acids and bases mindmap

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  • Created by: Jshke
  • Created on: 22-01-18 14:12
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  • chemistry acids, bases,salts
    • types of acids and bases
      • Bronsted-Lowry
        • acids are proton donors
        • bases are proton acceptors
      • Lewis
        • acids are electron pair acceptors
        • bases are electron pair donors
      • Arrhenius
        • acids give out H+ ions
        • bases give out OH- ions.
    • The ionic product of water
      • Kw is the symbol for the ionic product of water
        • the equilibrium constant for any reaction is the sum of the concentrations of the products divided by the sum of the concentrations of the reactants
        • For water Kc=([OH-]*[H+])/[H2O]
          • the concentration of water is a constant
            • so we form a new equation for Kw; Kw= Kc*[H2O]
              • therefore Kw=[H+]*[OH-]
                • Kw has a constant value at 25 degrees or 298 K
                  • Kw= 1*10^-14
    • calculating ph
      • strong acids
        • pH = -log[H+]
          • concentration of acid equals concentration of H+ unless acid is diprotic
      • weak acids
        • weak acids don't fully dissociate when dissolved in water
          • so an equilibrium exists between the acid and its salt + hydrogen ion
            • HA equilibrium H+  +  A-
            • so we use Ka to find the concentration of acid
              • Ka is the acid dissociation constant, Ka = (concentration of salt  X concentration of H+) / (concentration of acid)
                • PKa is the negative log of Ka and gives a measure of how strong a weak acid is
                  • the smaller pKa is the stronger the acid
              • For weak acids each H+ ion produced from the dissociation of the acid is produced alongside a molecule of salt
                • which means that the concentration of hydrogen ions is equal to the concentration of salt
                  • so therefore Ka is equal to (concentration of hydrogen ions)^2  / (concentration of acid)
                    • solve to find concentration of hydrogen ions and find ph
      • strong bases
        • Work out the concentration of the hydroxide ions.Use Kw to work out the hydrogen ion concentration.Convert the hydrogen ion concentration to a pH.
      • weak bases
        • use the percentage dissociation to find the concentration of hydroxide ions then same method as strong base
      • for mixtures calculate the moles of acid and base, find the excess species using the equilibrium equation then use excess moles to find pH
    • pH curves of titrations
      • phenolphthalein is used for strong acid strong base and weak acid strong base
        • methyl orange is used for strong acid weak base titrations
          • for weak acid weak base titrations no indicator can be used because the change in pH is not sharp enough
      • the indicator needs to change over the range of the curve where the gradient is steepest (equivalence points
    • Buffers
      • Buffers are solutions which reist changes in pH
        • Buffers are based on equilibrium reactions which oppose changes by shifting the equilibrium to remove added ions whether basic or acidic
          • acidic buffers are made from weak acids because the dissociation of a weak acid is an equilibrium
            • if acid is added to the equilibrium the salt reacts with the hydrogen ions to form the weak acid so the equilibrium shifts to the left because this acid being produced is the shift
              • if alkali is added the hydroxide ions react with the hydrogen ions to produce water and salt so the equilibrium shifts right as the acid breaks down to replace H+
        • acidic buffers are mixtures of a weak acid and a soluble salt of that acid which maintain a pH of below 7
          • Basic buffers are a mixture of a weak base and a salt of that weak base which maintain a pH greater than 7
            • Adding H+ to basic buffers the equilibrium shifts to RHS as H+ is removed by hydroxide and the base (NH3) breaks down more to replace the hydroxide ions
              • basic buffers are formed using the same method: weak base strong acid or weak base and it's salt
              • Adding OH- to basic buffers the ion is removed by the salt so the equilibrium shifts to the left hand side to remove the hydroxide because the salt reacting with the hydroxide forms the base
          • acidic buffers are formed by mixing a weak acid and a strong base or a weak acid and it's salt
            • basic buffers are formed using the same method: weak base strong acid or weak base and it's salt

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