GCSE AQA Chemistry unit 3

GCSE AQA Chemistry unit 3

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The Periodic Table

History of the Periodic Table

  • In early 1800s coulg only go on Atomic Mass, 2 ways to categorise elements: 1. Physical & Chemical properties. 2. Their Relative Atomic Mass.
  • Newland law of octaves in1864, was the first good effort. Noticed every 8th element had similar properties, however pattern broke on 3rd line with transition metals. His work was criticised since groups contained elements that didn't have similar properties, he mixed metals and non-metals and because he didn't leave gaps for undiscovered elements.
  • Mendeleev put elements in order of atomic mass, he left gaps to keep elements with similar properties in same vertical groups. The gaps predicted properties of undiscovered elements, when found they fitted the pattern.

The Modern Periodic Table

  • Late 19th Century, scientists discovered protons, neutrons & electrons.
  • Modern Periodic Table based on electronic structure, electrons in an atom set out in shells which each correspond to an energy level. Max number of electrons that can occupy an energy level given by 2xn². n is the number of energy level. Apart from transition metals, elements in same group have same number of electrons in the highest occupied energy level.
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Group 1, VII and Transition Metals.

Group 1- The Alkali Metals.

As go down the group: 1. atoms get bigger since there is more shells, they get more reactive since the outer electron is more easily lost. 2. Higher density since atoms have more mass. 3. Lower melting/boiling points. Alkali metals all form 1+ ions from ionic compounds. React with water to form hydrogen.

Group VII- Halogens.

As go down the group: 1. less reactive. 2. Higher melting/boiling points. Non-metals with coloured vapours. Fluorine- yellow gas, Chlorine dense green gas, Bromine red/brown liquid. Iodine dark grey solid or purple vapour.  Both bonding , reacts with metals to form salts. More reactive will displace less reactive ones.

Transition (bridge) Metals

Good conductors of heat & electricty, dense, strong, shiny, less reactive, high melting point. More than 1 ion. Compounds colourful, make good cataylsts. Energy levels overlap between 3rd-4th shell.

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Acids, Alkalis & Titrations.

Acids & Alkalis and Titrations

Arrhenius said acids release hydrogen ions in water. He also said that alkalis form hydroxide ions when in water. Idea worked well for acids/bases dissolved in water.  Lowry & Bronsted said acids are proton donors. Said Acids release H+ ions (proton donors), Bases accept H+ ions (proton accpetors)  Protons hydrated in water. In acidic solutions, acid molecules dissociate, releasing H+ ions. They become hydrated these make an acid an acid. In basic solutions, water molecules can dissociate into H+ & OH- ions= never do in pure water.  strong acids ionise almost completely in water. Weak acids ionise only slightly.  pH of an acid or alkali measure of concentration of H+(aq) ions in a solution. pH of an acid or alkali can be measured with a pH meter or with universal indicator.  titrations can be used to find out concentrations- how much acid is needed to neutralise a quantity of alkali or vice versa.

Titration calculations

Concentration (in mol/dm³ )=moles÷volume (in dm³). Step by step if ask for concentration in MOLES per dm³. Step 1. work out how many moles of known substance you have. 2. write balanced equation and work out how many moles you have of unknown substance. 3. work out concentration of unknown stuff. If ask for concentration in Grams per dm³.--> number of moles= mass÷relative formula mass. step 1, work out relative formula mass for acid. 2, convert concentration in moles into concentration in grams.

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Water and Solubility.

Water

Sun evaporates water from the sea, water vapour rises then cools causing it to condense into clouds. When condensed droplets get too big they fall as rain, water runs back into sea. At some stage the water comes into contact with rocks, then the cycle repeats.

Water's a solvent- it dissolves many other chemicals. Water dissolves most ionic compounds, as it surrounds the ions and disrupts the bonding so the structure falls apart.

Solubility- something soluble if it dissolves.

The solubility if a substance un a given solvent is the nom. of grams of the solute that dissolves in a 100g of the solvent at a particular temp. The solubility of solutes usually increase with temp. A saturated solution is 1 that cannot hold any more solid at that temp- and you have to be able to see solid on the bottom to be certain that it's saturated. All gases soluble to some extent.


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Hard Water and Water Quality

Hard water

Makes scum and scale. Caused by calcium and magnesium ions, it comes from areas with rocks like limestone, chalk and gypsum. The calcium is good for teeth and bones, and the scale forms a protective coating, stops poisonous metal ions getting into the water and protects iron pipes from rust. Can remove hardness by removing dissolved calcium and magnesium ions 1. by adding sodium carbonate. 2. by 'ion exchange columns'- sodium ions exchanged with calcium & magnesium ions. 3 scale mainly calcium carbonate dissolved by acid.

Water Quality

water from reservoirs goes to water treatment works for treatment: 1. water pass through mess screen to remove twigs. 2. treated with ozone/chlorine to kill microorganisms. 3. chemicals sdded to make solids stick together and fall to bottom. 4. Water filtered 5. pH corrected. 6. Water chlorinated to kill off anything else.To monitor water quality, samples taken of water.

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Energy, Bond Energy and Energy & Food.

Energy

Exothermic reaction one which gives out energy to surroundings, usually in form of heat & shown by rise in temperature. Endothermic reaction one which takes in energy from surroundings, usually in form of heat & shown by fall in temp. Energy transfer can be measured, can measure amount of energy produced by a chemical reaction by taking temp of reagents, mixing them then taking temp of solution at end of experiment. Biggest problem is losing energy to surroundings. Energy must be applied to break bonds & engery released when bonds form. bond breaking is endothermic and bond formation is exothermic.

Bond Energies

In exothermic reactions ΔH is -ve (ΔH is energy change). products at lower energy than reactants. in endothermic ΔH is +ve and is the opposite to exothermic reactions. Activation energy lowered by catalysts, activation energy represents the min energy needed by reacting particles for reaction to occur. 

Energy and Food- Food energy measured in calories. Energy in food released by respiration (glucose+oxygen-->water+energy).

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Test for Cations, Anions and Organic Compounds.

Test for Cations. (Cations are positive ions).

Flame tests identify metals ions- Lithium burns crimson red, Sodium burns yellow-orange, Potassium burns lilac, Calcium burns brick-red, Barium burns green. Some metals form coloured precipitate with NaOH. Calcium=white, Copper= blue, Iron (II)= green, Iron (III) red/brown, Aluminium=white at first but redissolves if more NaOH added. Magnesiu= white.  

Test for Anions (negative ions).

Testing for carbonates, first test for CO2, can test with limewater (if goes cloudy CO2 present). To test for Chloride, Bromide or Iodide ions add nitric acid then silver nitrate solution- chloride gives white precipitate, bromide gives cream precipitate, iodide gives yellow precipitate.

Test for Organic Compounds.

Can burn when heated , compounds with C=C bonds decolourise bromine water

To find unknown substance use tests for anions, cations and organic substances to see if can identify

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Instrumental Methods.

Advantages of machines. 1. can be operated by technicians. 2. more accurate than lab methods. 3. faster than lab methods. Disadvantages- very expensive to buy, run and maintain.

Atomic Absorption Spectroscopy Identifies metals- like flame test, patterns of light absorbed by metals in sample (each metal produce a different pattern).

Infrared Spectrometry- identifies which frequencies of infrared radiation are absorbed.

Ultraviolet Spectroscopy- similar to infrared spectroscopy but with ultraviolet light.

Nuclear Magnetic Resonance Spectroscopy- used for organic compounds, shows what atoms the hydrogen atoms are connected to , helps find the structure of the molecule.

Gas Liquid Chromatography- used to identify gases & identify.

Mass Spectrometry- method used for both elements & compounds. Tells the mass of each molecule/particle.

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