BTEC bonding and structure
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| What is a dative covalent bond? | Dative covalent bonding or coordinate bonding occurs when one atom provides both electrons need for a bond to form
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| What is ionic bonding | Ionic bonding is the attraction between a positive and a negative ion
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| Why can metals conduct electricity when solid | Delocalised electrons are mobile and therefore free to move around cations
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| Describe the structure in Sodium Chloride | 3D lattice of alternate positive sodium ions and negative chloride ions
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| 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.
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| 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.
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| Which bond is the most polarised and why: C-Cl or C-F? | C-F is the most polarised bond because F is more electronegative than Cl
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| Explain why lithium fluoride conducts electricity when molten but not when solid | The ions are able to move when molten and are fixed in place in a solid
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| 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
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| What is covalent bonding? | When a PAIR of electrons is attracted to two nuclei
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| Why does Methane have a low melting point? | there are weak VdW intermolecular forces
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| What must you remember to do when asked to draw dot and cross for Ammonium (NH4+)? | There is a dative covalent bond
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| Describe the dot and cross diagram for Ammonium ion | it is two non-metals so covalent bond; overlap 1 circle for C with 4 circles for Cl; dot-cross in overlap; BUT THREE dot-and-cross pairs between N and H and the FOURTH pair is made of two crosses, as it is a dative covalent bond
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| 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
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| 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
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| Define electronegativity | The ability to attract electrons in a PAIR of electrons from a COVALENT bonds. Remember the keywords.
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| 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
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| 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
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| 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.
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| 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.
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| 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
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| Explain HOW you can predict shapes and bond angles | Draw dor and cross diagram first; count bonding and lone pairs around central atom; pairs repel as far as possible
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| 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.
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| 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
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| 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
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| 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....
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| 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
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| 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 is easier 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
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| Describe the dot and cross diagram for CCl4 | it is two non-metals so covalent bond; overlap 1 circle for C with 4 circles for Cl; dot-cross in overlap; check group number for the rest
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| Describe the dot and cross diagram for NaCl | it is a metal with a non-metal so ionic: look up the periodic table to see how many electrons have been gained or lost; draw dot for one ion and crosses for other ion; REMEMBER CHARGES
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| Explain why NaCl has a lower melting point than MgO | Model answer: identify/compare/energy: both Giant ionic; Mg+2 more charge and smaller than Na+ so Mg+2 more attracted to O-2 than Na+ attracted to Cl-; more energy needed to break ionic bond
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