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AQA A1 bond struct

AQA bonding and structure

QuestionAnswer
What is a dative covalent bond? Dative covalent bonding or coordinate bonding occurs when one ATOM provides BOTH/a PAIR of electrons needed for a COVALENT bond to form
What is ionic bonding Ionic bonding is the attraction between two oppositely charged ions
Explain the general trends for the first ionisation energy that can be seen across the period As you go across the periods, ionisation energies increase because , whilst shielding remains constant, the nuclear charge increases, atomic radii decrease, electrons are more attracted to nucleus so energy need to remove electrons increases
Write the electronic configuration of magnesium ion 1s2 2s2 2p6 3s2 3p6
Why can metals conduct electricity when solid Model answer: CHARGED (particles) MOVE: Delocalised electrons are mobile and therefore free to move around cations
Describe the structure in Sodium Chloride Metal with non-metal so IONIC: 3D lattice of alternate positive sodium ions and negative chloride ions
Is an Na7- ion more stable than a sodium atom? The Na-7 is less stable as repulsion electrons outbalance nuclear attraction
Describe what happens to Sodium Chloride when it dissolves in water The ionic structure breaks, the ions are surrounded by water and each ion will form an ion-dipole with water: the ions are hydrated and dispersed through the solution
What are isoelectronic ions? Give isoelectronic ions to Argon Ions with the same electronic configuration for example K+ or Cl-
What is the difference between a covalent bond and a dative covalent bond? the two electrons shared in a dative covalent bond are both from one atom whereas in a covalent bond they are from one atom each. Once the bond is formed you cannot tell the difference.
Why do giant covalent lattices have high melting and boiling points? the covalent bonds are broken and these are very strong so require lots of energy to break.
Which bond is the most polarised and explain why: C-Cl or C-F? C-F is the most polarised bond because F is more electronegative than Cl
What shape is an ammonia molecule and the bond angle? Explain how you worked this out Trigonal pyramidal. 3 pairs of bonding electrons and one lone pair around the central Nitrogen atom. The lone pair (on the nitrogen) repels the N-H bonds more than the bonding pairs repel each other resulting in a bond angle of 107o.
Explain why lithium fluoride conducts electricity when molten but not when solid Model answer: CHARGED (particles) MOVE: The CHARGED ions are able to MOVE when molten and are fixed in place in a solid
Describe the structure and bonding in lithium and why it has a high boiling point Giant Metallic bonding: There's a strong attraction between the positive ions and delocalised electrons; a lot of energy is required
Why does covalent bonding happen? Covalent bonding happens because the electrons are more stable when attracted to two nuclei than when attracted to only one.
Why does Methane have a low melting point? there are weak VdW intermolecular forces
Why does Tetrachloromethane have a lower boiling point than Trichloromethane? C-Cl bonds are polarised (coz Cl is more electronegative than C); in CCl4, 4 dipoles that cancel each other, leaving only VdW IMF; whereas in CCl3 there are 3 dipoles and they do not cancel each other, the WHOLE olecule is polarised: the IMF are pd-pd.
What is the shape of the Ammonium ion? Tetrahedral, because there are 4 bonding pairs: 3 from the covalent bond in NH3 and the 4th is the dative covalent bond between the Ammonia and the Hydrogen ion
What must you remember to do when asked to draw dot and cross for Ammonium? There is a dative covalent bond
Describe the dot and cross diagram for Ammonium ion 3 dot-and-cross pairs between N and H and a 4th pair is made of two crosses, as it is a dative covalent bond (Clue H+)
Describe how VdW forces arise random movement of electron distribution leads to a temporary (aka instantaneous dipole) dipole in teh molecule; instantaneous dipole induces a dipole in teh neighbouring molecule; the two dipoles attract
Describe how pd-pd intermolecular forces arise in a molecule One or more bonds are polarised because one of the two atoms in the bond is more electronegative than the other; the dipoles/polarised bonds do NOT cancel each other out
Define electronegativity The ability to attract electrons in a PAIR of electrons from a COVALENT bonds. Remember the keywords.
Describe how to draw Hydrogen bonds Draw a Hydrogen sandwiched between two oxygens; draw a lone pair on one of the oxygen, facing the hydrogen; draw a covalent bond and the dashes for the hydrogen bond; draw delta+/- on teh covalent bond; draw the rest of the molecule
Why is SiO2 a solid but CO2 is a gas? Both have covalent bonds; SiO2 is a GIANT structure where each Si is linked to 4 Oxygens; CO2 is a SIMPLE molecule where each C is linked to 2 Oxygens
Explain why H2O has a much higher bp than H2S Model answer: compare/energy; keywords: between molecules=There is H-bonds (and vdW) in H2O but only pd-pd (and VdW) in H2S; it requires more energy to break the H-bonds BETWEEN MOLECULES.
Explain why H2O has a higher bp than NH3 Model answer: compare/energy; keywords: between molecules=Both have H-bonds (and vdW) but dipoles in H2O are stronger than in NH3 (coz O is a more electronegative element); it requires more energy to break the H-bonds BETWEEN MOLECULES of H2O than NH3.
Explain why ice is less dense than water In ice the water molecules are arranged so that the H-bonds hold the molecules in fixed position; in the liquid, the H-bonds break and reform easily without the rigid structure: the molecules can be closer together so water is more dense
Explain HOW you can predict shapes and bond angles Draw dot and cross diagram first; count bonding and lone pairs around central atom; “pairs repel as far as possible”; if no lone pairs: "pairs repel equally"; if one lone pair or more "lone pairs repel more than bonding pairs"
Why is bond angle in NH3 107 and not 109? the lone pair repels the bonding pairs MORE THAN THE BONDING PAIRS REPEL EACH OTHER.
Explain why Hexane is a liquid at room temperature but Butane is a gas Model answer: compare/energy; keywords: between molecules=both have only VdW BETWEEN MOLECULES but there are more electrons in the Hexane molecule than for the Butane so the VdW are stronger in Hexane and require more energy to break the IMF
Predict the shape and bond angle of ClF4- ion. Explain the reasoning 4 Fatoms around central Cl atom=4 dot-and-cross; leaving a pair of e- on Cl and another e-; the extra (-) charge indicates an EXTRA e-; so 6 pairs of e- around central (4 bonding+2lone) that repel as far as possible: octahedral shape with bond angle=90
Compare the shape of NH3 and BF3 NH3 has 4 pairs of e- around central atom (3bonding and 1 lone) so is trigonal pyramidal with bond angles of 107; BF3 only has 3 pairs so trigonal planar with angles of 120
CHALLENGE: How could you predict the shape of H2S without drawing a dot and cross diagram? S is in the same group as O so the molecule will have the same dot-and-cross as H2O....
CHALLENGE: Explain HOW you can work out the group number from a melting point vs atomic number graph where numbers on x-axis (atomic) have been rubbed off Spot the highest and lowest mp; the highest mp is for group 4 elements and lowest for group 8
When a transition metal forms an ion, why are the 4s sub shell electrons lost before the 3d sub shell? The 4s sub shell has a lower energy level than the 3d sub shell, hence it takes less energy to remove. To remember this, write the sub shells in ascending order e.g. Calcium: 1s2 2s2 2p6 3s2 3p6 4s2, not 1s2 2s2 2p6 3s2 4s2 3p6
Why has Ethanediol a higher boiling point than water Model answer: compare/energy; keywords: between molecules=There is H-bonds (and vdW) in both; Ethanediol has two OH groups so can form more OH bonds than water; it requires more energy to break two H-bonds BETWEEN MOLECULES than one
Describe the dot and cross diagram for Hydronium ion H3O+ 2 dot-and-cross pairs shared between O and H and a 3rd pair is made of two crosses, as it is a dative covalent bond (Clue H+)
Describe how VdW forces between molecules arise Random movement of electron so uneven distribution of electrons; this causes a temporary or instantaneous dipole; the dipole then repels the electron cloud in a neighbouring molecule; causing an induced dipole; the two dipoles attract
Describe how permanent dipoles arise When one element is more electronegative than another, a dipole is created: the bond is polarised
Explain why S8 is not a polarised molecule There is only one type of element!
Explain why CH4 is not a polarised molecule C-H bond is not polarised enough (LEARN this!); the difference of electronegativity between C and H is not high enough
Explain why DichloroEthene is NOT a polarised molecule C-Cl is polarised but the molecule is symmetrical; (draw a dot-and-cross to check): so the dipoles cancel
Explain why SF6 has an octahedral shape There are 6 pairs of electrons that repel as far as possible, equally; so the shape is octahedral
What is the strongest intermolecular force in Ethanoic acid? there is an OH group: the OH bond is polarised; the lone pair on an Oxygen atom can form a bond with the partially positive H of another molecule so Hydrogen bonding is the strongest IMF
What is the strongest intermolecular force in Trichloromethane? the C-H bond is NOT polarised so there is no partially positive H so no H-bond; however the C-Cl bond is polarised and the molecule is NOT symmetrical so the CHCl3 will have pd-pd interactions between molecules
What is the strongest intermolecular force in Ammonia? there is an NH group: the NH bond is polarised; the lone pair on an Nitrogen atom can form a bond with the partially positive H of another molecule so Hydrogen bonding is the strongest IMF
What is the strongest intermolecular force in H2S? draw a dot-and-cross diagram: the molecule is NOT symetrical: the SH bond is polarised (but is NOT OH, NH or HF); so pd-pd is the strongest IMF
What is the strongest intermolecular force in LiF? it is a metal with a non-metal: there is NO IMF in an IONIC compound!!!!
What is the model answer to a question on melting/boiling point? identify bonding and structure; describe bonding between particles; compare strength of bonding; discuss energy; IF ALL simp molec; identify IMF (dot-and-cross?); compare strength/energy; IF ALL vDW, discuss nb of e-; if ALL pd-pd, discuss electroneg
What is the strongest intermolecular force in CO2? The C=O is polarised but the molecule is symmetrical (draw a dot-and-cross to check): so the dipoles cancel out; the strongest IMF is vdW between molecules
Created by: UrsulineChem