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Acid and base: definitions
Acid and base: definitions
Recap
Recap
Acids have [H₃O⁺]>[OH⁻] with a pH under 7
Acids have [H₃O⁺]>[OH⁻] with a pH under 7
Bases have [OH⁻]>[H₃O⁺] with a pH > 7
Bases have [OH⁻]>[H₃O⁺] with a pH > 7
A pH of 7 is neutral where [OH⁻] =[H₃O⁺] .
A pH of 7 is neutral where [OH⁻] =[H₃O⁺] .
acid and base equilibria
acid and base equilibria
The dissociation of water is: H₂O ⇄ H⁺(aq) + OH⁻ (aq) , or more specifically:
The dissociation of water is: H₂O ⇄ H⁺(aq) + OH⁻ (aq) , or more specifically:
From this we can write an equilibrium constant expression, Kw:
From this we can write an equilibrium constant expression, Kw:
(remember we do not put liquids into these expressions)
(see the similarity between Kw and Ks!)
Kw = [H⁺] [ OH⁻] or [H₃O⁺] [OH⁻]
Kw = [H⁺] [ OH⁻] or [H₃O⁺] [OH⁻]
At a pH of 7:Kw = [OH⁻] =[H₃O⁺], where:[H₃O⁺] = 1 x 10⁻⁷ molL⁻¹,and[OH⁻]= 1 x 10⁻⁷molL⁻¹ Therefore Kw = 1 x10⁻¹⁴.
At a pH of 7:Kw = [OH⁻] =[H₃O⁺], where:[H₃O⁺] = 1 x 10⁻⁷ molL⁻¹,and[OH⁻]= 1 x 10⁻⁷molL⁻¹ Therefore Kw = 1 x10⁻¹⁴.
If we use the Kw expression we can work out the [OH⁻] or [H₃O⁺] provided we know the concentration of one of the ions, as we know the value for Kw.
If we use the Kw expression we can work out the [OH⁻] or [H₃O⁺] provided we know the concentration of one of the ions, as we know the value for Kw.
Logs,
Logs,
antilogs
antilogs
recap
recap
A recap on logs and antilogs can be found by scanning the QR code to the side or navigating to the acids and bases post under L2 Chemistry
Strong or Weak acids and bases
Strong or Weak acids and bases
Acid and base ionisation / dissociation
Acid and base ionisation / dissociation
RECAP: Due to the large amounts of ions produced by strong acids and bases they are much better conductors than weak acids or bases. In addition their reactions are faster due to there being more particles present.
RECAP: Due to the large amounts of ions produced by strong acids and bases they are much better conductors than weak acids or bases. In addition their reactions are faster due to there being more particles present.
species in solution
species in solution
Aqueous solutions are formed when solutes dissolve in water. A solution can be acidic, basic or neutral.
Aqueous solutions are formed when solutes dissolve in water. A solution can be acidic, basic or neutral.
In aqueous solutions the species present in the greatest concentration is water (55.6 molL⁻¹); however in most cases you will be asked to ignore the water.
In aqueous solutions the species present in the greatest concentration is water (55.6 molL⁻¹); however in most cases you will be asked to ignore the water.
Strong acids / weak acids in solution
Strong acids / weak acids in solution
This image shows the relative concentration of species when strong acids and weak acids dissociate. The same holds true for strong and weak bases.
This image shows the relative concentration of species when strong acids and weak acids dissociate. The same holds true for strong and weak bases.
Let's work with a couple of examples, using HCl as a strong acid and NaOH as a strong base, both with a concentration of 0.01 molL⁻¹.
Let's work with a couple of examples, using HCl as a strong acid and NaOH as a strong base, both with a concentration of 0.01 molL⁻¹.
strong acids and bases in solution
strong acids and bases in solution
weak acids and bases in solution
weak acids and bases in solution
ionic salts
ionic salts
The species present in solution depend whether the salt forms neutral solutions, acidic solutions or basic solutions. It is important to remember here that ionic salts when they dissolve, dissociate completely into their constituent ions- hence producing a lot of charges particles meaning they are good conductors of electricity. The pH of the solution depends on whether the ions produced hydrolyse further with water.
The species present in solution depend whether the salt forms neutral solutions, acidic solutions or basic solutions. It is important to remember here that ionic salts when they dissolve, dissociate completely into their constituent ions- hence producing a lot of charges particles meaning they are good conductors of electricity. The pH of the solution depends on whether the ions produced hydrolyse further with water.
neutral and acidic salts
neutral and acidic salts
basic salt
basic salt
A salt that forms basic solutions, such as sodium ethanoate would dissociate along the same lines at the acidic salt, ammonium chloride, with the main difference being that further hydrolysation of the appropriate ion would produce an excess of hydroxide ions. The ion in question being the ethanoate ion, which acts as the base when hydrolysing with water; ethanoic acid is the conjugate acid.
A salt that forms basic solutions, such as sodium ethanoate would dissociate along the same lines at the acidic salt, ammonium chloride, with the main difference being that further hydrolysation of the appropriate ion would produce an excess of hydroxide ions. The ion in question being the ethanoate ion, which acts as the base when hydrolysing with water; ethanoic acid is the conjugate acid.
molecular substances
molecular substances
When molecular substances dissolve they separate into aqueous molecules. Generally unless the molecule reacts with water a solution with [equal] of [OH⁻] and [H₃O⁺] = 1 x 10⁻⁷molL⁻¹
When molecular substances dissolve they separate into aqueous molecules. Generally unless the molecule reacts with water a solution with [equal] of [OH⁻] and [H₃O⁺] = 1 x 10⁻⁷molL⁻¹
- a neutral solution with a pH of 7. Additionally conductivity will be poor.
- a neutral solution with a pH of 7. Additionally conductivity will be poor.
species in solution questions
species in solution questions
weak acid and base calculations
weak acid and base calculations
RECAP: For strong acids and bases that completely dissociate then the pH is calculated by pH=log[H₃O⁺]
RECAP: For strong acids and bases that completely dissociate then the pH is calculated by pH=log[H₃O⁺]
pH is a measure of the [H₃O⁺] - the more there are the more acidic the solution and the lower the pH. As these numbers can be really small, eg 1.54 x10⁻⁴ the log (to the base 10) is used which gives a more manageable number, in this case 4
pH is a measure of the [H₃O⁺] - the more there are the more acidic the solution and the lower the pH. As these numbers can be really small, eg 1.54 x10⁻⁴ the log (to the base 10) is used which gives a more manageable number, in this case 4
Using the antilog of the pH returns us to the [H₃O⁺].
Using the antilog of the pH returns us to the [H₃O⁺].
The same is true for pOH and [OH⁻] . We need to give pH's but it is easy to convert from pOH to pH if we are starting with the [OH⁻] - just subtract pOH from 14.
The same is true for pOH and [OH⁻] . We need to give pH's but it is easy to convert from pOH to pH if we are starting with the [OH⁻] - just subtract pOH from 14.
Alternatively the Kw expression can be used to find out the [H₃O⁺] from the [OH⁻]
Alternatively the Kw expression can be used to find out the [H₃O⁺] from the [OH⁻]
Weak acids
Weak acids
pKa and Ka comparison
pKa and Ka comparison
kA is the acid dissociation constant and indicates how much an acid dissociates into its ions.
kA is the acid dissociation constant and indicates how much an acid dissociates into its ions.
Similar to Ks (equilibrium constant) a larger value favours the forward reaction. This means dissociation is greater and the acid is stronger.
Similar to Ks (equilibrium constant) a larger value favours the forward reaction. This means dissociation is greater and the acid is stronger.
Kb is the base dissociation constant.
Kb is the base dissociation constant.
pKa is the 'power' of Ka and is inverse to Ka. This means a lower pKa indicates a stronger acid
pKa is the 'power' of Ka and is inverse to Ka. This means a lower pKa indicates a stronger acid
pKa Definition. pKa is the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution. pKa = -log10Ka. The lower the pKa value, the stronger the acid. For example, the pKa of acetic acid is 4.8, while the pKa of lactic acid is 3.8
pKa Definition. pKa is the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution. pKa = -log10Ka. The lower the pKa value, the stronger the acid. For example, the pKa of acetic acid is 4.8, while the pKa of lactic acid is 3.8
You can think of the relationship between Ka and pKa as being the same as the relationship between [Hydronium] and pH. The equations for calculating each from each other are the same.
calculation examples
calculation examples
salt solutions and ph
salt solutions and ph
RECAP: Salts are ionic compounds made of metal cations and anions, apart from the oxide ion and the hydroxide ion.
RECAP: Salts are ionic compounds made of metal cations and anions, apart from the oxide ion and the hydroxide ion.
When an acid and a base react one of the products is a salt.
When an acid and a base react one of the products is a salt.
Salts in solution may be:
Salts in solution may be:
Acidic (pH < 7)
Acidic (pH < 7)
Neutral (pH = 7)
Neutral (pH = 7)
Basic (pH > 7)
Basic (pH > 7)
This is dependent on whether the ions in the salts further react with water - this is known as hydrolysis. If they do the solution produced will be either acidic or basic.
This is dependent on whether the ions in the salts further react with water - this is known as hydrolysis. If they do the solution produced will be either acidic or basic.
acidic and basic salts
acidic and basic salts
salt solution ph calculations
salt solution ph calculations
The pH of salt solutions are worked out in the same way as for weak acids and bases
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