Chemistry C3.4

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  • Chemistry C3.4 Will Pound
    • Tests for positive ions
      • Most group 1 & group 2 metal ions can be identified using flame tests
        • Lithium = Red
        • Sodium = Yellow
        • Potassium = Lilac
        • Calcium = Red
        • Barium = Green
      • Sodium hydroxide solution can be used to identify different metal ions, depending on the precipitate that is formed
    • Tests for negative ions
      • We can identify carbonates by adding dilute acid, which produces carbon dioxide gas
        • The gas turns the limewater cloudy
      • We identify halides bu adding nitric acid then silver nitrate solution. This produces a precipitate of silver halide
        • Chloride = White
        • Bromide = Cream
        • Iodide = Pale yellow
      • We identify sulfates by adding hydrochloric acid, then barium chloride solution
        • This produces a white precipitate of barium sulfate
    • Titrations
      • Is used to measure accurately how much acid and alkali react together completely
      • The point at which an acid-base reaction is complete is called the end point of the reaction
      • We use an indicator to show the end point of the reaction between an acid and an alkali
      • Calculations
        • Concentrations of solutions can be measured in g/dm^3 or mol/dm^3
        • Concentration can be calculated from the mass of solute dissolved in a known volume of solution
        • The mass of solute in any volume of solution can be calculated from its concentration
        • If the concentration of one of the solutions used in a titration is known, the results of the titration can be used to calculate the concentration of the other solution
        • Number of moles = Mass in grams/ relative formula mass
    • Chemical equilibrium
      • In a reversible reaction the products of the reaction can react to re-from the original reactants
      • In a closed system equilibrium is achieved when the rates of the forward and reverse reactions are equal
      • Changing the reaction conditions can change the amounts of products and reactants in a reaction mixture at equilibrium
    • Altering conditions
      • Changing the pressure can affect reversible reactions involving gases at equilibrium
        • Decreasing: Favours the reaction with the large number of molecules of gas formed
      • Changing the temperature at which we carry out a reversible reaction can change the amount of products formed at equilibrium
        • Increasing temperature favours the endothermic reaction
        • Decreasing temperature favours the exothermic reaction
    • The Haber process
      • Making Ammonia
        • Ammonia is an important chemical for making other chemicals including fertilisers
        • Ammonia is made from nitrogen and hydrogen in the Haber process
          • Any unreacted nitrogen and hydrogen are recycled in the Haber process
        • The Haber process is done using conditions which are chosen to give a reasonable yield of ammonia as quickly as possible
      • The Economics
        • Uses a pressure of around 200 atmospheres to increase the amount of ammonia produced
        • Higher pressures would make more ammonia
          • Would make the chemical plant too expensive to build and run
        • A temperature of 450 degrees is used for the reaction
          • A lower temperature would increase the yield of ammonia, it would be produced too slowly


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