A Level Chemistry - Structure and bonding Questions and Answers

A Level Chemistry - Structure and bonding Questions and Answers ionic bonding the strong electrostatic attraction between oppositely charged ions which two factors affect the strength of an ionic bond? ionic charge ionic radius (greater distance between ions = weaker bond) describe the trend in ionic radius down a group radius increases as atomic radius increases describe the trend in ionic radius for isoelectronic ions ionic radius decreases as atomic number increases because greater effective nuclear charge means stronger attraction between nucleus and outer shell of electrons - 'pulls' in outer electrons describe a procedure to show the migration of ions in copper (II) sulfate solution - electrolyse a solution of copper sulfate in a U-tube - solution around cathode turns blue as Cu2+ ions are attracted to the negative charge describe a procedure to show the migration of ions in potassium manganate (VII) - draw a faint pencil line across a piece of filter paper and moisten it with tap water - fasten the filter paper to a microscope slide with crocodile clips - use tweezers to place a small crystal of potassium manganate (VII) in the centre of the filter paper - connect the crocodile clips to the terminals of a 20V DC supply, noting which clip is connected to which terminal - after 15 minutes, a purple colour spreads towards the side of the filter paper connected to the positive terminal of the power supply due to the negatively charged MnO4- ions being electrostatically attracted to the positive terminal covalent bond the electrostatic attraction between two nuclei and the pair of electrons they share two types of covalent bond sigma and pi what is a sigma bond? head on/direct overlap of atomic orbitals, resulting in an area of electron density which is symmetrical about the axis between the nuclei e.g. in H2, Cl2 or HCl what is a pi bond? sideways overlap of p orbitals that occurs in a double or triple bond what is a dative covalent bond? a covalent bond in which a shared pair of electrons is supplied by one of the bonding atoms only to form a bond with an electron-deficient atom chemically the same as any other covalent bond positive charge is distributed all over the ion and all the bonds are equivalent how is a dative covalent bond represented? an arrow pointing away from the atom that donates the pair of electrons the Octet Rule the tendency in bonding of atoms to form an 'octet' (8 electrons in outer shell) two ways the octet rule breaks down 1. electron deficient molecules 2. expansion of the octet what happens if molecules are electron deficient? dative covalent bonds form between adjacent molecules describe the dative covalent bonding in beryllium chloride, BeCl2 the Be atom is electron deficient (only 4 electrons in outer shell) in order for the Be outer shell to be filled, a lone electron pair from Cl is shared with a Be atom from another molecule, forming a dative covalent bond chained polymer is formed describe the dative covalent bonding in aluminium trichloride the aluminium atom in the molecule is electron deficient (only 6 electrons in outer shell) when solid aluminium chloride is heated it sublimes to form a vapour consisting of the dimer Al2Cl6 because pairs of AlCl3 molecules bond together a lone electron pair from a chlorine atom in one molecule bonds dative covalently with aluminium from another molecule and so fills the aluminium outer shell when can expansion of the octet occur? in groups 5-7 from period 3 onwards, more outer shell electrons can be paired and shared in some elements these elements may end up with more than eight electrons in their outer shell e.g. in phosphorus pentafluoride why can expansion of the octet occur? the extra electrons are accommodated in an easily accessible (in terms of energy) 3d subshell does not occur in period 1 or period 2 because there is no such thing as a 1d or 2d subshell electron configuration of sulfate ion electron configuration and shape of phosphate ion tetrahedral with bond angle 109.5 four regions of electrons around central P atom with equal repulsion what are electron pairs that form covalent bonds called? bonding pairs what are electron pairs that are not involved in bonding called? non-bonding pairs or lone pairs first rule of VSEPR electron pairs move as far apart as possible to minimise their mutual repulsion bond angle of linear 180 bond angle of trigonal planar 120 bond angle of tetrahedral 109.5 bond angle in trigonal bipyramid 120 and 90 bond angle in octahedral 90 second rule of VSEPR lone electron pairs repel more strongly than bonding electron pairs why do lone pairs repel more strongly than bonding electron pairs? in bonding pairs, electrons are attracted by the positive nuclei of atoms at either end, so longer, thinner distribution of negative charge in space in lone pairs, electrons attracted by positive nuclei from one direction so shorter, fatter distribution of negative charge closer to parent atom bond angle in V shape (two lone pairs and two other bonds) 104.5 bond angle in pyramid shape (1 lone pair and three other atoms) 107 lone pair 'pushes' other atoms together shape of molecule with two lone pairs and three bonding pairs trigonal planar 120 bond angle shape of molecule with two lone pairs and four bonding pairs square planar/octahedral with lone pairs on poles 90 bond angle explain why a molecule has a particular shape - specify the number of bonding pairs of electrons (and lone pairs of electrons) around the central atom - state that electron pairs repel and will therefore move to be as far apart as possible - if there are lone electron pairs, state that these repel more strongly than bonding electron pairs - taking these factors into account, the molecule will adopt the _ shape with bond angles of _ bond length the average distance between the nuclei of two bonded atoms in a molecule which two factors does bond length depend on? the size of the atoms involved and the number of electron pairs shared why do larger atoms form longer bonds? larger atoms have more electrons that shield the nuclei and reduce the attraction between the nuclei and the electron cloud why are single bonds longer than double or triple bonds? the positively charged nuclei can stay closer together if the shared electron cloud contains more electrons to overcome the repulsion of the nuclei how is bond strength related to bond length? inversely proportional shorter bond is stronger with greater bond energy properties of simple molecular substances - usually gases, liquids or soft solids at room temperature - low melting and boiling points - do not conduct electricity in any state because they contain no ions or free electrons to carry the electric charge - usually more soluble in non-polar solvents such as hexane than water electronegativity the ability of an atom to attract the bonding pair of electrons in a covalent bond if there is a difference in electronegativity between two atoms in a covalent bond... a polar covalent bond is formed the shared electron pair is pulled more towards the atom with greater electronegativity Pauling scale measures electronegativity fluorine is highest francium is lowest factors affecting electronegativity atomic radius nuclear charge shielding trends in electronegativity increases across a period, decreases down a group what type of bond is formed when electronegativity difference is zero or very small? covalent each atom will have the same ability to attract the shared pair of electrons, so they will be shared evenly what type of bond is formed when electronegativity difference is very large? ionic electrons completely transferred from one atom to the other, significantly more electronegative atom what type of bond is formed when electronegativity difference between two atoms is neither very small nor very large? a polar covalent bond the shared electrons are more attracted towards the electronegative atom more electronegative atom is delta negative less electronegative atom is delta positive what is a permanent dipole? a small charge difference across a bond that results from a difference in the electronegativities of the bonded atoms How are polar molecules formed? if the molecule has polar bonds and is not symmetrical, there will be an unequal distribution of charge and the molecule will have a delta positive and delta negative end explain why a molecule is polar or not polar - identify if the molecule contains polar bonds and say which bonds are polar - state if the molecule is symmetrical or non-symmetrical - state if the dipoles within the molecule cancel out or not procedure for determining whether liquids are polar - place a small beaker underneath a burette of the liquid - rub a perspex rod several times with a cloth to give it a positive charge - open the burette tap - hold the charged rod near the jet of liquid - if the liquid is deflected, it is polar - the extent to which the liquid is deflected depends on how polar it is which two factors should be controlled as much as possible when testing polarity with a perspex rod? - how much the rod is rubbed - how close the rod is held to the jet of liquid London forces instantaneous dipole - induced dipole the weak attractive forces between molecules resulting from the small, instantaneous dipoles that occur because of the fluctuations in electron cloud factors affecting the size of London forces number of electrons in atom - more electrons means higher polarisability (the extent to which the electron cloud can be distorted by a nearby electric charge) shape of molecule (surface area of contact) - attraction between long, thin molecules can take place over a larger surface area what are permanent dipole-dipole interactions? attraction between oppositely-charged permanent dipoles in polar molecules hydrogen bond an extreme form of dipole-dipole attraction with the most electronegative elements (F, O, N) the strongest form of intermolecular force how do hydrogen bonds form? - molecule must contain a hydrogen atom covalently bonded to either fluorine, oxygen or nitrogen where the F, O or N atom has an available lone pair - the very electronegative atom makes the H electron deficient - the hydrogen bond is the force of attraction between the delta positive hydrogen atom in one molecule and the lone pair on the nitrogen, oxygen or fluorine why can hydrogen bonds only form with fluorine, oxygen or nitrogen? only fluorine, oxygen and nitrogen are electronegative enough to polarise the hydrogen sufficiently for it to accept a pair of electrons in a hydrogen bond why does water have a relatively high melting and boiling point? there are strong hydrogen bonds between water molecules which mean more energy is required to separate molecules why is ice less dense than liquid water? in liquid water, hydrogen bonds are constantly being formed and broken in each water molecule there are two O-H bonds and two lone pairs - each molecule can take part in up to four hydrogen bonds in ice the hexagonal structure of ice results in longer bond lengths between the molecules than in liquid water, resulting in a greater distance between water molecules and so lower density describe the trend in boiling points of alkanes boiling point increases as chain length increases what impact does branching have on the boiling points of alkanes? branched molecules have lower melting and boiling points because they are less compact and so can be less closely packed and because they have a smaller surface area intermolecular forces are weaker why are alkanes more volatile than alcohols? alcohols have an O-H group so each molecule can form hydrogen bonds with other molecules - more energy is required to separate these hydrogen bonds explain the trend in boiling points of hydrogen halides HF has highest boiling point because molecules can form hydrogen bonds then boiling point is much lower for HCl because it cannot form hydrogen bonds boiling point increases down the group as the number of electrons in the molecule increases and so the strength of the London forces increases general rule about solubility 'like dissolves like' when the intermolecular forces between solute molecules, solvent molecules and solute and solvent molecules are about the same strength, the solute dissolves freely in the solvent what is required for a solute to dissolve in a solvent? the energy released in forming bonds or intermolecular forces with solvent molecules is (usually) greater than the energy required to break bonds/IM forces in the solute and solvent why do non-polar molecules not dissolve in water? the non-polar molecules can separate fairly easily because their intermolecular forces are relatively weak but the strong hydrogen bonding between water molecules acts as a barrier to molecules that cannot themselves form hydrogen bonds why do polar organic molecules not dissolve in water? the weak dipole-dipole forces between organic molecules allow them to separate fairly easily but they cannot form hydrogen bonds with water so they cannot disturb the hydrogen bonding between water molecules why can ionic compounds dissolve in water? the ions are strongly hydrated by polar water molecules that form hydration shells around the ions the energy released when water molecules bind to the ions compensates for the energy needed to overcome the electrostatic attractions holding the ionic lattice together why can short-chain alcohols be dissolved in water? the O-H group can form a hydrogen bond with the water molecules metallic bonding the electrostatic attraction between positive metal cations and the sea of delocalised electrons between them what three factors affect the strength of a metallic bond? charge on the metal ions number of delocalised electrons per atom size of the ion (smaller ions = stronger attraction) Why do metals have high melting and boiling points? the strong electrostatic attraction between the cations and the delocalised electrons why are metals high-density? close-packing of metal atoms in layers why are metals malleable? layers of atoms can slide over each other as bonding is non-directional