CIE A Level Chemistry 3.6 Intermolecular Forces PDF

Head to www.savemyexams.com for more awesome resources CIE A Level Chemistry Your notes 3.6 Intermolecular Forces, Electronegativity & Bond Properties Contents Hydrogen Bonding Bond P...

Head to www.savemyexams.com for more awesome resources CIE A Level Chemistry Your notes 3.6 Intermolecular Forces, Electronegativity & Bond Properties Contents Hydrogen Bonding Bond Polarity & Dipole Moments Van der Waals' Forces Inter & Intramolecular Forces Page 1 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Hydrogen Bonding Your notes Hydrogen Bonding Hydrogen bonding Hydrogen bonding is the strongest form of intermolecular bonding Intermolecular bonds are bonds between molecules Hydrogen bonding is a type of permanent dipole – permanent dipole bonding For hydrogen bonding to take place the following is needed: A species which has an O or N (very electronegative) atom with an available lone pair of electrons A species with an -OH or -NH group When hydrogen is covalently bonded to an electronegative atom, such as O or N, the bond becomes very highly polarised The H becomes so δ+ charged that it can form a bond with the lone pair of an O or N atom in another molecule Polarity of the OH bond The electronegative atoms O or N have a stronger pull on the electrons in the covalent bond with hydrogen, causing the bond to become polarised For hydrogen bonding to take place, the angle between the -OH/-NH and the hydrogen bond is 180o The number of hydrogen bonds depends on: The number of hydrogen atoms attached to O or N in the molecule The number of lone pairs on the O or N Hydrogen bonding in ammonia Page 2 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Ammonia can form a maximum of one hydrogen bond per molecule Hydrogen bonding in water Water can form a maximum of four hydrogen bonds per molecule. Two hydrogen bonds on the δ- oxygen atom and one on each δ+ hydrogen atom Properties of water Hydrogen bonding in water, causes it to have anomalous properties such as high melting and boiling points, high surface tension and anomalous density of ice compared to water High melting & boiling points Page 3 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Water has high melting and boiling points which is caused by the strong intermolecular forces of hydrogen bonding between the molecules In ice (solid H2O) and water (liquid H2O) the molecules are tightly held together by hydrogen bonds Your notes A lot of energy is therefore required to break the water molecules apart and melt or boil them Changing states and hydrogen bonding Hydrogen bonds are strong intermolecular forces which are difficult to break causing water to have high melting and boiling points The graph below compares the enthalpy of vaporisation (energy required to boil a substance) of different hydrides The enthalpy changes increase going from H2S to H2Te due to the increased number of electrons in the Group 16 elements This causes an increased instantaneous dipole - induced dipole forces as the molecules become larger Based on this, H2O would have a much lower enthalpy change (around 17 kJ mol-1) However, the enthalpy change of vaporisation is almost 3 times larger which is caused by the hydrogen bonds present in water but not in the other hydrides Graph of enthalpy of vaporisation for different hydrides Page 4 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The high enthalpy change of evaporation of water suggests that instantaneous dipole-induced dipole forces are not the only forces present in the molecule – there are also those of the strong hydrogen bonds, which cause the high boiling points High surface tension Water has a high surface tension Surface tension is the ability of a liquid surface to resist any external forces (i.e. to stay unaffected by forces acting on the surface) The water molecules at the surface of liquid are bonded to other water molecules through hydrogen bonds These molecules pull downwards on the surface molecules causing the surface them to become compressed and more tightly together at the surface This increases water’s surface tension The effect of hydrogen bonding in water Page 5 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The surface molecules are pulled downwards due to the hydrogen bonds with other molecules, whereas the inner water molecules are pulled in all directions Density Solids are denser than their liquids as the particles in solids are more closely packed together than in their liquid state In ice however, the water molecules are packed in a 3D hydrogen-bonded network in a rigid lattice Each oxygen atom is surrounded by hydrogen atoms This way of packing the molecules in a solid and the relatively long bond lengths of the hydrogen bonds means that the water molecules are slightly further apart than in the liquid form Therefore, ice has a lower density than liquid water Structure and density of water Page 6 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The ‘more open’ structure of molecules in ice causes it to have a lower density than liquid water Exam Tip Ice floats on water because of ice's lower density. Page 7 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Bond Polarity & Dipole Moments Your notes Bond Polarity & Dipole Moments Electronegativity is the ability of an atom to draw a pair of electrons towards itself in a covalent bond Electronegativity increases across a Period and decreases going down a Group Polarity When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar Bonding electrons in a chlorine molecule The two chlorine atoms have similar electronegativities so the bonding electrons are shared equally between the two atoms When two atoms in a covalent bond have different electronegativities the covalent bond is polar and the electrons will be drawn towards the more electronegative atom As a result of this: The negative charge centre and positive charge centre do not coincide with each other This means that the electron distribution is asymmetric The less electronegative atom gets a partial charge of δ+ (delta positive) The more electronegative atom gets a partial charge of δ- (delta negative) The greater the difference in electronegativity the more polar the bond becomes Bonding electrons in a hydrogen chloride molecule Page 8 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive Dipole moment The dipole moment is a measure of how polar a bond is The direction of the dipole moment is shown by the following sign in which the arrow points to the partially negatively charged end of the dipole: Representing dipoles The sign shows the direction of the dipole moment and the arrow points to the delta negative end of the dipole Assigning polarity to molecules To determine whether a molecule with more than two atoms is polar, the following things have to be taken into consideration: The polarity of each bond How the bonds are arranged in the molecule Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such a way that the individual dipole moments cancel each other out Polarity in chloromethane Page 9 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes There are four polar covalent bonds in CH3Cl which do not cancel each other out causing CH3Cl to be a polar molecule; the overall dipole is towards the electronegative chlorine atom Polarity in tetrachloromethane Page 10 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Though CCl4 has four polar covalent bonds, the individual dipole moments cancel each other out causing CCl4 to be a nonpolar molecule Page 11 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Van der Waals' Forces Your notes Van der Waals' Forces & Dipoles Covalent bonds are strong intramolecular forces Molecules also contain weaker intermolecular forces which are forces between molecules These intermolecular forces are called van der Waals’ forces There are two types of van der Waals’ forces: Instantaneous (temporary) dipole – induced dipole forces also called London dispersion forces Permanent dipole – permanent dipole forces Intermolecular and intramolecular forces in water The polar covalent bonds between O and H atoms are intramolecular forces and the permanent dipole – permanent dipole forces between the molecules are intermolecular forces as they are a type of van der Waals’ force Instantaneous dipole - induced dipole (id - id) Instantaneous dipole - induced dipole forces or London dispersion forces exist between all atoms or molecules The electron charge cloud in non-polar molecules or atoms are constantly moving During this movement, the electron charge cloud can be more on one side of the atom or molecule than the other This causes a temporary dipole to arise This temporary dipole can induce a dipole on neighbouring molecules When this happens, the δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other Because the electron clouds are moving constantly, the dipoles are only temporary Page 12 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Instantaneous dipoles Your notes Page 13 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Id-id (London dispersion) forces between two non-polar molecules Id - id forces increase with: Your notes Increasing number of electrons (and atomic number) in the molecule Increasing the places where the molecules come close together Going down the Group, the id-id forces increase due to the increased number of electrons in the atoms Page 14 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The increased number of contact points in pentane means that it has more id-id forces and therefore a higher boiling point Permanent dipole - permanent dipole (pd - pd) Polar molecules have permanent dipoles The molecule will always have a negatively and positively charged end Forces between two molecules that have permanent dipoles are called permanent dipole - permanent dipole forces The δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other Page 15 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The delta negative end of one polar molecule will be attracted onwards the delta positive end of a neighbouring polar molecule Your notes For small molecules with the same number of electrons, pd - pd forces are stronger than id - id Butane and propanone have the same number of electrons Butane is a nonpolar molecule and will have id - id forces Propanone is a polar molecule and will have pd - pd forces Therefore, more energy is required to break the intermolecular forces between propanone molecules than between butane molecules So, propanone has a higher boiling point than butane Pd-pd forces are stronger than id-id forces in smaller molecules with an equal number of electrons Exam Tip Remember this difference: intramolecular forces are forces within a molecule, whereas intermolecular forces are forces between a molecule. Page 16 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Hydrogen Bonding as a Permanent Dipole Hydrogen bonding is an intermolecular force between molecules with an -OH/-NH group and Your notes molecules with an N/O atom Hydrogen bonding is a special case of a permanent dipole - dipole force between molecules Hydrogen bonds are stronger forces than pd - pd forces The hydrogen is bonded to an O/N atom which is so electronegative, that almost all the electron density from the covalent bond is drawn towards the O/N atom This leaves the H with a large delta positive and the O/N with a large delta negative charging resulting in the formation of a permanent dipole in the molecule A delta positive H in one molecule is electrostatically attracted to the delta negative O/N in a neighbouring molecule Hydrogen bonds in water molecules Hydrogen bonding in water occurs between the oxygen lone pair of one water molecule and the δ+ hydrogen atoms of another water molecule Page 17 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Inter & Intramolecular Forces Your notes Comparing Bonds & Intermolecular Forces Intramolecular forces Intramolecular forces are forces within a molecule Ionic bonding is the electrostatic attraction between positive (cations) and negative (anions) ions in an ionic crystal lattice These ions are formed by transferring the electrons from one species to the other Covalent bonds are formed when the outer electrons of two atoms are shared Metallic bonding is the electrostatic attraction of positively charged metal ions and their delocalised electrons in a metal lattice Intramolecular forces Page 18 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The three types of intramolecular forces are ionic, covalent and metallic bonding Intermolecular forces Intermolecular forces are forces between molecules and are also called van der Waals’ forces Permanent dipole - permanent dipole are the attractive forces between two neighbouring molecules with a permanent dipole Hydrogen bonds are a special type of permanent dipole - permanent dipole forces Instantaneous dipole - induced dipole (London dispersion) forces are the attractive forces between a temporary dipole and a neighbouring molecule with an induced dipole Permanent dipoles as intermolecular forces Page 19 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Permanent dipole - permanent dipole are the intermolecular forces that occur between two neighbouring molecules with a permanent dipole Instantaneous dipoles as intermolecular forces Page 20 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 21 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Instantaneous dipole - induced dipole (London dispersion) forces are the intermolecular forces that occur between a temporary dipole and a neighbouring molecule with an induced dipole Your notes Hydrogen bonding as an intermolecular force Hydrogen bonds are a special type of permanent dipole - permanent dipole forces In general, intramolecular forces are stronger than intermolecular forces The strengths of the types of bond or force are as follows: The varying strengths of different types of bonds Page 22 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes In general, ionic bonding is the strongest force while instantaneous dipole - induced dipole is the weakest force Page 23 of 23 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers