Acid and Base Strength - Online Tutor, Practice Problems & Exam Prep

Strong acids and bases are strong electrolytes, fully dissociating in water, while weak acids and bases are weak electrolytes, only partially dissociating. For example, hydrochloric acid (HCl) completely ionizes to form hydronium (H₃O⁺) and chloride ions (Cl^-), whereas hydrocyanic acid (HCN) only partially dissociates. This inverse relationship extends to conjugate acids and bases: strong acids yield weak conjugate bases, while weak acids yield stronger conjugate bases. Understanding these concepts is crucial for grasping acid-base chemistry and equilibrium dynamics.

Acid and Base Strength Concept 1

Now recall that strong acids and strong bases are classified as strong electrolytes, while weak acids and weak bases are weak electrolytes. For a strong electrolyte, that means an acid that dissociates or ionizes completely in water and donates a proton. When we say "proton," we mean H⁺ ion easily. We're going to say a weak acid though only partially dissociates and donates a proton less readily, and it favors reactants.

So if we take a look here at a strong acid versus a weak acid, here we have hydrochloric acid. Hydrochloric acid will basically donate an H⁺ to water, thereby creating H₃O⁺. Since HCl is giving away an H⁺, what's left of it is Cl^-. So basically think of this as a reaction, and what you have here on this side are your products, which is made up of H₃O⁺ ions and Cl^- ions. Realize that this is a complete dissociation of HCl, so there'll be 0% left of this, and you'll have 100% left of these products. It easily donates an H⁺ to the water and favors this product side because you're making 100% of it.

On the other side, we have HCN which is hydrocyanic acid, a weak acid. It will also donate an H⁺ to water, so you'll have the formation of some H₃O⁺, which will result in some CN^- being formed. But a vast majority of it will still be in the original form of HCN. So you're going to say here, we only dissociate partially. Only some of the H⁺ is donated over to the water, and the reactant of HCN is greatly favored. Because even if we look here, you can see a vast majority of it is still in this shading. Meaning a vast majority of it is still in the HCN form. So just keep this in mind when comparing a strong acid to a weak acid.

Acid and Base Strength Concept 2

We're going to say here that strong bases are bases that dissociate or ionize completely in water and have a high affinity for protons. That means they readily accept them; they want to accept H+ ions. Now weak bases, on the other hand, only partially dissociate and have a low affinity for protons and they favor reactants.

So here we have examples of a strong base and a weak base. Here we have NaOH, sodium hydroxide. It's a strong base, so when it's placed into water, it dissociates completely. The presence of all of these OH- ions, opposites attract. OH- is attracted to H+. And then we're making nothing but these ions, we make 100% of them. Products are highly favored.

On the other side, we have NH3 (ammonia), which is a weak base. It can accept H+ and become NH4+. It gets an H+ from water, water donating an H+ to it becomes OH-. But you don't make many of these ions. In fact, a vast majority of it is still in the form of ammonia. So it dissociates partially. We have a low affinity for H+ since there's not OH- being formed, and also because NH3 doesn't really accept an H+. And we have a majority of it still in NH3 form, so reactants are highly favored.

Now when it comes to strong acids and strong bases, here are some basic ones that you should keep in mind. For strong acids, they include HI, HBr, HCl (group 7A elements: iodine, bromine, and chlorine). When you combine these with H, they become strong acids. Their oxygen forms include bromate and perchlorate. When they combine with H+ they also make strong acids. Then, we have sulfuric acid, nitric acid, and the hydronium ion rounding it out as our strong acids.

For strong bases, it's easier to see the pattern; all of these are group 1A metals connected to OH-. So Li+ with OH- gives LiOH, K+ with OH- gives KOH. In group 2A, it involves calcium, strontium, and barium; when they combine with OH- they give a strong base. Keep this information in mind when looking at any questions dealing with acid or base strength.

Acid and Base Strength Example 1

Here, we're going to say the following represent aqueous acid solutions. Identify the strong acid, weak acid, and the weakest acid. Alright. So here, we have HA, which represents our weak acid, and it's reacting with water. Here, the products that would form should be H3O+ and A^-. If you are a strong acid, you should make 100% of both of these products. You shouldn't have any remaining. So, if we look, which one has no remaining? Remember, it is this form here. And we can see that's the middle one. In the middle one, all we have is H3O+ and A^-. So this is a strong acid. Then that would mean the other two are either a weak acid or the weakest acid. So remember, the weakest acid would form the least amount of these two ions. So, if we look, which one forms the least amount of those two ions? It looks like it is the last one. The last one forms the fewest amount of H3O+ that we can see. Right? H3O+ is this, and there's only one of it. And then here, this one is separate by itself; there it goes right there. This one will be the weakest acid. This one here is weak. It only makes 2 of the hydronium or the H3O+ ions here, and then it has 2 A^- ions. Alright. So remember, a strong acid completely ionizes, so we make 100% of these two ions. So we should have none of this remaining. And then to compare the other two, look at how much they make in terms of these products. The weakest one will make the fewest amounts of these particular ions as products.

Acid and Base Strength Concept 3

We're going to say there's an inverse relationship between strengths of acids and bases and their conjugates. A rule of thumb is a strong acid will have a relatively weak conjugate base. And in fact, the stronger the acid, the weaker the conjugate base. A weak conjugate base has a low affinity for a proton. That means it doesn't want to accept an H⁺. So here we have HCl reacting with water. HCl is the acid, so it's going to donate an H⁺ to the water. Water becomes H₃O⁺ as a result. HCl lost an H⁺, so what's left is Cl^-, its conjugate base. Remember, if you're a strong acid, that means your conjugate base will be weak. So here we're going to have a weak conjugate base as a product.

Acid and Base Strength Concept 4

Alright. So remember, there's an inverse relationship, so that means a weak acid will have a relatively strong conjugate base. And in fact, the weaker the acid, the stronger the conjugate base. A stronger conjugate base has a high affinity for protons, so it wants to more readily accept an H⁺. Here we have HCN, which is a weak acid, hydrocyanic acid; it reacts with water. Water accepts an H⁺ and becomes H₃O⁺ as a result. HCN gave away an H⁺, now it's CN^-. Here, it's a weak acid, so that means it's going to be a stronger conjugate base. It's still weak, but it's stronger because it came from somewhere else that's weak. Now notice also that we have reversible arrows here. The arrow that points to the reactant is longer; that means reactants are more highly favored. This makes sense because remember, weak acids and weak bases don't completely ionize. We don't make 100% of these ions. A vast majority of it is still in the reactant form, which is why the arrow is pointing towards the reactant. That side is more favored. There's more of it. So, we're going to say here, stronger the base, the weaker the conjugate acid. Weak conjugate acids less readily donate protons. The weaker the base, then the stronger the conjugate acid. Stronger acids more readily donate protons, or more easily donate protons. So just remember this inverse relationship. If you're strong in one particular way, you're weaker in the opposite way. A strong acid would equate to a weaker conjugate base.

Acid and Base Strength Example 2

Which of the following acids will have relatively strong conjugate bases? So remember, a strong conjugate base comes from a weak acid. So basically, we have to look and see which one here is a weak acid. Here we have perbromic acid, which is one of the strong acids, so this wouldn't work. Hydrocyanic acid is a weak acid, so this would be an answer. Nitric acid is a strong acid, so that wouldn't work. And then we have perchloric acid, which is a strong acid, so that wouldn't work. Remember, a weak acid equates to stronger or strong conjugate bases. So here, the only answer that works is option b.

Which of the following is the strongest base?

• A. NO₃^–
• B. F^–
• C. Cl^–
• D. ClO₄^–
• E. H₂O

Which of the following bases will have the weakest conjugate acid?

• A. CH₃COOH
• B. KOCH₃
• C. CH₃NH₂
• D. NH₄OH
• E. LiOH

Which of the following aqueous species will contain mostly reactants?

• A. CsOH
• B. HNO₃
• C. Sr(OH)₂
• D. HClO₃
• E. Mg(OH)₂

Determine [OH^–] in each base solution. If the base is weak, indicate the value that [OH^–] is less than.

Here it says determine the OH- concentration in each base solution. If the base is weak, indicate the value that OH- is less than. Alright. So here NaOH is a strong base. It breaks up 100% into Na+ aqueous, plus OH- aqueous. And because of that, because we break up 100% into OH-, that means the concentration of the compound is equal to the concentration of OH-. So here it would be 0.25 M. For b, this is a weak base ammonia, so that just means OH- concentration would be less than 0.25 M, because it doesn't break up 100%. Next, we have strontium hydroxide, which is a strong base. It breaks up into 1 strontium ion, plus 2 hydroxide ions. Each hydroxide ion has a concentration of 0.25 M. So because there are 2 of them, that would mean the concentration is 2 times 0.25 M, so it'd be 0.50 M. And then finally, d, we have potassium hydroxide. Potassium hydroxide is a strong base, so it breaks up 100% into potassium ion and hydroxide ion. Again, the concentration of the compound is equal to the concentration of the OH- ion, since it breaks up 100%. So it'd be equal to 0.125 M for the hydroxide ion concentration.

Predict the direction of the following reaction:

HC₂H₃O₂ (aq)  +  H₂O (liq)  ______________   H₃O⁺ (aq)  +  C₂H₃O₂^– (aq)

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the difference between strong acids and weak acids?

Strong acids are strong electrolytes that completely dissociate in water, donating all their protons (H⁺) to form hydronium ions (H₃O⁺). For example, hydrochloric acid (HCl) dissociates fully to produce H₃O⁺ and Cl^-. In contrast, weak acids are weak electrolytes that only partially dissociate in water, donating only some of their protons. An example is hydrocyanic acid (HCN), which only partially dissociates, leaving a significant amount of HCN in its original form. This partial dissociation means that weak acids favor the reactants in equilibrium, while strong acids favor the products.

How do strong bases differ from weak bases?

Strong bases are strong electrolytes that completely dissociate in water, readily accepting protons (H⁺). For instance, sodium hydroxide (NaOH) dissociates fully to produce OH^- ions. Weak bases, on the other hand, are weak electrolytes that only partially dissociate in water and have a low affinity for protons. Ammonia (NH₃) is an example of a weak base; it only partially accepts protons to form NH₄⁺ and OH^-, leaving a significant amount of NH₃ in its original form. This partial dissociation means weak bases favor the reactants in equilibrium, while strong bases favor the products.

What is the relationship between the strength of an acid and its conjugate base?

There is an inverse relationship between the strength of an acid and its conjugate base. A strong acid will have a weak conjugate base, meaning the conjugate base has a low affinity for protons and does not readily accept H⁺. For example, hydrochloric acid (HCl) is a strong acid, and its conjugate base, chloride ion (Cl^-), is weak. Conversely, a weak acid will have a relatively strong conjugate base, which has a higher affinity for protons. For instance, hydrocyanic acid (HCN) is a weak acid, and its conjugate base, cyanide ion (CN^-), is stronger.

Why do strong acids and bases favor products in equilibrium reactions?

Strong acids and bases favor products in equilibrium reactions because they completely dissociate in water. For strong acids, this means they donate all their protons (H⁺) to form hydronium ions (H₃O⁺), resulting in a 100% conversion to products. Similarly, strong bases fully dissociate to produce hydroxide ions (OH^-), leading to a complete conversion to products. This complete dissociation shifts the equilibrium towards the products, leaving no reactants behind. In contrast, weak acids and bases only partially dissociate, favoring the reactants in equilibrium.

What are some common examples of strong acids and strong bases?

Common examples of strong acids include hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and perchloric acid (HClO₄). These acids fully dissociate in water. Examples of strong bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), calcium hydroxide (Ca(OH)₂), strontium hydroxide (Sr(OH)₂), and barium hydroxide (Ba(OH)₂). These bases also fully dissociate in water, producing hydroxide ions (OH^-).