EL 5&6: Covalent bonding, shapes of molecules, amount of substance Flashcards
List the names for compounds containing 1-4 atoms of the same elements.
- Monoatomic
- Diatomic
- Triatomic
- Tetra-atomic
In covalent bonding, there is a balance between which forces?
- Repulsive forces between nuclei
- Attractive forces between nuclei + electrons
Draw dot and cross diagrams showing the bonding in:
- ammonia
- water
What is a dative covalent bond and how is it represented?
- Both bonding electrons come from one atom
- Arrow points away from donor atom
Tetrahedral bond angle
109.5o
Octahedral bond angle
90o
Pyramidal bond angle
107o
By what angle does a lone pair distort bonding pair angles?
2.5o
Bent bond angle
104.5o
Pyramidal - 2.5o since lone pair
- Trigonal bipyramidal bond angles
- Draw a generic shape
90o, 120o
Trig planar with groups vertically above + below
Shape of methane?
Tetrahedral
Shape of sulfur hexachloride?
Octahedral
Shape of water?
Bent
Shape of ammonium?
Tetrahedral
Shape of ammonia?
Pyramidal
Draw xenon tetrafluoride.
Draw ethene.
Trigonal planar around both carbons
Draw PCl4-, tetrachlorophosphanium ;)
Trigonal bipyramidal; one bonding pair replaced by lone pair
Draw chlorine pentafluoride / chloride (v) fluoride
Octahedral; one bonding pair replaced by lone pair
Urea has the formula CO(NH2)2. Draw it.
Trigonal planar (120)o around C. All bond angles around Ns = 107o
Use electron pair repulsion theory to deduce the shape of BF3 ← NH3.
NH3 = pyramidal
BF3 = trigonal planar
BF3NH3 = tetrahedral around both B and N
Draw a dot-and-cross diagram for NH2-. Use this to suggest and explain the bond angle in the ion.
Compare this bond angle with that in the ammonia molecule, giving your reasons.
- [Dot-and-cross]
- [Bent (2 lone pairs, bond angle 104.5o)]
- 4 electron groups repel to be as far apart as possible;
- Lone pairs repel more strongly than bonding pairs
- Ammonia is pyramidal: bond angle = 107o
- Angle greater because only 1 lone pair, so less repulsion
0.84 g of magnesium was burnt in 0.56 g of oxygen. What is the formula of the magnesium oxide produced?
Mol Mg = 0.84 / 24 = 0.035 mol
Mol O = 0.56 / 16 = 0.035
1 : 1 so MgO
Experimental results that a hydrocarbon is 75% carbon and 25% hydrogen, by mass. Derive its empirical formula.
Let mass of hydrocarbon = 100 g
Mol C = 75 / 12 = 6.25 mol
Mol H = 25 / 1 = 25 mol
C : H = 1 : 4 so CH4
Calculate the mass of aluminium oxide formed when 135 g of aluminium is burned in air.
4Al + 3O2 →2Al2O3
Mol Al = 135 / 27 = 5 mol
Mol Al2O3 = 5 x 2/4 = 2.5 mol
Mass Al2O3 = 2.5 x 102 = 255 g
2.53 g of hydrated magnesium chloride, MgCl2 • xH2O, was heated to constant mass. 1.17 g of solid remained.
What is the formula of the hydrated compound?
Mol pure solid = 1.17 / 95.3
Mass water in salt = 2.53 - 1.17 = 1.36 g
Mol water = 1.36 / 18
Mol salt to water in ratio 1 : x = 1 : 6.15
Formula = MgCl2 • 6H2O
What is the volume of 319.5 g of chlorine at room temperature and pressure?
Volume = mol x 24
= (319.5 / 35.5 x 2) x 24 = 108 dm3
What is the formula for percentage yield?
100 (experimental mass / theoretical mass)
Give 5 reasons why yields are inefficient.
- Impure reactants
- Side reactions
- Product lost during transfer between vessels
- Reversible reactions
- Changes in temperature / pressure
0.84 g of magnesium was burnt in excess oxygen, producing 1.1 g of magnesium oxide. Work out the percentage yield of the reaction.
Mg + 1/2O2 → MgO
Mol Mg = 0.84 / 24 = 0.035 mol
Mol MgO = 0.035 mol
Theoretical mass MgO = 0.035 x 40 = 1.4 g
% yield = 1.4 x 100 / 1.1 = 79%
