Chemistry Unit 2

?
  • Created by: YOIMO
  • Created on: 05-06-17 09:28

C2.1 Structure and bonding

  • Simple molecules- strong covalent bonds, low boiling and melting point, weak intermolecular forces, does not conduct electricity.
  • Giant covalent structures- carbon can make giant covalent structures AKA macromolecules eg silicon dioxide (similar structure to diamond), granite(3 other carbon atoms, giant flat 2D layers, no covalent bonds between layers so they slide over each other making graphite slippery and grey, delocalised electrons allow graphite to conduct heat and electricity, weak intermolecular forces so the layers can slide over easily) and diamonds (3D giant structure, 4 other carbon atoms, hard and transparent)
  • Fullerenes- large molecules formed from hexagonal rings of carbon atoms, the rings join together to form a 3D shape with different numbers of carbon atoms, some of which are nano-sized. scientists believe they can be used for drug delivery to the body, lubricants, catalysts
  • Metallic bonding: layers of atoms, regular patterns, when metal atoms pack together the electrons in the highest energy level (the outer electrons) delocalise and can move freely between atoms. this produces a lattice of positive ions in a 'sea' of moving electrons. the delocalised electrons strongly attract the positive ions and hold the giant structure together
1 of 8

C2.2 Structure and properties part1

  • Ionic bonding: Giant structure- each ion in the giant structure or lattice is surrounded by ions with the opposite charge and so is held firmly in place, strong electrostatic forces of attraction act in all directions therefore they are solids at room temp which requires large amounts of energy to melt the solids- high melting and boiling point, however when an ionic compound is melted the ions are free to move which allows them to carry electrical charge so the liquids conduct electricity, some ionic solids dissolve in water because water molecules split up the lattice so the ions are free to move in the solutions and conduct electricity
  • metal uses in wires: when stretched the atoms slide into new positions without breaking apart
  • shape memory alloys- can be bent or deformed into a different shape but when they're heated they return to their original shape eg dental braces
  • metal structures have delocalised electrons which move throughout the giant metallic lattice and can transfer energy quickly
  • LD polyethene and HD polyethene have different properties because they're made using different catalysts and different reaction conditions. HD polypropene has a higher softening temperature and is stronger than LD polypropene
  • polypropene softens at a higher temp than polyethene.
2 of 8

C2.2 Structure and properties part2

  • thermosetting polymers- covalent bonds, cross-links between their polymer chain, does not soften when heated
  • thermosoftening polymers- forces between the polymer chains are weak, when heated the weak intermolecular forces are broken and the polymers become soft, when the polymer cools down the intermolecular forces bring the polymer molecules back together so the polymer hardens again
  • a nanometre is 1 billionth of a metre (or 10-9 m) and nanoparticles are a few nanometres in size. they contain a few hundred atoms arranged in a particular way. their very small sizes gives them very large surface areas and new properties that can make them very useful materials
3 of 8

C2.3 How much?

  • the relative atomic mass of an element in grams is called one mole of atoms of the element
  • relative formula mass AKA Mr
  • foods can be checked by chemical analysis to ensure only safe, permitted additives have been used. the methods used include paper chromatography and mass spectrometry
  • paper chromatography can be used to analyse the artificial colours in food
  • compounds in a mixture can be separated using gas chromatography. once separated compounds can be identified using a mass spectrometer. the mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak
4 of 8

C2.4 Rates and energy

  • the minimum amount of energy that particles must have in order to react is called the activation energy
  • factors increasing the rate of reaction: temperature, concentration of solutions, pressure of gases, surface area of solids, using a catalyst
  • a powder reacts faster than large lumps of a substance
  • catalysts that speed up reactions lower the activation energy
  • some catalysts are used in industry involving transition metals and their compounds. some of these metals and their compounds are toxic and may cause harm if they get into the environment
  • exothermic reactions: combustion eg burning fuels, oxidisation eg respiration, neutralisation reactions involving acids and bases, eg hand warmers
  • endothermic reactions cool the surroundings but need to be heated continuously to keep the reaction going eg thermal decomposition, eg cold packs
  • when water is added to anhydrous copper sulfate the reaction is exothermic
5 of 8

C2.5 Salts and electrolysis part1

  • acids are substances which produce hydrogen ions H+ (aq) when they are added to water <7
  • alkalis dissolve in water to give hydroxide ions, OH- (aq), in the solution >7
  • when metals react with acids they produce a salt and hydrogen gas
  • salts can be crystallised from solutions by evaporating off water
  • acid + metal -> salt + hydrogen
  • bases react with acids and neutralise them and a salt and water are produced
  • acid + base -> salt + water
  • metal oxides and metal hydroxides are bases
  • HNO3 nitric acid, H2SO4 sulfuric acid
  • insoluble salts can be made by reacting 2 solutions to produce a precipitate that can be filtered from the solution and washed with distilled water and dried
  • some pollutants such as metal ions can be removed from water by precipitation. the water is treated by adding substances that react with the pollutant metal ions dissolved in the water to form insoluble salts
  • ionic compounds can only be electrolysed when they are molten or in solution because then their ions are free to move to the electrodes
  • inert- unreactive
6 of 8

C2.5 Salts and electrolysis part2

  • half equations: negative electrode- Pb2+  +  2e-  -> Pb  positive electrode- 2Br-  ->  Br2  + 2e-
  • MRCNG and NAPOL
  • hydrogen is produced at the negative electrode but only if the other positive ions in the solution are those of a metal more reactive than hydrogen
  • oxygen is usually produced at the positive electrode however if a reasonably high concentration of a halide ion, then a halogen will be produced
  • Aluminium is more reactive than carbon as it has to be melted and purified before it can be electrolysed from aluminium oxide. aluminium oxide melts at over 2000' which would need a lot of energy therefore it's mixed with cryolite at 850'. the mixture can be electrolysed at a lower temp now and produce aluminium and oxygen as the products. at the positive electrode oxide ions are oxidised to form oxygen atoms as they lost electrons. at the negative electrode aluminium ions are reduced to aluminium atoms by gaining electrons.
  • brine is a solution of sodium chloride in water. when its electrolysed hydrogen is produced at the negative electrode from the hydrogen ions. chlorine is produced at the positive electrode from the chloride ions which leaves a solution of sodium ions and hydroxide ions
  • sodium hydroxide is used in soap and bleaches. hydrogen is used to make margarine
7 of 8

C2.5 Salts and electrolysis part3

  • electroplating metal reasons: more, protect corroding, hardness of surface, reduce cost instead of a pure metal
  • the object to be electroplated is made at the negative electrode in an electrolysis cell. the plating metal is made at the positive electrode. the electrolyte contains ions of the plating metal
8 of 8

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Structure and bonding resources »