Edexcel Chemistry Unit 2 Notes

Edexcel chemistry unit 2 notes. 

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Shapes of molecules and ions
Demonstrate an understanding of the use of electron-pair repulsion theory to interpret and
predict the shapes of simple molecules and ions
pairs of electrons around the central atom repel each other to a position of maximum
separation and minimum energy
lone pairs have a greater repulsive effect than bonding pairs
o because they are not shared by two atoms so are attracted to a single nucleus and
so are closer to the central atom
lone pairs reduce the angle by ~2.5
Limitations of bonding model
dot-and-cross only show electrons shared, not the lengths of bonds or shape of molecule
most bonds are neither pure ionic or pure covalent, but an intermediate due to bond
Recall and explain the shapes of BeCl2, BCl3, CH4, NH3, NH4+, H2O, CO2, PCl5, SF6 and simple organic
o linear
o 180
o trigonal planar
o 120
o tetrahedral
o 109.5
o pyramidal
o 3 bonding pairs and 1 lone pair repel each other to a position of maximum
o the lone pair is more repulsive so decreases the bond angle slightly to 107
o 107
o tetrahedral
o 109.5
o bent
o 2 bonding pairs, 2 lone pairs
o 104.5
o linear
o 180
o trigonal bipyramid
o 120 and 90
o octahedral
o 90
Demonstrate an understanding of the terms bond length and bond angle and predict approximate
bond angles in simple molecules and ions

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Pairs of electrons Shape Bond angle
2 Linear 180
3 Trigonal planar 120
4 Tetrahedral 109.…read more

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each carbon atom forms four identical bonds to other carbon atoms
tetrahedral arrangement
strong covalent bonds
o very high melting point
o sublimes
o hard; used in drill tips and saws
o good thermal conductor as vibrations travel easily through the stiff lattice
o doesn't conduct electricity because electrons are held in covalent bonds
o insoluble in all solvents
exists in layers
carbon atoms bonded to three other carbon atoms at 120
o very high melting point; sublimes
o insoluble in all solvents…read more

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Between the extremes of pure ionic and pure covalent bonding there is a range of intermediate
bonds. Polarization of ions leads to distorted ionic bonds. If the polarization is large, the electron
density is distorted so much it will resemble a covalent bond.…read more

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Relate the physical properties of materials to the types of intermolecular force present, eg.…read more

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Demonstrate an understanding of:
Oxidation number - the rules for assigning oxidation numbers
o elements have an oxidation number of 0
o group 1 and 2 always have an oxidation number of +1 and +2
o hydrogen is always +1, except in hydrides, where it is -1
o oxygen is -2 except in peroxides where it forms a O-O single bond (-1) or is bonded
with Fluorine (which is always -1)
o fluorine is always -1
o other halogens are usually -1, except when…read more

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Recall the reaction of the elements in group 2 with oxygen, chlorine and water
All react to form solid metal oxides
Reactivity increases down group
Mg (s) + 0.5O2(g)--> MgO (s)
All react to from solid metal chlorides
Mg(s) + Cl2(g) --> MgCl2(s)
Beryllium won't react unless dust due to insoluble oxide layer.…read more

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Recall the characteristic flame colours formed by group 1 and 2 compounds and explain their
origin in terms of electron transitions
Sodium: yellow (sunny sodium)
Potassium/Caesium: lilac
Lithium/Calcium/Strontium: red
Barium: green (BaG)
Magnesium: colourless
Electrons are given energy and are excited to higher energy levels.…read more

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Red-brown gas/liquid (very volatile)
- Partially soluble in water
- Very soluble in a hydrocarbon solvent
- Red-brown solution in both cases
- Grey-black solid
- Sublimes on heating giving a purple gas
- Slightly soluble in water giving a brown solution
- Very soluble in hydrocarbon giving a pink solution
Describe and carry out the following chemical reactions of halogens:
Oxidation reactions with metal and non-metallic elements and ions such as iron(II) and
iron(III) ions in solution
Cl2 is the most reactive and…read more

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Evaluation of the results:
- difficult to determine accurately the volume of the liquid in a burette if the meniscus lies
between two graduation marks
- the material used to prepare a standard solution may not be 100% pure
- a burette is calibrated to be used at 20oC; a higher temperature could result in a
difference of the actual volume of liquid when filled to a calibration mark
- difficult to make an exact judgement on the exact end-point of a titration
Finding the…read more


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