IGCSE CHEMISTRY

IGCSE CHEMISTRY

The Particlate Nature of Matter
1. Particle arrangement in a solid, liquid and gas
  1. Solid
    1. When you apply pressure the particles can't move closer, so the volume does not change
    2. Particles are close together
    3. The arrangement is regular
    4. The forces between the particles are strong or quite strong so the particles can vibrate to an fro, but not move apart; that is why a solid has a fixed shape
  2. Liquid
    1. The particles are still close together
    2. The arrangement is irregular
    3. The force between the particles are less strong so the particles can move around, and slide past each other; that is why a liquid can be poured
    4. When you apply pressure the particles can't move closer, so the volume does not change
  3. Gas
    1. The particles are far apart
    2. The arrangement is random
    3. The forces between the particles are non-existent so the particles can move freely, collide with each other, and bounce away again ; that is why a gas spreads
    4. When you apply pressure the particles can move a lot closer, so the gas can be pushed into a much smaller volume
2. Changing State
  1. Heating
    1. In a solid, the particles are held in a regular structure. They don't move away, but they do vibrate to and fro
      1. As the particle take in heat energy, the vibrations get larger and stronger. So the solid expands a little.
        With more heat, the particles vibrate so much that the structure breaks down. The solid melts to a liquid.
        The particles continue to take in heat energy. So they move around more. So the liquid expands a little.
        Some particles gain enough energy to overcome the forces between them and escape. This is evaporation.
        At a certain point, all the remaining particles gain enough energy to escape. The liquid boils to a gas.
    2. The particles gain energy and move faster; in time they gain enough energy to overcome the forces of attraction between them
  2. When a substance changes state the particles do not change- only their arrangement
  3. Cooling
    1. The particles lose energy and move more slowly; as they get closer together, the forces of attraction take over
  4. During the process of melting and boiling the temperature remain constant, because the energy is used to break the bonds between the particles.
3. Diffusion
  1. It is the process by which particles mix and spread, through collisions with other particles
  2. The lighter the substance (lower Mr or Ar) the faster the gas will diffuse
  3. The more kinetic energy the particles have, the faster they move and diffuse.
Atoms elements and compounds
1. Atomic structure and Periodic Table
  1. Particle---mass, in relative atomic mass units--charge
    1. Proton-------------------1------------------1+ (positive)
    2. Neutron--------------------1------------------ no charge
    3. electron--1/1840(or almost nothing so usually ignored-- 1-(negative)
  2. Because they make up the atom, protons, neutrons and electrons are called subatomic partciles
  3. Nucleon number
    1. Proton and neutron form the nucleus and so are called nucleons. Nucleon number is total number of protons and neutrons. Nucleon number= mass number
  4. Proton number
    1. Number of electrons= number of protons= proton number
    2. Elements in the Periodic table are in order of their periodic number
  5. Isotopes
    1. Isotopes are atoms with the same proton number but different nucleon number.
    2. Radio isotopes are unstable atoms, which break down giving radiation
      1. cancer treatment (radiotherapy) –rays kill cancer cells using cobalt-60.
      2. to check for leaks – radioisotopes called tracers are added to oil or gas. At the leaks radiation is detected using a Geiger counter
2. Bonding: the structure of matter
  1. Element contains only one type of atom, compound contains more than one type of atom, held together by chemical bonds and mixture can contain any different number of different substances which are not chemically bonded but are just mixed
  2. Alloy: a mixture where at least one other substance is added to a metal, to improve its properties
    1. brass (70% copper and 30% zinc) is harder than copper, does not corrode, used in musical instruments.
    2. Stainless steel (70% iron, 20% chromium and 10% nickel) unlike iron does not corrode, used in car part and cutlery
  3. Ionic Bonding
    1. An ionic compound (in solid state) has a regular arrangement (lattice) of alternating positive and negative ions.
    2. The bond formed between ions of opposite charge. Also between metal and non-metal
  4. Covalent Bonding and macromolecules
    1. A covalent bond forms between two atoms and is the attraction of two atoms to a shared pair of electrons. Small groups of covalent bonded atoms can join together to form molecules
    2. Silicon (IV) oxide (SiO2 ) has a structure with each Si joined to 4 O and each O joined to 2 Si. It is the main ingredient in glass. It is also sand
    3. Silicon (used in microchips for computers) has the same structure as a diamond
    4. Diamond: The covalent bonds are very strong. Diamond is the hardest naturally occurring material. It does not conduct electricity.
    5. Graphite: Each layer is itself a giant molecule with very strong covalent bonds. Between the layers is delocalised (it is a free electron). Between the layers the bonds are weak. Free electrons between the layers allows graphite to conduct electricity and heat. The layers can easily slide over each other making graphite soft and slippery and an excellent lubricant (like oil)
  5. Metallic Bonding
    1. lattice of tightly packed positive ions in a sea of electrons, resulting in crystals, therefore: 1. Metals are malleable and ductile – the layers of ions can slide over each other 2. Metals are good conductors– free electrons take energy
Chemical energetics
1. Exothermic reaction: gives out energy to the surroundings Bond making is exothermic
  1. Combustion of fuels
  2. Neutralisation
  3. Adding water to anhydrous copper(II) sulfate
  4. Adding concentrated sulfuric acid to water
2. Endothermic reaction: takes in energy from the surroundings. Bond breaking is endothermic
  1. Photosynthesis.
  2. Thermal Decomposition
    1. Metal carbonates such as calcium carbonate break down when heated strongly
  3. ethanoic acid with sodium carbonate
  4. Melting, boiling and evaporation. These are endothermic processes, not reactions
3. Production of energy
  1. Fossil Fuels: oil (petroleum), natural gas, coal
    1. They burn in the oxygen in air
    2. Chemical energy is released as heat
    3. Natural gas is burned in homes for cooking and eating. Also in gas-fired power stations; the heat is used to boil water to make steam, which spins turbines for generating electricity
    4. Produces carbon dioxide which is linked to global warming
  2. Nuclear fuels: Radioisotopes such as uranium-235; these are unstable atoms which break down
    1. Nuclear fuels are not burned; instead the atoms are broken down by bombarding them with neutrons.
    2. Uranium breaks down to give smaller atoms such as lanthanum and bromine atoms: U-235 → la-145 + Br-88 + energy
    3. Atomic energy is released as heat
    4. Nuclear power stations use the heat generated by fission of a fuel (either uranium or plutonium) to boil water to make steam. The steam is used to turn a turbine to generate electricity.
    5. Harmful radiation is given out when nuclear fuels break down. The products are also radioactive.
  3. Hydrogen as a fuel
    1. Hydrogen reacts with oxygen to produce water plus energy. hydrogen + oxygen → water + energy 2H2(g) + O2(g) → 2H2O(l)
    2. Hydrogen can be made from the electrolysis of water. If the electricity used during electrolysis comes from a renewable resource (solar power for example) then the hydrogen fuel produced is also renewable.
    3. Hydrogen used as a fuel does not produce pollution or contribute to global warming because the only product of combustion is water
    4. Hydrogen is explosive and difficult to store.
    5. If the hydrogen does not come from a renewable resource but is made from methane, then the hydrogen fuel produced is also not renewable.
    6. Fuel cells
      1. A fuel cell needs to be continuously supplied with both a fuel and oxygen, which react together to produce electricity.
      2. A hydrogen fuel cell is made from an anode and a cathode with an electrolyte contained between them. The fuel cell generates electricity. It is not the same as an electrolysis cell which needs to be supplied with electricity.
      3. Electrolyte: Potassium hydroxide Anode and Cathode: Platinum catalyst
      4. 2H2(g) + O2(g) → 2H2O(g)
      5. Electricity is produced in a form of electric current
  4. Simple cell
    1. consists of a negative pole (the more reactive metal) and a positive pole (less reactive metal) and an electrolyte. The greater the difference in reactivity of the two metals, the greater the voltage will be. The electrons flow because one metal is more reactive, so it has a stronger drive to give up its electrons. The atoms give up electrons and enter the solution as ions.
4. Rate of reactions
  1. Rate of reaction = amount of reactant used ÷ time. Rate of reaction = amount of product formed ÷ time.
  2. Fill a gas jar with a mixture of hydrogen and oxygen, and cover it. Even if you leave it for hours, no reaction will happen. Then dip a platinum wire into the mouth of the jar. The gas mixture explodes immediately with a pop, producing water.
  3. Reactions with different sized particles (e.g. magnesium powder vs. ribbon + acid, marble chips vs. smaller chips). Time taken for a certain amount of gas to be produced is measured, or change in mass, because the gas escapes (e.g. hydrogen for Mg + acid reaction, carbon dioxide for marble chip + acid experiment.
  4. Hydrochloric acid and sodium thiosulphate solution are mixed in a flask, and a stopwatch starts. They react forming sulphur which is insoluble in water so precipitates. The flask is on top of a cross drawn on a piece of paper. You measure the amount of time taken for the cross to not be visible because there is enough sulphur. The diameter of the cylinder should be kept constant
  5. Large surface area can mean danger: Flour dust, wood dust, custard powder, instant coffee, sugar, and dried milk have large surface areas, and are combustible. A spark from a machine, or a lit match, can cause an explosion, this also applies to gases from mines
  6. Increasing the concentration of a substance in solution means that there will be more particles per dm3 of the substance. The more particles that there are in the same volume, the closer to each other the particles will be. This means that the particles collide more frequently with each other and the rate of the reaction increases.
  7. Raising the temperature makes the particles move faster. This means that the particles collide more frequently with each other and the rate of the reaction increases. Also, the faster the particles are traveling, the greater is the proportion of them which will have the required activation energy for the reaction to occur.
  8. Increasing the pressure of a reaction where the reactant is a gas is similar to increasing the concentration of a reactant in a solution. The gas particles (usually molecules) will be closer together when the pressure increases. This means that the particles collide more frequently with each other and the rate of the reaction increases.
  9. Use a catalyst

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