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A Level Chemistry Equilibria Notes
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  • A Level Chemistry
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  • Sixth Year / 12th Grade

I just completed my Cambridge a levels with A* chemistry. While i did use save my exams to study i realized there was so much unwanted and so much missing. So i went through the syllabus and made my own notes that covers every topic and everything you need for an A*. I have done 5 years of past pa...

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  • March 31, 2024
  • 11
  • 2023/2024
  • Class notes
  • Shyamala
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  • equilibria

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  • Secondary school
  • A Level Chemistry
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EQUILIBRIA
1.1 Acids and bases

A Brønsted acid is a species that can donate a proton

A Brønsted base is a species that can accept a proton

A conjugate acid-base pair is two species that differ from each other by an H+ ion

In the forward reaction:

o NH4+ is the conjugate acid of
the base NH3
o OH- is the conjugate base of
the acid H2O

In the reverse reaction

o NH3 is the conjugate base of
the acid NH4+
o H2O is the conjugate acid of
the base OH-




1.2 Calculating pH, Ka, pKa & Kw


pH – power of hydrogen

The pH can be calculated using: pH = -log10 [H+] where [H+] = concentration of H+ ions (mol dm-3)

The concentration of H+ ions in solution: [H+] = 10-pH

o pH of ethanoic acid when the hydrogen concentration is 1.32 × 10-3 mol dm-3
o pH = -log [H+] = -log 1.32 x 10-3 = 2.9

Ka & pKa – ionisation of weak acids

The Ka is the stability constant of the dissociation of a weak acid at 298 K

HA (aq) ⇌ H+ (aq) + A- (aq)

, A high value of Ka means that:

o The equilibrium position lies to the right
o The acid is almost completely ionised
o The acid is strongly acidic – lower pH

A low value of Ka means that:

o The equilibrium position lies to the left
o The acid is only slightly ionised (there are mainly HA and only a few H+ and A- ions)
o The acid is weakly acidic – higher pH

Since Ka values of weak acids are very low, pKa values are used instead to compare the strengths of
weak acids with each other

o pKa = -logKa
o The less positive the pKa value is, stronger the acid is




Kw – ionisation of water

The Kw is the stability constant for the ionisation of water at 298 K

o Its value is 1.00 x 10-14 mol2 dm-6
o H2O (l) ⇌ H+ (aq) + OH- (aq)

Kw = [H+] [OH-] concentration is same for both, so simplified to Kw = [H+]2

Concentration of water is regarded as constant and removed

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