# pH,Acids,Bases,Buffers,Neutralisation.

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

Kw=[OH-][H+]

pH strong base:

[H+}=Kw/[OH-]

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## Buffer

• 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]) http://www.youtube.com/watch?v=XzB-czYbNmc
• 5marker exam Q include the following

HA<----->H+ + A-        [1mark]

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

• 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

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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