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# Introduction to Acids and Bases

If you've ever had heartburn, you know that the name is accurate. It feels like your chest is literally burning. But why is this the case? Heartburn is really just a result of your stomach acid having too much hydrochloric acid, which causes it to back up into your esophagus. This causes the "burn in your chest" that we described earlier. So, naturally, to relieve your spicy ramen-induced heartburn, you reach for an antacid and take one. This antacid is actually a base, and after about 20 minutes, your heartburn ends because the antacid has neutralized all the backed-up hydrochloric acid. This demonstrates this topic: an introduction to acids and bases!

• In this lesson, we'll cover the introduction of what acids, bases, and salts are.
• After, we'll list some acids and bases you might be familiar with and distinguish between strong and weak acids and bases.
• Then, we will look over an introduction to titrations of acids and bases.
• Lastly, we will briefly discuss acids and bases as electrolytes found in body fluids.

## Basic Introduction to Acids and Bases

Let's start our introduction to acids, bases, and salts by talking about the definition of acids and bases. There are different definitions associated with acids and bases, and each these definitions comes from different chemists.

An Arrhenius acid is a substance that increases the concentration of hydrogen ions (H+) in water, whereas an Arrhenius base increases the concentration of hydroxide ions (OH-) in water.

Hydrochloric acid (with a formula HCl), for instance, is an Arrhenius acid because, in water, it separates into its individuals ions, increasing the H+ concentration in water.

$$\text{HCl } \xrightarrow{\text{ H}_{2}\text{O}} \text{H}^{+} \text{ + Cl}^{-}$$

On the other hand, sodium hydroxide (NaOH) is considered an Arrhenius base because, in water, it separates into OH- and Na+, increasing the OH- concentration in water.

$$\text{NaOH } \xrightarrow{\text{ H}_{2}\text{O}} \text{Na}^{+} \text{ + OH}^{-}$$

Next, we have the Brønsted-Lowry definition of acids and bases.

A Brønsted-Lowry acid is a substance that donates an H+ (proton) to another substance, while a Brønsted-Lowry base is a substance that accepts an H+ ion from another substance.

For example, in a chemical reaction between ammonia (NH3) and hydrochloric acid (HCl), HCl will donate a H+ ion to NH3 and become Cl-, and NH3 will accept a H+ ion to become NH4+.

$$\mathop{\text{NH}_{3}}_{Base} \text{ + } \mathop{\text{HCl}}_{Acid}\longrightarrow \text{NH}_{4}^{+} \text{ + Cl}^{-}$$

Brønsted-Lowry acids are grouped bases of the amount of protons (H+) they are able to donate.

• Monoprotic acids can only donate one proton (H+). For example, HF, HCl and HClO3.
• Diprotic acids can donate two protons. Diprotic acids include H2SO4 and H2CO3.
• Triprotic acids can donate three protons. A common example of a triprotic acid is H3PO4.

Although not always, acids tend to start with an H, and bases tend to end in an OH in a chemical formula.

## Introduction to Acids, Bases, and Salts

Now that we learned what acids and bases are, let's talk about salts! Basically, when an acid and a base react together, the products of the chemical reaction are salt and water. The chemical reaction between acids and bases is called a neutralization reaction.

In a neutralization reaction, an acid reacts with a base to produce salt and water.

• A salt is defined as an ionic compound formed by the neutralization reaction between an acid and a base.

The reaction below shows an example of a neutralization reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH), forming table salt (NaCl) and water (H2O).

$$\mathop{\text{NaOH}}_{Base} \text{ + } \mathop{\text{HCl}}_{Acid}\longrightarrow \text{NaCl}\text{ + H}_{2}\text{O}$$

Some examples of common salts include baking soda (NaHCO3), and bleaching powder (CaOCl2).

## Introduction to Acid and Base Concepts

When dealing with an introduction to acid and base, we need to explore some important concepts. One of them is is acid-base strength. The strength of acids and bases is based on how readily acids and bases perform their respective roles.

The strength of an acid depends on how well it dissociates and forms protons (H+ ions) in water. This means that a strong acid would more readily give up a proton than a weak acid, and a strong base more readily accepts a proton than a weak base.

• Strong acids will completely dissociate and yield nearly 100% H+ ions in water, while weak acids will partially dissociate in H2O and not yield as many protons.

$$\text{HA}_{(aq)} \text{ + H}_{2}\text{O}_{(l)}\longrightarrow \text{H}^{+}_{(aq)} \text{ + A}^{-}_{(aq)}$$

So, what happens to the strong acid after it has given up its proton? Because it wants to avoid accepting the proton back, it becomes a weak base! This is a trend that is seen in all strong acids and bases: they form a weak variant of the opposite once they have reacted in an acid-base reaction. We call these variants conjugates. Strong acids donate a proton and become a weak conjugate base. Weak acids donate a proton and become strong conjugate bases.

Similarly, the strength of a base depend on how well it dissociates in water, yielding hydroxide ions (OH-). Strong bases accept a proton and become weak conjugate acids. Weak bases accept a proton and become strong conjugate acids.

Let's look at an example showing this! In chemical reaction below, HCl donates an H+ to the weak base (H2O) and becomes a conjugate base, whereas the weak base (H2O) accepts a proton from HCl and becomes a conjugate acid.

$$\mathop{\text{HCl}}_{\text{Strong Acid}} \text{ + } \mathop{\text{H}_{2}\text{O}}_{\text{Weak base}}\longrightarrow \mathop{\text{Cl}^{-}}_{\text{Weak conjugate base}} \text{ + } \mathop{\text{H}_{3}\text{O}^{+}}_{\text{Strong conjugate acid}}$$

Water is considered an amphoteric molecule, meaning that it can behave as an acid (proton donator) or as a base (proton acceptor)!

For AP Chemistry, you should familiarize yourself with the following strong acids and bases (table 1).

Table 1. Strong acids and Strong bases.

 Strong acids Strong bases Hydrochloric acid ( $$\text{HCl}$$) Group 1A (Alkali metal) hydroxides - $$\text{LiOH, NaOH, KOH, RbOH, and CsOH}$$ Hydrobromic acid ( $$\text{HBr}$$) $$\text{Ca(OH)}_{2}$$ Hydroiodic acid ( $$\text{HI}$$) $$\text{Sr(OH)}_{2}$$ Nitric acid ($$\text{HNO}_{3}$$) $$\text{Ba(OH)}_{2}$$ Chloric acid ($$\text{HClO}_{3}$$) Perchloric acid ($$\text{HClO}_{4}$$) Sulfuric acid ($$\text{H}_{2}\text{SO}_{4}$$)

The second concept we need to kind in mind is pH and the pH scale.

A solution's pH is a measurement of the amount of hydrogen ions (H+) and hydroxide ions (OH-) found in solution.

Figure 1 shows the pH scale. The pH scale is used by chemists to measure the tendency of molecules to be acidic or basic. On the left, we have the quality of "acidic," and on the right, we have "basic". The pH scale scale ranges from 0 up to 14, with zero being extremely acidic, 14 being extremely basic, and 7 representing a neutral solution.

Figure 1. pH scale, Isadora Santos - StudySmarter Originals.

For example, water (H2O) has a pH of 7, so we know it’s neutral. Black coffee, however, has a pH of 5, meaning that it is a weak acid. Baking soda, with a pH of around 9.5, is considered a weak base!

When strong base is added to a strong acid, the recently added OH- ions will react with the dissociated H+ ions from the strong acid, changing the concentration of H+ ions in solution and causing the pH to change!

However, if you added a strong base to a weak acid, the pH would not undergo major changes because the strong base would mostly react with the HA molecules that did not dissociate with its ions.

Interested in learning more about the pH scale? Browse through this detailed explanation on the "pH scale"!

## Introduction to Titration of Acids and Bases

Now, let's make an introduction to titration of acids and bases, and explore the basic concepts related to it.

Acid-base titration involves using an acid or base with a known concentration to find out the unknown concentration of another acid or base.

This means that if we wanted to figure out an acid's unknown concentration, we would use a base with a known concentration, and slowly add it to the acid until the mixture is neutralized. If we know the exact amount of base that we added, as well as its concentration, we can deduce the acid's unknown concentration!

Figure 2 shows the common laboratory setup used for acid/base titrations. The solution of unknown concentration is usually placed in a flask and a few drops of an indicator is added to it. The solution of known concentration is placed in the buret and added drop-wise to the sample in the flask until the solution changes color.

For an in-depth explanation on titrations of acids and base, check out "Acid-base Titration"!

## Fluid Electrolyte and Acid Base Balance Introduction to Body Fluids

To finish off, let's explore fluid, electrolyte and the acid-base balance of homeostasis. Although this is more important in biology and you most likely won't encounter it in your chemistry exam, it is a very important topic!

In the body of an average adult, there is an average of 40 L of body fluids (figure 3). The intracellular fluid is the fluid inside body cells, and it consists of mostly water and electrolytes like potassium (K+), magnesium (Mg2+), and HPO42-. The extracellular fluid is found outside body cells and contains electrolytes such as Na+, Cl-, HCO3- and Ca2+.

Electrolytes are basically chemical substances that, when dissolved in water, release cations and anions.

One of the many function of electrolytes is to help maintain acid-base balance in homeostasis. In this case, acids are considered electrolytes that release H+ ions in water, while bases are electrolytes that release OH- ions in water.

Homeostasis is the tendency of our bodies to return to a steady state after an environmental change. A body's ability to maintain homeostasis is essential to life. For instance, if changes in blood pH occur and the body is unable to return the blood pH to its normal range, it can lead to fatal consequences!

If you understand the concepts discussed in this explanation, you'll have a really strong foundation that will help you during the AP chemistry test and also in higher-level chemistry courses!

## Introduction to Acids and Bases - Key takeaways

• An Arrhenius acid is a substance that increases the concentration of hydrogen ions (H+) in water, whereas an Arrhenius base increases the concentration of hydroxide ions (OH-) in water.
• A Brønsted-Lowry acid is a substance that donates an H+ (proton) to another substance, while a Brønsted-Lowry base is a substance that accepts an H+ ion from another substance.
• A solution's pH is a measurement of the amount of hydrogen ions (H+) and hydroxide ions (OH-) found in solution.
• Acid-base titration involves using an acid or base with a known concentration to find out the unknown concentration of another acid or base.

## References

1. Moore, J. T. (2022). 5 Steps To A 5 Ap Chemistry 2023. Mcgraw-Hill Education.
2. N Saunders, Kat Day, Iain Brand, Claybourne, A., Scott, G., & Smithsonian Books (Publisher. (2020). Supersimple chemistry : the ultimate bite-size study guide. Dk Publishing.
3. Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.
4. Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson.

Arrhenius acids and bases are explained based on whether they increase the concentration of H+ or OHin water.

Brønsted-Lowry acids and bases are explained because of a substance's ability to donate or accept a proton.

Acids and bases are an important part of the chemistry curriculum, and you also encounter acids and bases every day.

An Arrhenius acid (for example, HCl) is a substance that increases the concentration of hydrogen ions (H+) in water.

An Arrhenius base increases the concentration of hydroxide ions (OH-) in water (for example, NaOH).

Acids have many uses, but three uses of acids include using acids for food preparation (ex. vinegar and lemon), in the manufacture of paints and fertilizers, and also in industries to dissolve metals.

## Final Introduction to Acids and Bases Quiz

Question

True or false: Titrations are used to find the unknown concentration of an acid or base known as analyte.

True

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Question

A _______ is a technique used to find the unknown concentration of an acid or base by adding an acid or base with a known concentration.

Titration

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Question

At the _______, the weak acid and its conjugate base will have the same number of moles.

half-equivalence point

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Question

Think about a strong acid/strong base titration. At the ______, the titrant and the analyte will have the same number of moles.

equivalence point

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Question

True or false: The equivalence point is the point where a sufficient amount of titrant has been added to neutralize the analyte.

True

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Question

What is added to the solution with known concentration to tell us when the equivalence point has been reached?

An indicator solution

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Question

A _____ curve (also known as pH curve) shows the changes in pH of a solution that happens when an acid or a base is added to it.

titration

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Question

Three types of acid-base titrations:

• Strong Acid/Strong Base Titrations
• Weak Acid/Strong Base Titrations
• Weak Base/Strong Acid Titrations

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Question

In a strong acid/strong base titration, the pH at the equivalence point is _______.

equal to 7

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Question

In weak acid/strong base titrations, the pH at the equivalence point will be ______

a little higher than 7.

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Question

In a weak base/strong acid titration the pH at the equivalence point will be _______.

slightly less than 7

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Question

True or false: an acid-base indicator is a substance that shows a distinct observable change (usually, a color change) when there is a change in the pH of an aqueous solution.

True

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Question

A weak acid indicator (HIn) partially dissociates in water to form:

H+ and In-

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Question

In acid/base indicators, the color of the lower pH form and the higher pH form are ______.

different

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Question

A solution will have the color of the _____ pH form, if most of the indicator molecules exist in the form HIn.

lower

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Question

The ______ is referred to as the point where the indicator changes color.

end-point

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Question

______ is an indicator commonly used in weak acid/strong base titrations because it changes color at around a pH of 8.2.

Phenolphthalein

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Question

Blue litmus paper turns ____ in the presence of an acid.

Red

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Question

True or false: A universal indicator is made up of several indicators.

True

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Question

The lower pH color of phenolphthalein is:

Colorless

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Question

According to ______, the color change of an indicator occurs due to the partial ionization of the indicator and the different colors that the un-ionized form and the conjugate base (ionized) form has.

Ostwald's theory

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Question

The _____ is based on the idea that acid-base indicators exist in two tautomeric forms (benzenoid and quinonoid), and these two forms have different colors.

Quinonoid theory

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Question

For _______ titrations, the acid-base indicators commonly used are those that change color at pH 7.

Strong acid/strong base

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Question

• For ______ titrations, the acid-base indicators commonly used are those that change color at pH> 7.

Weak acid/strong base

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Question

For _______  titrations, the acid-base indicators commonly used are those that change color at pH < 7.

weak base/strong acid

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Question

______ are acids that can yield only one proton per molecule of acid.

Monoprotic acids

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Question

Which of the following are examples of monoprotic acids?

HCl

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Question

Which of the following are considered polyprotic acids?

H3PO4

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Question

How many dissociation reactions happen in H3PO4

1

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Question

In the first dissociation reaction of phosphoric acid, water ____ a proton (H+) to become a hydronium ion (H3O+), whereas phosphoric acid ____ a proton and becomes a conjugate base!

gains; loses

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Question

A titration curve is a graph that shows how the pH of a solution _______ when an acid or base is added to it.

changes

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Question

In a polyprotic titration, for each successive dissociation, the acid dissociation constant (Ka) value gets _____.

bigger

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Question

The______  is the point in a titration where there are equal concentrations of each species.

half-equivalence point

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Question

In a weak acid, _____ at the half-equivalence point.

[HA] = [A-]

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Question

The equivalence point is the point where the number of moles of titrant added is ____  the number of moles of analyte that was originally present.

equal to

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Question

How many equivalence points does a titration graph of a titration of 0.1 M carbonic acid with a strong base show?

2

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Question

If the pH at the half-equivalence point is 3.13, what is the pKa value?

3.13

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Question

How many equivalence points would you expect to see in the titration curve of HCl?

Only one because HCl is a monoprotic acid

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Question

Acid + Base _____  + _____

salt, water

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Question

What type of reaction is an Acid-Base reaction?

Neutralization Reaction

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Question

What are the four types of Acid-Base Reactions?

Strong Acid-Strong Base

Weak Acid-Strong Base

Weak Base- Strong Acid

Weak Acid- Weak Base

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Question

Which of the below reactions is between a strong acid and weak base?

HNO3 and Ba(OH)2

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Question

Which of the following generic equation is for a weak base - strong acid reaction?

BH+(aq) + H₃O⁺(aq) → B-(aq) + H₂O(l)

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Question

What will the salt be if a weak acid-strong base reacts with an equal amount of moles?

neutral

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Question

What is the generic equation for a weak acid-strong base equation?

HA(aq) + OH⁻(aq) → A⁻(aq) + H₂O(l

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Question

If a ____ acid and _____ base react together, the salt will be _____.

strong, strong, acidic

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Question

Calculate the pH of the solution when 75 mL of 2 M of HNO3  is mixed with 45 mL of 2 M NaOH.

0.301

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Question

Which of the following is a balanced net ionic equation between HBR and NH3

NH3 (aq)  +  HBr (aq) NH3Br( aq) + H2O(l)

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Question

What are binary acids?

Binary acids are acids with the formula H-X, where a hydrogen atom is bonded to an electronegative nonmetal atom, X. X is typically a halogen.

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Question

What are oxyacids?

Oxyacids are acids that have one (or multiple) H-O bonds, where a hydrogen atom is bonded to an oxygen atom.

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