AQA New Specification C2B

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Rates of reactions

  • Collision theory states that particles must collide with sufficient energy in order to react
  • The minimum energy required for a reaction to take place (a successful collision) is the activation energy, 
  • The rate of reaction can be measured as: - the amount of reactant used up in a set time. -or the amount of product formed in a set time. 
  • Collisions that result in a reaction are more frequent with an increase in:
  • tempreture
  • concentration of a solution
  • pressure in gas(es)
  • surface area of a solid. 
  • Using a catalyst lowers the activation energy required for particles to react. 
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Types of reaction

  • In an exothermic reaction, energy is transferred to the surroundings. Exothermic reactions give OUT energy. 
  • In an endorthermic reaction, energy is transferred from the surroundings. Endothermic reactions take IN energy. 
  • If a reversible reaction is exothermic in direction, it is endothermic in the opposite direction, e.g: ammonium chlorideEquilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif)ammonia + hydrogen chloride OR  hydrated copper sulfate (blue)Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif)anhydrous copper sulfate (white) + water
  • In reversible reactions, equilibrium is reached at a point when the rate of the reverse reaction balances the rate of the forward reaction. 
  • The relative amounts of all the reacting substances at equilibrium depend on the reaction conditions.
  • Optimum conditions for the Haber process are:
  • temperature at 450°C 
  • pressure 200 atmospheres
  • iron catalyst
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Ions

  • When molten or dissolved in water, the ions in the ionic compounds are free to move. 
  • In electrolysis, an electric current passed through an ionic compound in which ions are free to move breaks it down into its elements.
  • During electrolysispositive metal ions gain electrons to form atoms, negative non-metal ions lose electrons to form atoms and molecules. 
  • Electrolysis of sodium chloride (common salt) solution makes hydrogen, chlorine and sodium hydroxide:
  • Each positive sodium ion is reduced to form an uncharged sodium metal:
  • Na^+ (aq) + e^- ----> Na (s)
  • Each negative chloride ion is oxidised to form an uncharged chlorine atom that shares electrons with another chlorine atom to form chlorine gas:
  • 2Cl^- (aq) ---->Cl2(g) + 2e^2
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain- of electrons
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Making salts

  • Metal oxides and hydroxides are bases and react with acids to form salts. Soluble bases are alkalies.
  • In neutralisation reactions, hydrogen ions and hydroxide ions react to form water:---------------------------------------> H^+ (aq) + OH^- (aq) ---> H20 (l)
  • Neutralisations always produce a salt:
  • acid + metal ---> metal salt + hydrogen
  • acid + ammonia ---> ammonium salt
  • acid _ soluble base ---> metal salt + water
  • acids + insoluble base ---> metal salt + water
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Making salts 2

  • Some metals can be reacted with acids to make salts. The reactivity series of metals shows wether a metal will react with an acid. 
  • Salts from acids: sulfuric acid --> sulfates; nitric acids --> nitrates; hydrochloric acids ---> chlorides 
  • Insoluble salts can be made as precipitates when two solutions are mixed together.
  • Ion impurities can be removed from the water supply by making them into insoluble salts, e.g. phosphate ions from washing powders can be removed using calcium ions to form insoluble calcium phosphate. 
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