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 reactions and their applications in chemistry.

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. Now, 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 will be 0% left of this, and you'll have a 100% left of these products. It easily donates an H⁺ to the water, and it 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 it's the reactant of HCN that's 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 they 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 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 these OH^- ions, opposites attract. OH^- is attracted to H⁺. And then, we're making nothing but these ions; we make 100% of them. So, products are highly favored; there'd be 0% left of this.

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, if you look, a vast majority of it is still in the bluish spheres which represent ammonia. So we're not making very much of this. So, we're making a very low percentage of these products. So a vast majority of it will stay in the NH₃ form. 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 when looking at strong acids and strong bases. So here, we have HI, HBr, HCl. These are your Group 7A elements, iodine, bromine, and chlorine. So, if you look on the periodic table, you put them next to an H, they become an acid. They represent strong acids. Here we have their oxygen forms. So, we have BrO₄, which is perbromate. Here, bromate, perchlorate, and chlorate. When they combine with H⁺, they also make strong acids. Then we have sulfuric acid, nitric acid, and then the hydronium ion, rounding it out as strong acids. For strong bases, it's easier to see the pattern. Here, all of these are Group 1A metals connected to OH^-. So, Li⁺ with OH^- gives me LiOH, K⁺ with OH^- gives me KOH. So these are all your Group 1A metals with an OH. Here in Group 2A, it's a little trickier; it's not all the metals in Group 2A, it's just calcium, strontium, and barium. When they combine with OH^-, they give us a strong base. So, keep in mind, this is a direct list of the strong acids and strong bases that you'll see when you're doing questions on acids and bases. So keep these 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. 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's 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 weak acid or weakest acid. 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 by itself; there it goes right there. This one would be the weakest acid. This one here is weak. It only makes two 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. 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 amount 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. And 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 some place 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, the 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. So 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 would equate to strong or stronger 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) + 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 molar. For b, this is a weak base, ammonia, so that just means OH- concentration would be less than 0.25 molar, because it doesn't break up 100%. Next, we have strontium hydroxide, which is a strong base. It breaks up into one strontium ion, plus two hydroxide ions. Each hydroxide ion has a concentration of 0.25 molar. So, because there are two of them, that would mean the concentration is 2 times 0.25 molar, so it would be 0.50 molar. And then finally d, we have potassium hydroxide. Potassium hydroxide is a strong base, so it breaks up 100% into a potassium ion and a hydroxide ion. Again, the concentration of the compound is equal to the concentration of the OH- ion, since it breaks up 100%. So, it would be equal to 0.125 molar 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 completely dissociate in water, meaning they donate all their H⁺ ions to form H₃O⁺ ions. For example, hydrochloric acid (HCl) dissociates fully to form H₃O⁺ and Cl^- ions. In contrast, weak acids only partially dissociate in water, donating only some of their H⁺ ions. An example is hydrocyanic acid (HCN), which only partially dissociates, leaving a significant amount of HCN in its original form. This means strong acids are strong electrolytes, while weak acids are weak electrolytes.

What are some examples of strong bases and weak bases?

Strong bases completely dissociate in water and have a high affinity for protons (H⁺). Examples include sodium hydroxide (NaOH) and potassium hydroxide (KOH), which dissociate fully to form OH^- ions. Weak bases, on the other hand, only partially dissociate and have a low affinity for protons. An example is ammonia (NH₃), which only partially accepts H⁺ ions to form NH₄⁺ and OH^- ions, leaving a significant amount of NH₃ in its original form.

How does the strength of an acid relate to 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 (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 stronger conjugate base. For instance, hydrocyanic acid (HCN) is a weak acid, and its conjugate base, cyanide ion (CN^-), is stronger compared to the conjugate base of a strong acid.

What is the relationship between strong bases and their conjugate acids?

Similar to acids, there is an inverse relationship between the strength of a base and its conjugate acid. A strong base will have a weak conjugate acid, meaning the conjugate acid does not readily donate protons (H⁺). For example, sodium hydroxide (NaOH) is a strong base, and its conjugate acid, water (H₂O), is weak. Conversely, a weak base will have a relatively stronger conjugate acid. For instance, ammonia (NH₃) is a weak base, and its conjugate acid, ammonium ion (NH₄⁺), is stronger compared to the conjugate acid of a strong base.

Why do strong acids and bases completely dissociate in water?

Strong acids and bases completely dissociate in water because they have a high tendency to donate or accept protons (H⁺). For strong acids, this means they readily donate all their H⁺ ions to water, forming H₃O⁺ ions. For strong bases, they readily accept H⁺ ions, forming OH^- ions. This complete dissociation is due to the strong electrolytic nature of these substances, which makes them highly reactive in aqueous solutions.