Chemistry revision IGCSE


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Chemistry revision IGCSE

  1. 1. Chemistry Revision
  2. 2. Atoms All chemical elements are made up of atoms Every atom is made up of smaller particles; neutrons and protons in the atom’s nucleus which is surrounded by electrons Atoms become ions when they gain/lose an electron The atomic number is the number of protons in the nucleus ( which is the same as the number of electrons) The mass number is the protons + neutrons in the nucleus, the mass of an electron is so small it is considered to be negligible
  3. 3. Atom Mass A is the relative atomic mass M is the relative molecular mass, For example- H₂O has a relative molecular mass of 18 [(2x1) + 16) For ionic compounds the term relative formula mass is used as there is no separate molecules r r
  4. 4. Isotopes Isotopes are different forms of the same element If an isotope has more neutrons than protons, it can sometimes be unstable and is likely to decay
  5. 5. Alkali Metals Group 1 in the periodic table More reactive as you go down the group Low densities Ionic compounds +1 charge Alkaline solution Soft shiny metals React with the oxygen in the air and become tarnished so they are stored in oil React vigorously with water, they fizz and produce hydrogen gas Identifying Alkali Metals- identified using the flame test Lithium burns with a red colour Sodium burns with a yellow/orange colour Potassium burns with a lilac/purple colour
  6. 6. Halogens Group 7 in the periodic table More reactive as you go up the group Reacts with alkali metals Halogens react with iron- iron wool bursts into flames when it reacts with fluorine gas Halogen displacement reactions mean the more reactive halogen displaces the less reactive halogen Identifying Halides- identified using the precipitation test using silver nitrate Chlorine- white/silver Bromine- Pale yellow/cream Iodine- yellow
  7. 7. Metal – Physical Properties The following properties applies to copper High electrical conductivity High thermal conductivity High melting point and boiling point Ductile Corrosion resistant Antibacterial Malleable (can be hammered/squashed into different shapes) Easy to alloy
  8. 8. Metallic Structure positive The structure can easily pass the vibration of hot particles along and the free electrons move faster which is why metals are such good conductors
  9. 9. Ionic Compound- Physical Properties The following properties applies to Sodium Chloride High melting point- because they have strong attractive forces ( lot of energy to separate) Solid sodium chloride does not conduct electricity because the lattice holds to sodium and chloride ions in fixed positions ( Not free to move) but it can conduct electricity does if dissolved in water as the lattice breaks down so the ions move freely Brittle- if a stress is applied will make the layers move Soluble in water
  10. 10. Ionic Structure The sodium lost an electron and the chlorine gained an electron
  11. 11. Simple Covalent Molecule- Physical properties Low melting point( strong bonds but weak attractive forces) Liquid at room temperature Poor conductor of electricity Water soluble
  12. 12. Simple Covalent Structure A covalent bond happens when electrons are shared between to atoms
  13. 13. Giant Covalent Substance Diamond Transparent + crystalline (used for jewellery) Extremely hard ( lot of energy needed to break down lattice) Electrical insulator High melting point Graphite Grey/black shiny solid Very soft- used in pencils Conducts electricity( along the layer as electrons are free to move but does not conduct across the layers) High melting point Slippery( hexagonal layers slide across each other Both different forms of carbon
  14. 14. Structure of diamond and graphite Carbon atoms each connected to 4 other atoms 3-d Lattice based on a tetrahedral unit cell graphite Diamond Made up of layers Arranged in hexagonal rings Each carbon atom is connected to 3 other atoms Bonds between layers are weak Layers can slide over each other
  15. 15. Carbon Nanotubes One of the stiffest and strongest fibres known High electrical conductivity Formed when graphite layers are rolled up into tubes Covalently bonded hexagonal carbon sheets make it very strong Proposed uses for electrical connections in smaller circuits Used in bike components, boat hulls and bonding of things like wind-turbines Could be used for cancer treatment
  16. 16. Smart Materials Smart materials are materials that change when there is a change in their surroundings Thermochromic Pigments Special paints which change colour when there is a change in temperature Most are base on liquid crystal technology Used in mugs to see hot temperatures Also used in T-shirts Photochromic Pigments Contain organic molecules which will change colour when exposed to light, especially ultraviolet light Used in T-shirts and glasses
  17. 17. Smart Materials Shape-memory polymers Plastics that regain their shape when heated Somewhere between thermoplastics and thermosets Used for sealing around window frames Could be used for car bodies (fix the dent) or stitches which will adjust to tension Shape-memory alloys Metal alloys that regain their shape when heated Used for spectacle frames, surgical plates and wires Hydrogels Polymers that absorb or expel water and swell or shrink the size it swells/shrinks to depends on changes in PH or temperature Used in artificial muscles
  18. 18. Rates of reaction We can measure the rate of reaction by : -Capturing & measuring the volume of gas produced by a reaction - Measuring the change in mass - Measuring the amount of light passing through a reaction that is producing a precipitate
  19. 19. Chemical Reactions Chemical reactions occur when molecules/atoms/ions collide but it only happens there is enough energy for the bonds to break and be reformed. Successful collisions per second = collision frequency Factors that affect the rate of reaction 1. Temperature 2. Surface Area 3. Concentration 4. Use of a catalyst
  20. 20. Catalysts These are substances that increase the rate of reaction but remain chemically unchanged It reduces the amount of energy needed which will increase the collision frequency Used in the production of bulk materials eg. Sulphuric acid and food processing
  21. 21. Percentage Composition of Compounds Calculate the % of copper in copper sulphate, CuSO4 ◦Relative atomic masses: Cu = 64, S = 32 and O = 16 ◦relative formula mass = 64 + 32 + 4x16 = 160 ◦only one copper atom of relative atomic mass 64 ◦% Cu = 64 x 100 / 160 = 40% copper by mass in the compound Example
  22. 22. Masses of reactants and products The total mass of the reactants equals the total mass of the products
  23. 23. The Yield of a Chemical reaction Yield= What was obtained What was expected X 100 % Example: Mass of carbon monoxide you would expect to reduce 1000g of iron oxide Fe₂O₃ + 3CO 2Fe + 3CO₂ Fe₂O₃ CO Mass(g) 1000 ? = 525 Mr 160 40 Mass/Mr 6.25 18.75 (3x6.25) 1 : 3
  24. 24. Combustion of methane The breaking of the bong is endothermic (gives out heat The formation of the bond is exothermic (takes in heat)
  25. 25. Oil Crude oil is a mixture of hydrocarbons Fractional distillation is how you separate different fractions of oil The larger the molecule the higher the boiling point
  26. 26. Alkanes and Alkenes Alkanes are saturated hydrocarbons which means they only contain single bonds Alkenes are unsaturated hydrocarbons they have a double bond Alkenes can be produced from alkanes by cracking ( heating the alkane with a catalyst)
  27. 27. Polymers Examples: polythene , polypropene Monomers (ie ethene) are used to form polymers Ethene can become polyethene by heating it under pressure
  28. 28. Thermosets and thermoplastics Thermosets are plastics that are resistant to heat. Used for kettles, saucepan handles etc. Have strong cross linkages which hold the structure together and make them resistant to heat make them resistant to heat Thermoplastics are plastics that soften when heated. Used for packaging, containers. Made up of polymer chains not linked together so they can slide over each other
  29. 29. Water Treatment When the water is in the reservoir, the large particles will sink to the bottom (sedimentation), the smaller particles will be filtered. The water is chlorinated to kill the bacteria to make it safe to drink. Fluoride is also added It is then stored until it is needed
  30. 30. Desalination Definition: The removal of salt from sea water so it can be used for drinking This is usually done by reverse osmosis which goes through a membrane that lets water molecules through but not salt Problems - Lots of energy required - Produces a lot more greenhouse gases - Expensive - Very salty water is left when fresh water is used - Poor countries that have droughts cannot afford this - Some countries don’t have access to the sea
  31. 31. Extracting water If something is dissolved in water, you can extract it by evaporating & condensing To separate a liquid from water you need to use distillation. Different liquids have different boiling points . If you heat an ethanol and water mixture, the boiling point of ethanol is 78C so by heating the mixture at this temperature the ethanol will evaporate and when the ethanol vapour reaches the condenser it will be cooled and become a liquid
  32. 32. Chromatography Paper Chromatography- a drop of mixture is placed on the Chromatography paper and placed in a solvent used to dissolve pigment. The solvent soaks into the paper and moves upwards. The more soluble will travel with the solvent and move further up the paper. The R value is used to measure this (distance substance travelled/ distance solvent travelled) f Gas Chromatography- can be used to detect pollutants in water or air and it is also used to test for banned substances in sports players. The mixture must be in the form of a gas (vaporised or naturally occurring.) The gas passes through a column and different substances are absorbed into an inert solid or liquid. The position the chemical moves along the column is detected electronically
  33. 33. Hard water Hard water contains dissolved calcium and magnesium ions. Can be temporary or permanent Temporary hard water contains magnesium/calcium hydrogen carbonates . When heated the hardness is removed but it forms calcium carbonate (scale) which clogs up pipes. Permanent hard water contains chlorides/sulphates of calcium and magnesium which does not soften when heated Advantages of hard water -Some people prefer the taste -Calcium is good for teeth and bones -Helps reduce heart illness - Some brewers use hard water in beer Disadvantages of hard water -Difficult to form lather with soap -Scum forms with soap - Scale forms, clogs pipes and kettles
  34. 34. How to soften hard water 1. Boiling- easy and cheap but doesn’t work for permanent hardness 2. Adding Sodium Chloride- prevents calcium and magnesium ions bonding with washing detergent. Cheap, removes permanent hardness but deposits are still formed 3. Ion exchange column- Column filled with resin which has sodium ions. The calcium/magnesium ions are swapped for the sodium ions and come out of the tube with the water where as the calcium ions remain in the resin. Can treat a large amount of water but are more expensive.