Weak Acids and Bases
Acids are molecules that give up protons when dissolved in water. Hydrochloric acid and ethanoic acid are both acids, but hydrochloric acid is strong while ethanoic acid is weak. In this chemistry article, we'll focus on weak acids and bases.
So, what exactly are weak acids and bases? We'll explain that, and then we'll dive into the pH of weak acids and bases. We'll even compare strong and weak acids and bases to help you remember what you've learned. Plus, we'll include a chart of weak acids and bases, and talk about Ka and Kb. Finally, we'll explore titrations for weak acids and bases. By the end of this article, you'll have a better understanding of weak acids and bases and how they differ from their stronger counterparts. So, let's get started!
Defining weak acids and bases
What are weak acids and weak bases? Let's look at a few definitions to make this clearer.
Weak acids
To understand weak acids, it's important to know what makes an acid strong. A strong acid is one that completely breaks apart in water, giving up all of its protons. In contrast, a weak acid doesn't fully break apart in water. When a weak acid reacts, it reaches a balance where most of the molecules remain as acid molecules, and only a small number give up their protons and break apart into ions. The more hydrogen ions are in solution, the stronger the acid is. Keep in mind that when acids are in water, they actually react with water to create the H3O+ ion, also known as the hydronium ion.
For example, hydrofluoric acid (HF) is a weak acid. When dissolved in water, it exists in equilibrium with H+ and F- ions. Since it doesn't fully dissociate into its ionic components, it's considered a weak acid. This equation shows the reaction: HA + H2O ⇌ H3O+ + A-.
By understanding the difference between strong and weak acids and how they react in water, you'll have a better grasp on the properties of weak acids.
Weak bases
Just like with acids, we can also distinguish between strong and weak bases. It's important to remember that a base is a proton acceptor.
A strong base is one that fully dissociates in solution. For example, sodium hydroxide (NaOH) is a strong base because it completely dissociates into Na+ and OH- ions in water.
In contrast, a weak base is one that only partially dissociates in solution. Ammonia (NH3) is an example of a weak base. When dissolved in water, only a small proportion of the molecules ionize and accept a proton. This means that not every molecule of the weak base accepts a proton.
We represent the dissociation of ammonia with the following equation: B + H2O ⇌ BH+ + OH-. As with weak acids, the backward reaction is strongly favored and a mixture of BH+ and OH- ions is present. This equation can also be simplified by omitting the water molecule.
By understanding the difference between strong and weak bases and how they dissociate in water, you can better comprehend the properties of these chemical compounds.
It's important to note the difference between concentration and strength when it comes to acids and bases. Concentration refers to the amount of acid or base molecules that are dissolved in a solution, while strength refers to the proportion of these molecules that dissociate into ions.
A strong acid or base is one that fully dissociates in solution, regardless of its concentration. Similarly, a weak acid or base is one that only partially dissociates in solution, regardless of its concentration.
It's also important to understand that concentration and strength are independent properties. For example, you can have a concentrated solution of a weak acid or a dilute solution of a strong acid. Concentration and strength are determined by different factors and can vary independently of each other.
So, when considering strong and weak acids, it's important to keep in mind that their strength is determined by the proportion of molecules that dissociate, while their concentration is determined by the amount of acid molecules dissolved in solution.
pH of weak acids and bases
Weak acids have higher pH values than strong acids. Likewise, weak bases have lower pH values than strong bases. This is because weak acids and bases only partially ionise in solution. Remember that pH is a measure of hydrogen ion concentration in solution. Furthermore, a lower pH means there is a higher concentration of hydrogen ions. Let's now take some time to compare strong and weak acids and bases. This table should recap the differences we've already mentioned, as well as introduce you to their reactions with metals and conductivities.
Weak acids and bases chart
Here is a chart with common examples of strong and weak acids and bases:
As you can see, common examples of strong acids include hydrochloric acid, sulfuric acid, and nitric acid, while common examples of weak acids include acetic acid, citric acid, and carbonic acid. Strong bases include sodium hydroxide, potassium hydroxide, and calcium hydroxide, while weak bases include ammonia, sodium bicarbonate, and aluminum hydroxide.
Weak acids and Ka
To clarify, the equation for Ka for a weak acid HA in solution is:
Ka = ([H+][A-])/[HA]
Where [H+] is the concentration of hydrogen ions, [A-] is the concentration of the conjugate base of the acid (in this case, A- is the conjugate base of HA), and [HA] is the concentration of the undissociated weak acid.
The concentration of water is not included in the equation for Ka because it is present in such high abundance that its concentration remains essentially constant.
It's important to note that the value of Ka is a measure of the strength of the weak acid. A larger value of Ka indicates a stronger acid, while a smaller value of Ka indicates a weaker acid. Additionally, the pKa is defined as the negative logarithm of Ka, and is a more convenient way of expressing the acidity of a weak acid.
Calculating the pH of a solution containing a weak acid involves using the value of Ka (or pKa) and the initial concentration of the acid. By solving the equilibrium expression for [H+], we can determine the concentration of hydrogen ions in solution and use the pH equation (pH = -log[H+]) to calculate the pH of the solution.
The units of Ka
To find the units of Ka, we multiply and cancel down the units of all the species involved in the equation. All three species, [H+], [A-], and [HA], have the units mol dm-3. The equation now looks like this: One of the mol dm-3 from the top of the fraction cancels out with the one on the bottom, leaving just one mol dm-3:
Ka and Ka
Just as PKw is the negative log of Kw, PKa is the negative log of Ka:
You should note the following relationships between Ka, PKa, acid strength and pH:
As Ka increases, PKa and pH both decrease.As Ka increases, acid strength increases.
Finding the pH of weak acids
Using the example of ethanoic acid, CH3COOH, with a Ka of 1.74 x 10^-5 and a concentration of 0.100 mol dm^-3, we can calculate the pH of the solution as follows:
- Write out the equation for the dissociation of ethanoic acid: CH3COOH ⇌ H+ + CH3COO-
- Write out the expression for Ka: Ka = [H+][CH3COO-]/[CH3COOH]
- Since ethanoic acid is a weak acid, we can assume that the concentration of undissociated CH3COOH at equilibrium is approximately equal to the initial concentration of 0.100 mol dm^-3.
- Substitute the values into the expression for Ka: 1.74 x 10^-5 = [H+]^2 / 0.100
- Rearrange to solve for [H+]: [H+] = √(1.74 x 10^-5 x 0.100) = 1.32 x 10^-3 mol dm^-3
- Calculate the pH using the pH equation: pH = -log[H+] = -log(1.32 x 10^-3) = 2.88
Therefore, the pH of a 0.100 mol dm^-3 solution of ethanoic acid with a Ka of 1.74 x 10^-5 is 2.88.
Weak bases and 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 Kb. The equation for Kb is given below using B to represent the base:
Units of Kb
Like Ka , Kb has the units mol dm-3.
Kb and pKb
You can probably guess how we calculate PKb. It is simply the negative log of Kb:
There’s a special relationship between PKw , PKa and PKb. It is very similar to the relationship we saw between pKw, pH and pOH in The Ionic Product of Water:
Finding the pH of weak bases
We can use the equation [OH-] = Kb x [NH3] to calculate the concentration of OH- ions in the solution. Substituting in the values of Kb and [NH3], we get [OH-] = 1.77 x 10-5 x 0.15 = 2.655 x 10-6 mol dm-3. We can then use the equation pOH = -log[OH-] to calculate the pOH of the solution, which is 5.57. Finally, we can use the equation pH + pOH = 14 to calculate the pH of the solution, which is 8.43.
Steps for finding the pH of weak acids and bases
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 all of the other articles we mentioned here for more information about various acid-base calculations.
Weak acid and base titrations
We've learned that weak acids have higher pH values than strong acids, whilst weak bases have lower pH values than storng bases. 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 [HA] ↔ [H+] + [A-]. A weak base is a base that only partially dissociates in solution. These form an equilibrium represented by the equation [B] ↔ [H+] + [BH+].
Ka is a modified equilibrium constant for the dissociation of weak acids. It is represented by the equation Ka = [H+][A-]/[HA]. Kb is a modified equilibrium constant for the dissociation of weak bases. It is represented by the equation Kb = [H+][BH+]/[B]. Both Ka and Kb take the units mol dm-3.
We can use Ka and Kb to find the pH of solutions containing weak acids and bases. This can be done by setting up an ICE table and using the K a expression to solve for x and calculate the pH.
Weak Acids and Bases
How do you identify 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.
What are weak acids and bases?
Weak acids and bases are, as the name suggests, types of acids and bases. They differ from strong acids and bases in that they only partially dissociate in solution.
How do you find the pH of weak acids and weak bases?
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.
What indicator is used for a weak acid and strong base?
Phenolphthalein would be a suitable indicator for a titration reaction between a weak acid and a strong base.
What is the difference between strong and weak acids?
Strong acids fully dissociate in solution whereas weak acids only partially dissociate.
