Chemistry Unit 5 OCR

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  • Created by: shannon
  • Created on: 07-06-13 14:21

Rate, Equilibrium and pH

  • An acid is a proton donor
  • A base is a proton acceptor 

A strong acid will completely dissociate 

A weak acid only partially dissociates (this is seen as a reversible reaction)

The extent of the dissociation is equal to the strength of the acid 

Ka = (conc of products)/(conc of reactants)               Pka = -log(ka

A stronger acid means a higher value of Ka

A strong acid means a lower Pka value

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Rate, Equilibrium and pH

Kc = (conc of products)/(conc of reactants)

Remember - if the product or reactant has more than one mole, you will have to times the concentration by the power of this number 

  • Kc only changes with changing temperature
  • If the forward reaction is endothermic, the Kc will increase with temperature
  • If the forward reaction is exothermic, the Kc value will decrease as temperature increases

HA + H2O → H3O+ + A-

Where HA  is acid 1

H20 is base 2

H30 is acid 2

A- is base 1 

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Rate, Equilibrium and pH

  • In a strong acid, the proton concentration (H+) is equal to the concentration of the acid
  • In a weak acid, the proton concentration has to be worked out using a formula:

(H+) = Ka X (concentration of HA) You would then square root this to find the answer

  • In a strong base, the proton concentration is equal to:

Kw/(concentration of OH)

  • In a buffer, the proton concentration is equal to:

Ka X (concentration of acid 1/concentration of its conjugate base)

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Rate, Equilibrium and pH

  • A buffer minimises any pH changes

The addition of an alkali will shift the position of the equilibrium to the right

The addition of an acid will shift the position of the equilibrium to the left

Addition of an Acid to a buffer solution

  • The proton concentration increases
  • The conjugate base will react with the protons
  • Equilibrium will shift to the left, removing the protons

Addition of an Alkali to a buffer solution

  • The OH concentration increases
  • Small concentrations of protons react with the OH ions
  • HA dissociates, shifting the position of equilibrium to restore the protons that have been lost
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Energy

Entropy = sum of the products - sum of the reactants

  • In a system that is more random the value of entropy will be more positive
  • In a system that is more ordered the value of entropy will be more negative 

G = H - TS

For a reaction to be feasible, the value of G has to be lower than 1 

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Energy

Half cells

Ecell = Epostive - Enegative 

  • The half cell with the more negative E value supplies the electrons
  • The half cell with the more positive E value receives the electrons
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Transition Elements

Transition metal ion - Ions with a partially filled d sub shell 

Remember not to use the word incomplete instead of partially filled

  • Ligand - an electron pair donor to a metal ion 
  • Substitution - Replacement of one ligand by another 

Precipitation reactions colours

  • Fe(II) - Green
  • Fe(III) - Brown 
  • Cu(II) - Blue 
  • Co (II) - Pink

Properties of transition metals 

  • Catalytic properties
  • Coloured compounds
  • Variable Oxidation states


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