Rates, Equilibrium and pH Summary Cards for OCR A2 Chemistry


Rates, Strong & Weak Acids, Acid-Base Pairs, Equilibrium constant Kc, pH Calculations, Buffers

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  • Created by: Matt Gill
  • Created on: 14-04-11 17:46


For reaction :

X + Y + Z = products

rate equation determined experimentally as rate = k[X]2[Y]

where k is the rate constant

  • 2nd order with respect to X
  • 1st order with respect to Y
  • Zero order with respect to Z

On a concentration-time graph, 1st order has a constant half life.

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Strong and Weak Acids

In accordance with the Bronstead-Lowry theory:

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

A strong acid is one which will completely dissociate into its ions:

  • HCl ------> H+ + Cl-

A weak acid is one which only partially dissociates into its ions:

  • HCOOH <-----> H+ + HCOO-

Extent of dissociation is the 'strength' of an acid, and is measure by the acid dissociation constant, Ka.

Ka = ( [H+] x [HCOO-] ) / [HCOOH],  pKa = -log(Ka)

The stronger the acid, the greater the value of Ka, and the smaller the value of pKa.

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Acid-base Pairs

Acid base pairs differ by H+

  • HA + H2O <-----> H3O+ + A-

In the above:

  • HA is acid 1.
  • H2O is base 2.
  • H3O+ is acid 2.
  • A- is base 1.

Acid 2 and Base 2 can also be known as conjugate, as they are accepting or donating H+.

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Equilibrium Constant, Kc

  • N2(g) + 3H2(g) <-----> 2NH3(g)

             reactants                   products

Kc = [NH3]2 / ( [NH2] x [H2]3 )

           products over reactants

Kc only changes with temperature

Depends on sign of deltaH for forward reaction:

deltaH +ve (endo), Kc increases with an increase in temperature

deltaH -ve (exo), Kc decreases with an increase in temperature

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pH Calculations


WEAK ACID, [H+] = square root of (Ka x [HA])

STRONG BASE, [H+] = (Kw) / [OH-]

BUFFER, [H+] = Ka x ( [HA] / [A-] )

  • Ka is the acid dissociation constant
  • Kw is the ionic product of water, and is equal to 1.00 x 10-14


  • pH = -log[H+(aq)]
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A buffer is a solution which is able to resist changes to its pH when small amounts of acid and alkali impurities are added to it.

It is usually comprised of a weak acid, HA, and its conjugate base, A-. For example,


The mixture forms two equations:

  • CH3COOH <----> H+ + CH3COO-
  • CH3COO-Na+ ----> CH3COO- + Na+

When impurites are added:

  • Acid added: H+ reacts with CH3COO- to form CH3COOH, causing equilibrium to shift to the left.
  • Alkali added: OH- reacts with H+ to form water, causing equilibrium to shift to right, as H+ ions need to be replaced.
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This sums it all up really nicely! I've got a test next week on this - so thanks! :)

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