molecule in which the bond dipoles present do not cancel each other out and thus results in a molecular dipole Cancellation depends on the shape of the molecule and the orientation of the polar bonds. Molecular Dipole A result of the bond dipoles in a molecule. Bond dipoles may or may not cancel out thereby producing either molecules that are nonpolar, if they cancel, or polar, if they do not cancel v Objectives Ø Provide explanation of properties based on particle views Dispersion Has partial positive and negative areas in the electron cloud based on attraction to other molecules and electronegativity causing distortion Abby Carroll 7
Ø Dipole- dipole Molecular dipoles occur due to the unequal sharing of electrons between atoms in a molecule. Those atoms that are more electronegative pull the bonded electrons closer to themselves. The buildup of electron density around an atom or discreet region of a molecule can result in a molecular dipole in which one side of the molecule possesses a partially negative charge and the other side a partially positive charge. Molecules with dipoles that are not canceled by their molecular geometry are said to be polar. Ø Hydrogen bonding A hydrogen bond is a special type of dipole- dipole attraction, which occurs when a hydrogen atom bonded to a strongly electronegative atom exists in the vicinity of another electronegative atom with a lone pair of electrons. These bonds are generally stronger than ordinary dipole- dipole and dispersion forces, but weaker than true covalent and ionic bonds. Intra- molecular hydrogen bonds: Intra- molecular hydrogen bonds are those that occur within one single molecule. This occurs when two functional groups of a molecule can form hydrogen bonds with each other. In order for this to happen, both a hydrogen donor an acceptor must be present within one molecule, and they must be within close proximity of each other in the molecule. Intermolecular hydrogen bonds: Intermolecular hydrogen bonds occur between separate molecules in a substance. They can occur between any numbers of like or unlike molecules as long as hydrogen donors and acceptors are present in position in which they can interact Ø Ion dipole An ion-dipole force is an attractive force that results from the electrostatic attraction between an ion and a neutral molecule that has a dipole. Abby Carroll 8
Most commonly found in solutions. Especially important for solutions of ionic compounds in polar liquids. A positive ion (cation) attracts the partially negative end of a neutral polar molecule. A negative ion (anion) attracts the partially positive end of a neutral polar molecule. Ø Use aspects of particulate models (spacing motion and forces of attraction) to reason about observed differences between solid and liquid phases and among solid and liquid materials Intramolecular forces are those forces between the atoms found inside a single molecule. Example: Metallic, ionic and covalent bonds. Stronger Within inside Intermolecular forces are those forces between molecules. These forces determine the boiling point of substances thus the state. (ie solid, liquid or gas). The stronger the IMF the stronger the boiling point. Examples: dipole- dipole forces London dispersion forces hydrogen bonding between Space - the farther the molecules are away from each other the less dense it is, meaning that gases molecules will be spread out and far apart, and solid molecules will be packed in close together Motion the more space that molecules have around each other means that they can move more freely. Gas for example can move quickly because the are so far apart, while In a sold, there is little movement because molecules are packed so tightly together Forces of attraction There are forces of attraction and repulsion that exist between molecules of all substances. These intermolecular forces allow molecules to pack together in the solid and liquid states. When a pot of water is placed on a burner, it will soon boil. A phase change is occurring; the liquid water is changing to gaseous water, or steam. On a molecular level, the intermolecular forces between the Abby Carroll 9
water molecules are decreasing. The heat is providing enough energy for the water molecules to overcome these attractive forces. Ø Explain trends in properties and/or predict properties of samples consisting of particles with no permanent dipole on the basis of London dispersion forces Things that affect the strength of dispersion Larger mass larger mass means more electrons meaning a larger electron cloud which means increased polarizability and stronger attractions Polarizability the ability for a molecule to be polarized or the tendency for an atoms electron cloud to be distorted from its normal shape by an external electric field, basically if something is polar then they will have unequal sharing meaning one atom is going to be far more electronegative, making it harder to pull them apart Surface contact more electrons have contact with more electrons which would cause a greater attraction making them have a higher boiling point because they are harder to pull away Ø Identifying difference between intra and inter molecular forces Find under objective 2 Ø Describe the relationships between the structural features of polar molecules and forces of attraction between the particles Polar molecules mean that there is unequal sharing, so one will be partial positive and one will partial negative based on the amount of electronegativity of the atoms causing distortion Ø Attractions between ions and solvents explain the factors that contribute to the solubility of ionic compounds Water attraction to an ion, if the ion is positive, then oxygen will be attracted to it because of the partial negative charge it has. Hydrogen will be attracted to an anion because of the partial positive charge that it has Intermolecular forces dictate the state of matter (solid, liquid or gas), but they also dictate solubility of a solute in a solvent. In order for a solution to form, the intermolecular attractions between the solute and solvent must be greater than the attraction between solute molecules or the attraction between solvent molecules. Many types of intermolecular interactions can contribute to solvation, including ion-dipole forces. Ø Explain the properties (phase) of small and large molecular compounds terms of the strength and types of intermolecular forces The strength of attractions determine the state Intermolecular stronger attraction means higher melting or boiling point Based on negative and positive attraction Larger attraction means stronger force Larger distance means weaker Abby Carroll 10
Ø Rank and justify the ranking bond polarity on the basis of location of the bonded atoms in the periodic table Basically electronegativity Ø Make claims and/or predictions regarding relative magnitudes of the forces acting within collections of interacting molecules based on distributions of electrons within the molecules and the types of intermolecular forces through which the molecules interact Ø List and rank the strength the different type of intermolecular forces and be able to explain discrepancies to the rank in terms of strength Ion- dipole Hydrogen bond Dipole- dipole London dispersion Non- polar could be greater than polar if it has greater surface area also the straight chains would be greater for the same reason Ø Is something a chemical or a physical change based on the process involved with intra and intermolecular interactions Intermolecular forces have to do with physical changes Think of water Ø Describe intermolecular forces and the chemistry concepts (solubility dispersion polarity molecular structure) related to the shaving cream print activity and form a conclusion based on results using the concepts Solubility like dissolves like Food coloring and water Dispersion - spread of molecules over are Polarity greater attractions between things can break intermolecular forces Greater attraction to the paper than the shaving cream Cellulose is a mixture of polar and non-polar parts including hydroxyl groups which are also on the water coloring so they form hydrogen bonds There was a greater attraction (the food coloring) to the paper due to intermolecular forces that were stronger between paper and food coloring Abby Carroll 11