Arrhenius Acids and Bases - Online Tutor, Practice Problems & Exam Prep

The Arrhenius model defines acids as compounds that increase hydrogen ion (H⁺) concentration in aqueous solutions, exemplified by HCl, which dissociates to form H⁺ and Cl^-. Bases, like NaOH, increase hydroxide ion (OH^-) concentration. This model, while foundational, is limited as it does not account for acid-base behavior in non-aqueous media or compounds lacking H⁺ or OH^-. Understanding these definitions is crucial for grasping acid-base reactions and their applications in various chemical contexts.

Arrhenius Acids and Bases

When it comes to our understanding of acids and bases, one of the first models that came into fruition was the idea of the Arrhenius acid-base model. Here, the most general definition for acids and bases was developed by Arrhenius himself near the end of the 19th century. According to Arrhenius, the H⁺ or hydronium ion cation and the hydroxide anion are fundamental to the concept of acids and bases. We know now that there are acids that don't have H⁺ and there are bases that definitely do not have OH^-. We've expanded our understanding of acids. His definition failed to adequately describe acidic and basic behavior in nonaqueous media. According to Arrhenius' understanding of acids and bases, they had to be dissolved within an aqueous solution, such as water, in order to free up these ions and be classified as an acid or a base.

Following the Arrhenius definition, it states that an acid is a compound that increases your hydrogen ion or hydronium ion concentration when dissolved in a solvent, typically water. For example, we could have HCl, and when it dissolves in water, it gives us H⁺ Cl^-, aqueous. In actuality, we have our HCl ion or HCl compound donating an H⁺ to water to create the hydronium ion and the chloride ion. And remember that H⁺ and H₃O⁺ are synonymous with one another; they're basically the same thing. This is what happens according to his definition, yet in actuality, it is more so this.

According to the Arrhenius' definition for bases, they increase OH^- concentration when dissolved in a solvent. Typically, under this definition, we'd have a metal like NaOH connected to OH. It would dissolve in water because it is soluble in water, breaking up into Na⁺ aqueousand OH^- aqueous. According to Arrhenius' definition, HCl is an Arrhenius acid because it increases the amount of free-floating H⁺ in our solution. And NaOH would be an Arrhenius base because we freed up the amount of OH^- found within our solution. Realize that these are just very general, very basic reasons for what constitutes an acid and a base.

Now that you've seen these examples, take a look at example 1 and see if you can get the final answer that's asked of you.

Arrhenius Acid-Base

So here, for this example 1, it says, "Which ions are formed from the dissociation of the following compound?" We're dealing with sulfuric acid, which is a strong acid. In this form, it'll completely ionize into its ions. Here, if we looked at the quick version of the products, we'd form, you could say it has 2 acidic hydrogens. So we'd have 2 H+ aqueous + SO42− your sulfate ion. But this would not paint an accurate picture of what's really going on with sulfuric acid. It is a strong diprotic acid. Because it's diprotic, it's going to relinquish or give up 2 H+ ions that we have here. But, it doesn't do them both at the same time. It releases 1 at a time. The sulfuric acid form is its strong acid form. Here, we produce initially H2HSO4−. In this form, it's strong so that's why we have a single arrow going forward. Now, this product here that's formed still possesses another acidic hydrogen. But here, it's in its much weaker state. That could also give up an H+. But now, because it's weak, it doesn't go to completion and form 100% of these products. Here, we're going to have double arrows. Here, once it gives up that H+ right here, it becomes your sulfate ion. And if you want the overall equation, here we have our hydrogen sulfate ions. They're intermediates and they cancel out. That's how we're left at the end with this overall equation. These 2 are on the same side so they don't cancel out. They add up. So that's what's really going on in terms of sulfuric acid. The H+ pluses are released in stages, not all at one time. Seeing it that way is how we got this final equation at the end. Now that you've seen example 1, see if you can do example 2. Don't worry about any tricks that happened with that. All you got to do is just break that up into its ions and see what's produced. Pretty simple because the Arrhenius definition of acids and bases is one of the first ways of understanding acids and bases, so it's pretty simple.

Arrhenius Acid-Base

Let's finish the last question left at the bottom of the page. Here it says, which ions are formed from the dissociation of the following compound? Pretty straightforward question. Here we have calcium hydroxide, a strong base that undergoes complete ionization. It's composed of calcium ions and hydroxide ions. Here, it breaks up into a calcium ion, which is Ca²⁺. And then here we have 2 hydroxide ions. Now, we don't have to worry about going into further detail because when it comes to a strong base, both of those OH^- groups are of equal strength. It just breaks up into this one easy equation. When it comes to diprotic or triprotic acids, each one of those acidic hydrogens is of varying strength. Therefore, each one requires their own separate equation. Then, later on, we combine them all together to get our final overall equation. Just remember, for acids, there are different K_as and sometimes different strengths attached to each one of the acidic hydrogens. But for strong bases, in terms of definition, each OH^- is of equal strength. You can do it all in one shot with one simple equation like we did in example 2. Now that we've looked at this basic understanding of acids and bases, we'll continue onward with a more accurate, more in-depth look at how acids and bases truly react when they are in an aqueous solution.

Here’s what students ask on this topic:

What is the Arrhenius definition of acids and bases?

The Arrhenius definition of acids and bases is one of the earliest models to describe these substances. According to Arrhenius, an acid is a compound that increases the concentration of hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. For example, hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl^-. A base, on the other hand, is a compound that increases the concentration of hydroxide ions (OH^-) when dissolved in water. Sodium hydroxide (NaOH) is a typical example, as it dissociates in water to form Na⁺ and OH^-. This model is limited to aqueous solutions and does not account for acid-base behavior in non-aqueous media.

Why is the Arrhenius model limited in describing acids and bases?

The Arrhenius model is limited because it only applies to aqueous solutions, meaning it requires the presence of water for acids and bases to dissociate and exhibit their properties. This model does not account for acid-base behavior in non-aqueous media. Additionally, it restricts acids to compounds that produce H⁺ ions and bases to those that produce OH^- ions. However, there are acids and bases that do not fit this criterion, such as ammonia (NH₃), which acts as a base but does not produce OH^- ions directly. These limitations led to the development of more comprehensive models like the Brønsted-Lowry and Lewis definitions.

How does HCl behave as an Arrhenius acid in water?

When hydrochloric acid (HCl) is dissolved in water, it dissociates completely to form hydrogen ions (H⁺) and chloride ions (Cl^-). The dissociation can be represented by the equation:

${\mathrm{HCl}}_{\left(\mathrm{aq}\right)}\to H_{\left(\mathrm{aq}\right)}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}$

In reality, the H⁺ ions quickly associate with water molecules to form hydronium ions (H₃O⁺):

$H_{\left(\mathrm{aq}\right)}+H_{2O}\to H_{3O}+$

Thus, HCl increases the concentration of H⁺ (or H₃O⁺) ions in the solution, classifying it as an Arrhenius acid.

Can you give an example of an Arrhenius base and explain its behavior in water?

Sodium hydroxide (NaOH) is a classic example of an Arrhenius base. When NaOH is dissolved in water, it dissociates completely to form sodium ions (Na⁺) and hydroxide ions (OH^-). The dissociation can be represented by the equation:

${\mathrm{NaOH}}_{\left(\mathrm{aq}\right)}\to {\mathrm{Na}}_{\left(\mathrm{aq}\right)}+{\mathrm{OH}}_{\left(\mathrm{aq}\right)}$

This increase in OH^- ions in the solution is what classifies NaOH as an Arrhenius base. The presence of OH^- ions makes the solution basic, as they can neutralize H⁺ ions, forming water (H₂O).

What are the limitations of the Arrhenius definition of acids and bases?

The Arrhenius definition of acids and bases has several limitations. Firstly, it only applies to aqueous solutions, meaning it cannot describe acid-base behavior in non-aqueous media. Secondly, it restricts acids to compounds that produce H⁺ ions and bases to those that produce OH^- ions. This excludes many substances that exhibit acidic or basic properties but do not fit these criteria. For example, ammonia (NH₃) acts as a base but does not produce OH^- ions directly. These limitations led to the development of more comprehensive models like the Brønsted-Lowry and Lewis definitions, which can describe a wider range of acid-base reactions.