AQA A Level Chemistry PDF

Head to www.savemyexams.com for more awesome resources AQA A Level Chemistry Your notes Molecules: Shapes & Forces Contents Shapes of Simple Molecules & Ions Bond Polarity Types of Forces between Molecules Eff...

Head to www.savemyexams.com for more awesome resources AQA A Level Chemistry Your notes Molecules: Shapes & Forces Contents Shapes of Simple Molecules & Ions Bond Polarity Types of Forces between Molecules Effects of Forces Between Molecules Page 1 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Shapes of Simple Molecules & Ions Your notes Electron Pairs The valence shell electron pair repulsion theory (VSEPR) predicts the shape and bond angles of molecules Electrons are negatively charged and will repel other electrons when close to each other In a molecule, the bonding pairs of electrons will repel other electrons around the central atom forcing the molecule to adopt a shape in which these repulsive forces are minimised When determining the shape and bond angles of a molecule, the following VSEPR rules should be considered: Valence shell electrons are those electrons that are found in the outer shell Electron pairs repel each other as they have the same charge Lone pair electrons repel each other more than bonded pairs Repulsion between multiple and single bonds is treated the same as for repulsion between single bonds Repulsion between pairs of double bonds are greater The most stable shape is adopted to minimize the repulsion forces Different types of electron pairs have different repulsive forces Lone pairs of electrons have a more concentrated electron charge cloud than bonding pairs of electrons The cloud charges are wider and closer to the central atom’s nucleus The order of repulsion is therefore: lone pair – lone pair > lone pair – bond pair > bond pair – bond pair Page 2 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Different types of electron pairs have different repulsive forces Shapes of Molecules & Ions Molecules can adapt the following shapes and bond angles: Page 3 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Molecules of different shapes can adapt with their corresponding bond angles Examples Your notes Page 4 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 5 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 6 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examples of molecules with different shapes and bond angles Worked Example VSEPR & shapes of molecules Draw the shape of the following molecules: 1. Phosphorus(V) chloride 2. N(CH3)3 3. CCl4 Answer 1: Phosphorus is in group 15, so has 5 valence electrons; Cl is in group 17, so has 17 valence electrons All 5 electrons are used to form covalent bonds with Cl and there are no lone pairs This gives a trigonal (or triangular) bipyramidal shape: Page 7 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Phosphorus pentachloride or phosphorus (V) chloride Answer 2: Nitrogen is in group 15, so has 5 valence electrons; carbon is in group 14, so has 4 valence electrons, 3 of which are already used in the covalent bonds with hydrogen Three of the valence electrons in N are used to form bonding pairs, so there is one lone pair left N(CH3)3 has a triangular pyramid shape: Trimethylamine Answer 3: Carbon is in group 14, so has 4 valence electrons; chlorine is in group 17, so has 7 valence electrons All four valence electrons are use to bond with chlorine and there are no lone pairs The shape of CCl4 is tetrahedral Page 8 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Tetrachloromethane Page 9 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Bond Polarity Your notes Electronegativity Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself The electron distribution in a covalent bond between elements with different electronegativities will be unsymmetrical This phenomenon arises from the positive nucleus’s ability to attract the negatively charged electrons, in the outer shells, towards itself The Pauling scale is used to assign a value of electronegativity for each atom First three rows of the periodic table showing electronegativity values Fluorine is the most electronegative atom on the Periodic Table, with a value of 4.0 on the Pauling Scale It is best at attracting electron density towards itself when covalently bonded to another atom Page 10 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Electron distribution in the C-F bond of fluoromethane Nuclear charge Attraction exists between the positively charged protons in the nucleus and negatively charged electrons found in the energy levels of an atom An increase in the number of protons leads to an increase in nuclear attraction for the electrons in the outer shells Therefore, an increased nuclear charge results in an increased electronegativity Atomic radius The atomic radius is the distance between the nucleus and electrons in the outermost shell Electrons closer to the nucleus are more strongly attracted towards its positive nucleus Those electrons further away from the nucleus are less strongly attracted towards the nucleus Therefore, an increased atomic radius results in a decreased electronegativity Page 11 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Shielding Filled energy levels can shield (mask) the effect of the nuclear charge causing the outer electrons to be Your notes less attracted to the nucleus Therefore, the addition of extra shells and subshells in an atom will cause the outer electrons to experience less of the attractive force of the nucleus Sodium (period 3, group 1) has higher electronegativity than caesium (period 6, group 1) as it has fewer shells and therefore the outer electrons experience less shielding than in caesium Thus, an increased number of inner shells and subshells will result in a decreased electronegativity Trends in Electronegativity Electronegativity varies across periods and down the groups of the periodic table Down a group There is a decrease in electronegativity going down the group The nuclear charge increases as more protons are being added to the nucleus However, each element has an extra filled electron shell, which increases shielding The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in larger atomic radii Overall, there is decrease in attraction between the nucleus and outer bonding electrons Page 12 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Electronegativity decreases going down the groups of the periodic table Across a period Electronegativity increases across a period The nuclear charge increases with the addition of protons to the nucleus Shielding remains relatively constant across the period as no new shells are being added to the atoms The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the period of the periodic table This results in smaller atomic radii Page 13 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Electronegativity increases going across the periods of the Periodic Table Examiner Tips and Tricks Remember: The general trend is an increase in electronegativity towards the top right of the periodic table. Fluorine is the most electronegative element in the periodic table. Page 14 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Types of Forces between Molecules Your notes Intramolecular Forces vs. Intermolecular Forces Intramolecular forces Intramolecular forces are forces within a molecule and are usually covalent bonds Covalent bonds are formed when the outer electrons of two atoms are shared Single, double, triple and co-ordinate bonds are all types of intramolecular forces Intermolecular forces Molecules also contain weaker intermolecular forces which are forces between the molecules There are three types of intermolecular forces: Induced dipole – dipole forces also called van der Waals or London dispersion forces Permanent dipole – dipole forces are the attractive forces between two neighbouring molecules with a permanent dipole Hydrogen Bonding are a special type of permanent dipole - permanent dipole forces Intramolecular forces are stronger than intermolecular forces For example, a hydrogen bond is about one tenth the strength of a covalent bond The strengths of the types of bond or force are as follows: Page 15 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The varying strengths of different types of bonds Polar Bonds Polarity When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar The two chlorine atoms have the same electronegativities so the bonding electrons are shared equally between the two atoms Page 16 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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 Your notes 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 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 Permanent dipole - dipole forces: Polar molecules have permanent dipoles The molecule will always have a negatively and positively charged end Page 17 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Forces between two molecules that have permanent dipoles are called permanent dipole - 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 Induced Dipole-Dipole Forces Induced dipole-dipole forces: Induced dipole - dipole forces exist between all atoms or molecules They are also known as van der Waals forces or London dispersion forces Page 18 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes 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 Page 19 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 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 Your notes neighbouring molecule are attracted towards each other Because the electron clouds are moving constantly, the dipoles are only temporary Relative strength For small molecules with the same number of electrons, permanent dipoles are stronger than induced dipoles Butane and propanone have the same number of electrons Butane is a nonpolar molecule and will have induced dipole forces Propanone is a polar molecule and will have permanent dipole 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 Examiner Tips and Tricks Remember: Intramolecular forces are forces within a molecule, whereas intermolecular forces are forces between a molecule. Hydrogen Bonds Hydrogen bonding Page 20 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Hydrogen bonding is the strongest form of intermolecular bonding Intermolecular bonds are bonds between molecules Your notes 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, N or F (very electronegative) atom bonded to a hydrogen When hydrogen is covalently bonded to an O, N or F, the bond becomes highly polarised The H becomes so δ+ charged that it can form a bond with the lone pair of an O, N or F atom in another molecule For example, in water Water can form two hydrogen bonds, because the O has two lone pairs Hydrogen bonding in water Examiner Tips and Tricks Make sure to use a dashed, straight line when drawing your intermolecular forces!Hydrogen bonds should start at the lone pair and go right up to the delta positive atom - it must be really clear where your H bond starts and ends. Page 21 of 25 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Effects of Forces Between Molecules Your notes Influence of Intermolecular Forces Properties of water Hydrogen bonding in water, causes it to have anomalous properties such as high melting and boiling points, high surface tension and a higher density in the liquid than the solid High melting & boiling points Water has high melting and boiling points due to the the strong intermolecular forces of hydrogen bonding between the molecules in both ice (solid H2O) and water (liquid H2O) A lot of energy is therefore required to separate the water molecules and melt or boil them Hydrogen bonds are strong intermolecular forces which are harder to break causing water to have a higher melting and boiling point than would be expected for a molecule of such a small size 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 increase in the instantaneous dipole - induced dipole forces (dispersion forces) as the molecules become larger Based on this, H2O should 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 Page 22 of 25 © 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 strong hydrogen bonds, which cause the high boiling point 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 the surface molecules causing the surface of them to become compressed and more tightly together at the surface This increases water’s surface tension Page 23 of 25 © 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 The water molecules are packed into an open lattice This way of packing the molecules 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 by about 9% Page 24 of 25 © 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 Examiner Tips and Tricks Ice floats on water because of ice's lower density. 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