Ionic Equations & Precipitation (OCR Gateway Science B)

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  • Ionic equations and precipitation
    • ionic lattices and precipitation reactions
      • ionic substances for ionic lattices. The ions are fixed in position within the solid lattice.
        • The lattice of ionic substances breaks apart in water and the separated ions can move
        • in precipitation reactions, the reactant ions can move in solution but must collide to react
      • in a precipitation reaction involving mixing two different ionic solutions, there is an extremely high collision frequency between the ions of the two solution, so the reaction is extremely fast. The insoluble precipitate appears so fast is seems instant.
    • constructing word equations
      • precipitation reactions can be summarised as:           AB + CD -> AD +CB
        • Eg: when barium chloride is added to sodium sulfate this is the word equation:         Barium chloride + sodium sulfate -> barium sulfate +sodium chloride
      • precipitation reactions of barium chlorde with sulfate ions is used to test for sulfate ions
        • Eg: when barium chloride is added to sodium sulfate this is the word equation:         Barium chloride + sodium sulfate -> barium sulfate +sodium chloride
      • precipitation reactions of lead nitrate with halide ions is used to test  for iodide and chloride ions.
    • preparing a clean and dry sample of an insoluble salt
      • STAGE ONE: MIX. the two solutions to make barium sulfate and sodium cholride (eg)
      • STAGE TWO: FILTER. the precipitate of barium sulfate, which stays in the filter paper
      • STAGE 3: WASH. with distilled water to remove traces of the sodium chloride solution.
      • STAGE FOUR: DRY. by leaving the precipitate in a warm place for the water to evaporate.
    • spectator ions
      • when lead nitrate and sodium iodide react, only lead ions react with iodide ions as these are precipitated out of the solution. the other ions do not directly take part in the reaction. these are spectator ions.
    • ionic equations
      • An equation can be written out fully in words or symbols
        • lead nitrate (aq) + sodium iodide (aq) -> lead iodide (s) + sodium nitrate (aq)
        • Pb(NO3)2 + 2NaI -> PbI + 2NaNO3
      • In an ionic equation only the reacting ions are shown. the spectator ions are left out.
        • PB2+     +    2I-               -> PbI2
        • Like all equations they MUST be balanced

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