Published on January 13, 2010
Unit 2.3 - Intermolecular forces Intermolecular forces • Inter molecular forces are the forces acting between molecules • They are much weaker than the bonds within molecules • They do, however, significantly affect the physical properties of substances (melting point etc…) • We will be looking at three different types of intermolecular force 1. Dipole- dipole interactions • Polar molecules like _____ have a permanent dipole • In other words: they have ____________ end and a ______________ end • Opposite charges ______________ so polar molecules are drawn together by so-called permanent dipole- permanent dipole interactions • These forces are about 100 times weaker than covalent bonds • They do, however, have a big affect on physical properties: Species Boiling point / °C Hydrogen chloride (HCl) -85 Dipole-dipole forces Polar Hydrogen sulphide (H2S) -61 mean that polar Fluorine (F2) -188 molecules have Non-Polar Argon (Ar) -186 relatively ________ melting and boiling temperatures Use the space below to sketch a quick graph of the data in the above table: 2. London forces • Permanent dipole-permanent dipole forces only exist between __________ molecules • There are, however, forces between non- polar molecules • We know that these forces exist because we can liquefy Fritz London enjoying a cocktail
non-polar substances like nitrogen • These forces are known as London forces • They are relatively weak Instantaneous dipole Induced dipole London Forces • The electron density • The electric field of the • There is attraction between the in an atom can instantaneous dipole instantaneous and induced fluctuate effects the electron dipoles • This can lead to density in nearby atoms • The attraction is weak and charge separation • This causes induced fluctuates , it is sometimes zero • This happens very dipoles briefly and fluctuates Strength of London forces Noble gas Boiling temp / London forces are K • Bigger atoms hold onto their outer the only Helium 4 Neon 27 electrons _________ strongly _____________ Argon 87 • Induced dipoles occur more easily in forces that act Krypton 121 Xenon 166 __________ atoms or molecules between noble gas Radon 211 • The ___________ the atom or molecule (group ___) atoms the stronger the London forces 3. Hydrogen bonds The third and final type of intermolecular force we will look at is hydrogen bonding, which is present in many substances including water :
A water molecule Hydrogen bonding in water • Water molecules have a ___________ • The hydrogens have a partially shape _______________ charge • Oxygen is __________ electronegative than • The negatively charged lone-pair electrons hydrogen so it pulls electrons away from of _______________ are attracted to the it hydrogens • Oxygen has _____ lone pairs of electrons, • This attraction is a hydrogen bond which are not involved in covalent bonding Hydrogen bonding doesn’t just happen in water! Hydrogen bonding significantly affects physical properties, we will cover this shortly, but first we will look at what types of compound hydrogen bonding happens in: Molecules that Conditions needed for H can form H bonding… bonds… 1. 2. 3. Relative strength of intermolecular forces: H bonds Dipole-dipole London forces Trends in physical properties • The physical properties of a substance ( _____________ and ______________ temperature) are decided by the strength of the ________________ forces present in the substance • Strong intermolecular forces mean… • Weak intermolecular forces mean… View slide
Melting and boiling temperatures of alkanes • The table above clearly shows that as Meltin Boilin Molecular g g State at Formula Point Point 25oC Name (oC) (oC) methane CH4 -183 -164 gas ethane C2H6 -183 -89 propane C3H8 -190 -42 butane C4H10 -138 -0.5 pentane C5H12 -130 36 hexane C6H14 -95 69 heptane C7H16 -91 98 octane C8H18 -57 125 nonane C9H20 -51 151 liquid decane C10H22 -30 174 undecane C11H24 -25 196 dodecane C12H26 -10 216 the length of the hydrocarbon chain eicosane C20H42 37 343 increases, melting and boiling triacontane C30H62 66 450 solid temperatures __________________ • This is because the intermolecular forces are getting ______________ • The intermolecular forces between alkanes are _______________ forces Sketch an alkane melting/boiling temp graph: The longer the chain, the _____________ the ______________ forces Effect of branching Displayed formula Boiling temperature / °C 36 28 10 • Boiling temperature of alkanes _______________ with branching • They become __________ volatile View slide
• This is because... Alkanes vs alcohols Propane Propanol • Alcohols have much ___________ boiling Boils at -42 °C Boils at 97 °C temperatures than alkanes because they contain oxygen and can form _____________ bonds, whereas alkanes only have weaker ____________ forces Some research suggests that geckos’ feet have complex surface structures that allow them to use London forces to stick to smooth surfaces and even walk on ceilings! Boiling temperatures of the hydrogen halides We saw earlier that hydrogen bonds can form between molecules that contain _____________ bonded to a more __________________ atom, which draws electron density away from hydrogen leaving it with a partial ______________ charge that attracts a lone pair of electrons from another molecule: Hydrogen bonding in water H bonding in hydrogen H bonding in ammonia fluoride (HF) (NH 3 ) The hydrogen bonding in hydrogen fluoride has a big effect on its physical properties : Halide HF HCl HBr HI Boiling temperature / 19.5 -85.1 -66.4 -34.4 °C Sketch a graph of the above table… Electronegativities Fluorine 4.0 Chlorine 3.0 Bromine 2.8 Iodine 2.5
• Fluorine is _________ electronegative than the other halogens • Because of its high electronegativity fluorine can strongly draw ____________ density away from hydrogen making it partially _______________ • This means that ______________ bonds can form between HF molecules, which makes the boiling temperature of hydrogen fluoride unexpectedly _____________ Solubility Intermolecular forces can also exist between different types of molecule, therefore they have big part in the dissolving of solids and the mixing of liquids… Key words Solubility - The mass of a solute that will dissolve in 100 g of a solvent at a particular temperature Non-aqueous solvent - A solvent that is not water e.g. hexane or paraffin Miscible - Liquids that mix completely are miscible Immiscible - Liquids that do not mix at all and form separate layers are immiscible Dissolving ionic solids in water • Ionic solids like _____ and _____ can be soluble in water • Ionic solids are made up of positive and negative ______, which are held together by strong _________________ forces of attraction • The energy required to break down an ionic lattice is known as the _____________ energy • When ionic solids dissolve the energy to break the lattice up comes from new forces of attraction between the ions and water molecules: Hydration - Energy is released Attractive forces between water Water surrounds Na+ ions Water surrounds Cl- ions
and ions break down the lattice • The process of water molecules surrounding ions is called hydration • The energy released when this happens is the hydration enthalpy • If hydration enthalpy is greater than lattice energy a substance will dissolve in water, if it is not then it will not: Substance NaCl LiF Lattice energy / kJmol -1 -770 -1031 Hydration enthalpy / +770 +1025 kJmol -1 Soluble/insoluble in water Soluble Insoluble Solubility of alcohols in water • Alcohols and carboxylic acids are soluble in water • This is because they have polar OH groups, which can form _____________ bonds with water molecules • They become less soluble as chain length increases Why are non- polar substances insoluble in water? • Oil and water do not mix, they are ___________________ • Two separate layers form with the Weak _________ on top forces between oil • The same happens with other non- molecules polar organic liquids like ____________ or paraffin Strong forces • This is because the ____________ between water bonds between water molecules are so molecules much stronger than the ____________ forces between oil molecules • The oil molecules cannot disrupt the structure of the water, they cannot squeeze between water molecules so the liquids are __________________ Mixing two organic liquids • Simple organic liquids like hexane or octane are ____ - ____________
• The only forces between molecules are the weak _____________ forces • When two non-polar organic liquids are mixed these forces extend throughout the mixture • Non-polar liquids like hexane and octane are ________________
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