pH of acids, bases and buffers, definitions, concepts, acid-base pairs, neautralisation curves summary.

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Acid and Bases

  • Acid: Proton Donor [H+]
  • base: proton acceptor
  • Alkali is a base that dissolves in water to form OH- ions

pH= -log[H+] 

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Strong acid

Strong acids: dissociate completely 100% no equilibirum sign needed

HA----> H+ + A-

Equal amount of [HA] and [H]

so pH= -log[HA]

for strong acids

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Weak acids

  • Ka=([H+][A-])/[HA]
  • Ka= 10^-pka
  • pka=-logKa

Weak acids partially dissociate, with equilibirum sign.

HA<------->H+ + A-

[H+]=[A-].... H^2 on Ka equation at top instead of both [H+]and [A-] as they are equal

  • so [H+]=  Ka X [HA]
  • then Use pH= -log[H+]
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Kw ionic product of water

at 25degreesC is 1x10^-14 this is given in the formula booklet for OCRA


pH strong base:


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  • buffer consists of weak acid and it's conjugate base
  • [A-] can be formed by weak acid + alkali or salt of weak acids dissocates completely into [A-]
  • pH of buffer solution pH= pka+ log([A-]/[HA])
  • 5marker exam Q include the following

HA<----->H+ + A-        [1mark]

On addition of alkali:

  • reacts with [HA] 
  • moves equilibrium to right
  • H+ + OH- ----> H20                 [2marks]

On addition of acid:

  • reacts with [A-] reducing excess H+ ions
  • moves equilibirum to left           [2marks]
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conjugate base pairs

Definition: a pair of species that transform into eachother by gain or loss of a proton

  • more reactive acid is acid 1 loses a H and becomes base 1 on the other side.
  • water acts as an acid with NH3 or another base
  • water acts as a base with an acid

ACID1 + BASE2---------> BASE1(-ivecharge as it has lost a H) + ACID 2 (+ivecharge as it has an extra H)

acid one becomes base 1 by losing a H (as acids donate H to the base)

acid 2 becomes base 2 by losing a H

  • in the exam show charges and which one is which acid/base
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Neutralisation: when an acid and base react together to produce salt and water

  • the neutralisation curve has a steady increase then a sharp straight line showing increase and another steady increase, always show this in exams
  • end point when indicator [HA] form=[A-] form the inbetween acid and alkali form in colour
  • equivalance point (sharp increase on curve) is the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution. matches stoichiometry of reaction taking place.
  • We choose an indicator which has it's end point as close as possible to the equivalance point in the graph
  • weak acid and weak base has no sharp increase/no equivalence point- no indicator can be chosen
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Enthalpy change of Neutralisation

Enthalpy change of neutralisation: enthalpy change that accompanies the reacton of neutralisation of an aqueous acid by an aqeous base forming 1 mole of water under standard conditions.

Determing enthalpy change of neutralisation:

  • use Q=m x c x deltaT to work out enthalpy 
  • mass= total volumes e.g 50cm^3acid and 50^cm3 of base...is100g as we assume density =1g/dm^3
  • then divide this by moles we have using n=v x c of one that is not in excess (i.e base)
  •  to get 1 mole of water (as the definition states) 
  • remember it is exothermic so add a negative sign
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reactions involving acids

  • Acid and alkali always makes salt + water H+ + OH----->H2O this is why the delta H is the same/similar no matter which acid we use
  • Acid + Base: salt and water :MgO + 2H+---> Mg2+ +H2O                                                                                        Na2O + 2H+--->2Na+ +H20

acid and metal makes salt and hydrogen :group 2 metal: 2H+ + Mg----> Mg2+ +H2                                                                          or group 1 metal: 2H+ + 2Na--->2Na+ + H2

  • Acid and carbonate: salt, water and carbondioxide produced:                         

CaCO3(s) + 2H------> H2O +CO2 + CO3-2 +Ca2+ 

in solution so CaCO3 can release it's ions they are cancelle out on either side of the ionic equation: CO3-2 +2H---> CO2 +H2O

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