C2a Topic 3 - Covalent Compounds and Separation Techniques Flashcards
Simple molecular Covalent Substances have
1) Very strong covalent bonds.
2) Weak forces of attraction between molecules.
3) Weak inter-molecular forces mean they have low melting and boiling points.
4) Often liquids or gases at room temperature.
5) Molecular substances don’t conduct electricity, simply because there are no ions.
6) Examples include hydrogen, oxygen and water.
Giant Molecular Covalent Substances have
1) No charged ions.
2) All the atoms are bonded to each other by strong covalent bonds.
3) They have very high melting and boiling points.
4) Except for graphite, they don’t conduct electricity - not even when molten.
5) They are usually insoluble in water.
6) Diamond and Graphite are the main examples, which are both made only from carbon atoms.
Structure and properties of diamond
1) Each carbon atom forms four covalent bonds in a very rigid giant covalent structure, which makes diamond really hard. This makes diamond great as cutting tools.
2) It doesn’t conduct electricity because there are no free electrons.
Structure and properties of Graphite
1) Each carbon atom only forms three covalent bonds, creating sheets of carbon atoms which are free to slide over each other. This makes graphite useful as a lubricant.
2) The layers are held together so loosely that they can be rubbed off onto paper - that’s how pencil works.
3) As only three out of each carbon’s four outer electrons are used in bonds, there are lots of delocalised electrons. These electrons can move, so graphite is a good conductor of electricity. This means that graphite can be used in electrodes.
Sodium chloride, magnesium sulphate and copper sulphate all conduct electricity when dissolved, but not when solid. So they must be…
Ionic lattices!
Hexane and liquid paraffin don’t conduct electricity at all and have low melting and boiling points. So they must be..
Simple molecular covalent substances!
Silicone(IV) oxide doesn’t conduct electricity and has high melting and boiling points, so it must be a…
Giant molecular substance!
Sucrose doesn’t conduct electricity even in solution so it can’t be an ionic lattice. It has a low melting point and is soluble in water, so it must be a …
Simple molecular substance!
What equipment would you use to separate immiscible liquids
A separating funnel
Explain how separating immiscible liquids works
1) If two immiscible liquids are shaken together, when they are allowed to stand they will separate out into layers.
2) The denser liquid sinks to the bottom and the less dense one forms a layer at the top.
3) These layers can be separated using a separating funnel with a tap. The tap can be opened to drain off the denser liquid into a beaker.
4) E.g. A mixture of oil and water can be separated in this way.
Fractional distillation separates out
Miscible liquids
Describe how we can fractionally distil liquid air
1) Air is filtered to remove dust.
2) It’s then cooled to around -200*C and becomes a miscible mixture of liquids.
3) During cooling water vapour condenses and is removed.
4) Carbon dioxide freezes and is removed.
5) The liquified air then enters the fractionating column and is heated slowly.
6) The remaining gases are separated by fractional distillation. Oxygen and argon come out together so another column is used to separate them.
7) Liquid Oxygen and argon leave at the bottom of the fractionating column, whereas nitrogen gas leaves at the top.
You can use paper chromatography to identify different substances in a
Mixture
Steps involved in chromatography
1) Put spots of each mixture be in tested on a pencil baseline on filter paper.
2) Put the paper in a beaker containing a solvent e.g. ethanol or water. The baseline must be kept above the level of the solvent!
3) The solvent seeps up the paper, taking the samples with it.
4) The different chemicals in the sample form separate spots on the paper.
5) The result of chromatography analysis is called a chromatogram. A chromatogram with four spots means there are at least four different substances in the sample mixture.
How do you calculate Rf value
Rf = distance travelled by substance DIVIDED BY distance travelled by solvent
