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Strength of Acids

Key Concepts

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Acid Dissociation
(Acid Ionisation or Acid Ionization)

When an acid dissociates (or ionises) in water it forms hydrogen ions2 and anions which are in equilibrium with the undissociated acid molecules:

  acid hydrogen
ions
+ anions
general form HA H+(aq) + A-(aq)
hydrochloric acid HCl H+(aq) + Cl-(aq)
nitric acid HNO3 H+(aq) + NO3-(aq)
acetic (ethanoic) acid CH3COOH H+(aq) + CH3COO-(aq)
formic (methanoic) acid HCOOH H+(aq) + HCOO-(aq)

For a strong acid, the equilibrium position lies very far to the right, so that there are almost no undissociated acid molecules present.
We write the balanced chemical equation for the dissociation of a strong acid as going to completion and use a single arrow to show the direction of the reaction:

  acid hydrogen
ions
+ anions
strong acid HA H+(aq) + A-(aq)
hydrochloric acid HCl H+(aq) + Cl-(aq)
nitric acid HNO3 H+(aq) + NO3-(aq)

For a 0.010 mol L-1 aqueous solution of monoprotic strong acid:

  acid hydrogen
ions
+ anions
strong acid dissociation equation: HA H+(aq) + A-(aq)
concentration of species in solution ≈ 0 0.010 mol L-1 0.010 mol L-1

We say that the monoprotic strong acid has fully dissociated (or fully ionised or fully ionized).

  acid hydrogen
ions
+ anions
strong acid dissociation equation: HA H+(aq) + A-(aq)
concentration of species in solution ≈ 0 0.010 mol L-1 0.010 mol L-1
% of undissociated acid molecules in solution 0%

There has been 100% dissociation of the acid (or 100% ionisation or 100% ionization).

A weak acid does not fully dissociate in water, the undissociated acid molecules are in equilibrium with their hydrogen ions and anions.
The balanced chemical equation for the dissociation of a weak acid is written using an "equilibrium arrow" to show that both the forward and reverse reactions are occurring:

  acid hydrogen
ions
+ anions
weak acid HA H+(aq) + A-(aq)
acetic acid (ethanoic acid) CH3COOH H+(aq) + CH3COO-(aq)
formic acid (methanoic acid) HCOOH H+(aq) + HCOO-(aq)

For a 0.010 mol L-1 aqueous solution of monoprotic weak acid which dissociates to produce n moles per litre of H+(aq):

  acid hydrogen
ions
+ anions
weak acid HA H+(aq) + A-(aq)
concentration of species in solution 0.010 - n n + n
% of undissociated acid molecules in solution: more than 0%
less than 100%

A weak acid only partially dissociates (ionises) to form hydrogen ions and anions, some undissociated acid molecules still exist in the solution.

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Comparing the Strength of Acids

Two common methods used to compare the strength of acids are:

  1. electrical conductivity of solutions
    More ions in solution means greater electrical conductivity.
  2. pH of solutions
    More hydrogen ions in solution results in lower pH

BUT beware!

In order to compare the strengths of aqueous solutions of acids they must both be:

  1. at the same temperature
    The equilibrium position is affected by changes in temperature.
  2. the same concentration
    The concentration of ions in solution depends on the initial concentration of the acid.

Let′s take a look at each method for comparing the strength of acids:

  1. Electrical Conducitivity of Dilute Aqueous Solutions

    A strong acid dissociates fully producing the maximum number of hydrogen ions and anions in solutions.
    A weak acid only partially dissociates so it produces fewer ions in solution than a strong acid.

    How well an aquous solution conducts electricity is dependent on the number of mobile ions available3.
    The more mobile ions there are in solution, the better the electrical conductivity of the solution.

    In order to determine the comparative (or relative) strengths of monoprotic acids, we could measure the electrical conductivity of each solution as long as we use the same concentration for each acid, and all the measurements are taken at the same temperature.

    Monoprotic acids at the same
    concentration and temperature
    stronger acid ← ← weaker acid
    better electrical
    conductor
    ← ← poorer electrical
    conductor

    Note that the solutions must have the same concentration.
    For a water soluble acid, the more concentrated the solution, the more hydrogen ions there will be in solution and the better the solution will conduct electricity.
    Similarly the solubility of an acid is dependent on temperature, and the mobility of ions in solution is temperature dependent, so the electrical conductivity measurements must be made at the same temperature.

  2. pH of Dilute Aqueous Solutions

    pH is a measure of the concentration of hydrogen ions in solution.

    A strong acid dissociates (ionizes) fully producing the maximum number of hydrogen ions and anions in solutions.
    A weak acid only partially dissociates so it produces fewer ions in solution than a strong acid.

    The more hydrogen ions there are in the solution, the lower the pH will be.

    In order to determine the comparative (or relative) strengths of monoprotic acids, we could measure the pH of each solution as long as we use the same concentration for each acid, and all the measurements are taken at the same temperature.

    Monoprotic acids at the same
    concentration and temperature
    stronger acid ← ← weaker acid
    lower pH ← ← high pH

    Note that the solutions must have the same concentration of acid.
    For a water soluble acid, the more dilute the solution, the fewer hydrogen ions there will be in solution and the higher its pH will be.
    Similarly the solubility of an acid is dependent on temperature which will effect the number of ions in solution, so the pH measurements must be made at the same temperature.

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Worked Examples

Question 1. Which acid will have a higher pH at 25°C, 0.010 mol L-1 HCl(aq) or 0.010 mol L-1 HF(aq)?

  1. Note that the solutions are both monoprotic:
    HCl H+(aq) + Cl-(aq)
    HF H+(aq) + F-(aq)
  2. Note that both solutions are at the same concentration and temperature.
    This means we can compare the strength of each acid in order to determine which acid will have the higher pH.
  3. Compare the strength of each acid:
    HCl is a strong acid, it will fully dissociate in water:
    HCl → H+(aq) + Cl-(aq)
    HF is not a strong acid, it will not fully dissociate in water:
    HF H+(aq) + F-(aq)
  4. Compare the concentration of hydrogen ions in solution:
      undissociated acid
    concentration
    hydrogen ion
    concentration
    + anion
    concentration
    HCl 0   0.010 mol L-1 0.010 mol L-1
    HF 0.010 - n mol L-1 n mol L-1 n mol L-1

    The concentration of H+(aq) in HF(aq) must be less than the concentration of H+(aq) in HCl(aq).
  5. Compare the pH of each solution:
    More H+(aq) in solution = lower pH
    Fewer H+(aq) in solution = higher pH
    0.010 mol L-1 HF(aq) produces fewer H+(aq) in solution than 0.010 mol L-1 HCl(aq).
    At 25°C, 0.010 mol L-1 HF(aq) will have a higher pH than 0.010 mol L-1 HCl(aq).

Question 2. An aqueous solution of acetic acid (ethanoic acid) has a pH of 2.4
At the same temperature, an aqueous solution of hydrochloric acid has a pH of 3.4
Which is the stronger acid?

  1. Is this a trick question?
  2. Hydrochloric acid is a strong acid, it fully dissociates in water.
    Acetic acid (ethanoic acid) is not a strong acid, it does not fully dissociate in water.
    (see the list of 6 strong acids at the top of the page)
    Note: a concentrated weak acid can have a lower pH than a dilute strong acid!

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1. You will see the terms "dissociate" and "ionise" (or "ionize") both used to describe the "breaking up" of a Brønsted-Lowry acid into a hydrogen ion (proton or hydron) and an anion.
Which term is correct?
Both terms are appropriate, they simply emphasize different aspects of the reaction.
"Dissociation" emphasizes the "separation" of the ions from the molecule.
Ionisation (or ionization) emphasizes the production of ions that result from this separation.
Which term should you use?
Play it safe and use whichever term your teacher uses.

2. A hydrogen ion, H+, is a naked proton!
A naked proton is very reactive, so, in practice an H+ ion "jumps" onto a water molecule to form the hydronium (or oxonium) ion, H3O+.
For this reason H3O+ is also known as a hydrated hydrogen ion or hydrated proton.
When Chemists refer to hydrogen ions or H+ in aqueous solutions, they really mean H3O+.
Should you write H+ or H3O+?
Generally speaking, it doesn′t matter, but it would be better to refer to H+(aq), rather than H+, so that there is no confusion.
We use H+ and H+(aq) here because it highlights the fact that pH relates to H+ concentration.

3. As long as we are discussing dilute aqueous solutions at 25oC, this is generally true.