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Weak Acids and Bases

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Chemistry

Hydrochloric acid and ethanoic acid are both acids, as their names suggest. Acids are molecules that donate protons when in solution by dissociating into positive hydrogen ions and negative ions. However, whilst hydrochloric acid is a strong acid, ethanoic acid is weak.

Defining weak acids and bases

Let's look at a few definitions to make this clear.

A strong acid is an acid that dissociates fully in solution.

We can represent a strong acid using the following equation. Note how this reaction isn’t reversible:

On the other hand, weak acids behave a little differently.

A weak acid is an acid that only partially dissociates in solution.

Weak acids form an equilibrium, in which the majority of the molecules present are acid molecules and only a tiny fraction are dissociated into ions. The stronger an acid is, the more the equilibrium shifts to the right and the greater the concentration of hydrogen ions in solution.

Remember that acids in solution actually dissociate and react with water to form the hydronium ion, . In order to simplify the equation, we omit the water and replace the hydronium ion with the hydrogen ion:

Simplified version:

In fact, whenever you see the hydrogen ion in acid-base reactions, you should know that it actually represents the hydronium ion.

We can also find strong and weak bases. Remember that a base is a proton acceptor.

A strong base is a base that dissociates fully in solution.

An example of a strong base is sodium hydroxide, . It dissociates in solution to form sodium ions and hydroxide ions:

However, ammonia is only a weak base.

A weak base is a base that only partially dissociates in solution.

Like with weak acids, weak bases form an equilibrium in which the backward reaction is strongly favoured, and only a small proportion of the molecules are ionised. We can represent the dissociation of ammonia with the following equation:

The general equation for the dissociation of a weak base is shown below:

Don’t get strong and concentrated mixed up - they mean totally different things. Concentration is a measure of how many acid or base molecules are dissolved in solution, whilst strength is a measure of what proportion of these molecules is dissociated into ions. You can get both concentrated weak acids and dilute strong acids!

Strong and weak concentrated and dilute acids. Anna Brewer, StudySmarter Originals

You’ll encounter both strong and weak acids and bases in everyday life. For example, concentrated hydrochloric acid, a strong acid, is used to remove algae from the bottoms of boats, whilst a more dilute solution is used in toilet cleaners. It is also the acid found in our stomachs that helps digest our food. Citric acid and ethanoic acid, the acids found in lemons and malt vinegar respectively, are both weak acids. Sodium bicarbonate is also known as baking soda, a useful leavening agent in baking, whilst calcium hydroxide is used to help neutralise acidic soil.

What is Ka?

In The Ionic Product of Water, we discussed . is a modified equilibrium constant for the dissociation of water. We can also get , a modified equilibrium constant for the dissociation of weak acids.

The general equation of the equilibrium constant for the reaction is shown below:

Square brackets represent concentration, and the small letter represents the number of moles of each species in the chemical equation. For example, the equilibrium reaction, , has the equilibrium constant .

Let’s look at this from the point of view of a weak acid. It dissociates in solution with the equation . The reactant is the acid and the products are the hydrogen ions and the negative ions. This gives us the following equilibrium constant, known as :

You can remove the state symbols to simplify the equation if you want.

“But hang on!” We hear you cry. “How about the equation of a weak acid involving water and hydronium ions! Where have they gone?”

If we write out that equation, we get . This produces the equilibrium constant . However, the concentration of water is so large that it would completely dominate all the other values in the equation. To form we simply omit it.

You should also remember that we can use the hydrogen ion to represent the hydronium ion in acid-base reactions. This gives us the familiar equation for

The units of Ka

To find the units of , we multiply and cancel down the units of all the species involved in the equation. All three species, , and , have the units . The equation therefore looks like this:

One of the from the top of the fraction cancels out with the one on the bottom, leaving just one :

Ka and pKa

Just as is the negative log of , is the negative log of :

You should note the following relationships between , , acid strength and pH:

  • As increases, and pH both decrease.
  • As increases, acid strength increases.

How do you find the pH of weak acids?

To calculate the pH of weak acids, you use the relationships between , , and the concentration of the acid in solution. You’ll be given information about the acid’s or . There are a few more steps compared to working out the pH of a strong acid, but it isn’t too tricky. Let’s have a look at an example together.

Ethanoic acid, , has . Calculate the pH of a solution of this weak acid.

First of all, let’s look at the equation for the dissociation of ethanoic acid:

To find pH, we need to know , the concentration of hydrogen ions in solution. Well, what do we know about ? It is a modified equilibrium constant for the dissociation of a weak acid such as ethanoic acid and involves . For ethanoic acid, it looks like this:

Let’s think about those values. The concentration of ethanoic acid was originally . At equilibrium, it will be a little less than that because some of the molecules will dissociate into ions. However, ethanoic is a weak acid and hardly any of the molecules dissociate - the equilibrium lies far to the left. We can therefore say that the concentration of ethanoic acid at equilibrium is still roughly . Let’s put that value into our equation:

Look back at the equation. When one mole of ethanoic acid dissociates, it forms one mole of positive hydrogen ions, , and one mole of acetate ions, . This means that the number of hydrogen ions in solution equals the number of acetate ions in solution, and thus they have the same concentrations:

We can replace with in our equation for :

The question gives us , so we can substitute that in. Now we have an equation where the only unknown is . We can solve it normally, as shown:

You should remember the equation for pH. Substituting our value for we get our final answer:

Remember to always give pH to two decimal places.

If you get a question that gives you instead of , all you have to do is convert into . You can do this using the equation we learn earlier in the article, .

What is Kb?

We know what a weak base is - a base that only partially dissociates in solution. It forms an equilibrium reaction. Just like for acids, we can find an equilibrium constant, this time known as . The equation for is given below using B to represent the base:

Like , it has the units .

Kb and pKb

You can probably guess how we calculate . It is simply the negative log of , just as is the negative log of :

There’s a special relationship between , and . It is very similar to the relationship we saw between , pH and pOH in The Ionic Product of Water:

How do you find the pH of weak bases?

Finding the pH of a weak base is similar to finding the pH of a weak acid. However, there are a few differences. Let’s look at an example together.

A solution of has . What is its pH at 25℃?

First of all, let’s take our equation for the dissociation of ammonia and our equation for :

We know from the first equation that the amounts of ammonium ions, , and hydroxide ions, , are equal. They therefore have equal concentrations. This simplifies the equation for :

We know the value for and we know the concentration of ammonia, . Our original solution had a concentration of . Though the equilibrium solution will have a slightly lower concentration, the proportion of molecules that have dissociated into ions is so small that we can largely ignore it. Therefore, the equilibrium concentration of molecules still roughly equals . We can substitute these values in and rearrange to find :

We can then use the relationships between and pH to work out the pH, just like we did to find the pH of a strong base. (If you aren’t too sure about this, check out Brønsted-Lowry Acids and Bases for a detailed explanation.)

Congratulations! You made it through some tricky calculations. You should now be able to work out the pH values for all sorts of acids, bases, and mixtures.

The following flowchart provides a summary of the steps taken to work out the pH of weak acids and bases. Remember to check out the previous articles for more information about other sorts of acid-base calculations.

Weak acids and bases flow chart pH StudySmarterFinding the pH of weak acids and bases. Anna Brewer, StudySmarter Originals

Titrating weak acids and bases

Weak acids have higher pH values than strong acids. Likewise, weak bases have lower pH values than strong bases.

Weak acids and bases strong weak acids bases pH StudySmarterThe pH of strong and weak acids and bases. Anna Brewer, StudySmarter Originals

This means that they produce slightly different pH curves in titration experiments. You can explore this more in pH Curves and Titrations.

Weak Acids and Bases - Key takeaways

  • A weak acid is an acid that only partially dissociates in solution. These form an equilibrium represented by the equation .
  • A weak base is a base that only partially dissociates in solution. These form an equilibrium represented by the equation .
  • is a modified equilibrium constant for the dissociation of weak acids. It is represented by the equation .
  • is a modified equilibrium constant for the dissociation of weak bases. It is represented by the equation .
  • Both and take the units .
  • and
  • We can use and to find the pH of solutions containing weak acids and bases.

Weak Acids and Bases

You can identify weak acids and bases based on the fact that they only partially dissociate in solution. They tend to have pH values closer to 7 than strong acids and bases.

Weak acids and bases only partially dissociate in solution.

To work out the pH of weak acids and bases, you use the equilibrium constants Ka and Kb to work out the concentrations of hydrogen ions or hydroxide ions in solution. You can then calculate pH. 

Phenolphthalein would be a suitable indicator for a titration reaction between a weak acid and a strong base.

Strong acids fully dissociate in solution whereas weak acids only partially dissociate.

Final Weak Acids and Bases Quiz

Question

What is a weak acid?

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Answer

An acid that only partially dissociates in solution.

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Question

What is a weak base?

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Answer

A base that only partially dissociates in solution.

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Question

Compare the terms concentrated and strong in terms of an acid.


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Answer

  • Concentrated means that there are lots of acid molecules dissolved in the solution. 
  • Strong means that all of the acid molecules have dissociated into ions.

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