8.4 Acid-Base Reactions and Buffers
4 min read•january 10, 2023
Dylan Black
Jillian Holbrook
Dylan Black
Jillian Holbrook
Now that we understand with acids and bases, it is time to get into the fun stuff: mixing acids and bases together! In this section, we introduce the idea of reactions between acids and bases, such as the reaction CH3COOH + NaOH → CH3COONa + H2O, and how it relates to a new topic: .
Defining Buffers
are awesome. are a mixture of an acid (HA) and its (A-) in a solution. What makes a buffer interesting is that it is resistant to changes in . When adding some acid or base to a buffer, you will find that the (for a range) will not change much. Why? If you add acid, the will take care of it, and vice versa.
are important both in chemistry and biology. are found everywhere in organisms to maintain , a healthy state. In fact, your own blood is a buffer! 🩸
are incredibly important not only because of their biological ramifications but also in their creation via chemical reactions between acids and bases.
Reactions with Acids and Bases
Strong Acid Strong Base Reactions
A strong acid strong base reaction is one that you are most likely familiar with. This is the type of acid-base reaction students see in Unit 4 when acids and bases are first introduced without the ✨magic✨ of .
An example of a strong acid strong base is the reaction between HCl and NaOH to form NaCl and H2O. However, when we look closer, we find that for this reaction (and any strong acid strong base reaction), the ions are spectators, so the net ionic is just H+ + OH- <=> H2O.
Take a look at an example problem with a strong acid strong base reaction:
What is the after the addition of 10.0 mL of 0.100 M NaOH to 25.0 mL of 0.100 M HCl?
Start by writing out our reaction:
HCl + NaOH <=> NaCl + H2O
Writing the net ionic, we find that Cl- and Na+ are spectators, giving us:
H+ + OH- <=> H2O
By multiplying mL times molarity, we can find the number of millimoles of each species:
10.0 * 0.1 = 1mmol OH-
25.0 * 0.1 = 2.5mmol H+
We use to find how much H+ is left over (we know that all of the OH- will be used up because H+ is in excess).
Finally, we divide 1.5mmol H+ by the volume (35mL) to find [H+] = 4.8 * 10^-2, which corresponds to a of 1.37.
It is worth noting that with (and other reactions we will discuss), that there are three "zones": the zone where the acid is in excess, where the base is in excess, or where the two are equimolar. This thinking will come in handy when we apply these reactions in the next section about titrations.
Weak Acid Strong Base Reactions
are very similar to , except the acid does not fully , meaning our will be slightly different. For the example in this section, we will use the reaction CH3COOH + NaOH <=> CH3COONa + H2O.
Like before, we see that Na+ is a spectator. However, we cannot say the same about CH3COO- because CH3COOH is a weak acid and does not fully . Our for this reaction will be CH3COOH + OH- <=> CH3COO- + H2O, which we will use for calculations.
In general, you usually want to start problems with by writing out the . With these problems, we are going to start seeing , mixtures of an acid and its . We will discuss calculating the of these in detail, but for now, just know that we can use this equation:
Let's take a look at an example!
Calculate the in the of 25.0 mL of 0.100 M acetic acid with 0.100 M NaOH after adding 10.00 mL of 0.100 M NaOH.
We can use the same procedure for solving our strong acid strong base problem for this problem. First, calculate millimoles, and then use to find what's left.
However, what's different here is that instead of having H+ or OH- at the end, we have a mixture of CH3COOH and its CH3COO-. We can use the above equation to solve for :
Note that the equation calls for concentrations of the and the acid. Because we have the same volume of each, the divisions would cancel out.
Again, like the strong acid strong base reaction, you will have points with only acid, a buffer region (this is unique to weak acid/base reactions), an (where mmol acid = mmol base), and a post-equivalence region.
The strategies shown in this example can be applied to weak base strong acid reactions. Just remember to be careful about managing conjugate acids/bases and distinguishing between or pOH. On the AP Exam, a weak acid strong base example is much more likely, but be ready for either.
Key Terms to Review (14)
Acid-Base Reactions
: Also known as neutralization reactions, these involve an acid reacting with a base to produce water plus a salt.Buffers
: Buffers are solutions that resist changes in their pH when small amounts of an acid or a base are added.Conjugate Acid/Base
: A conjugate acid is the particle formed when a base gains a hydrogen ion, and a conjugate base is what remains after an acid has donated a proton during a chemical reaction.Conjugate Base
: The conjugate base is what remains of an acid after it has donated its proton during an acid-base reaction.Dissociate
: Dissociation in chemistry refers to the process where compounds split into smaller particles, usually ions.Equilibrium
: Equilibrium refers to the state in which both reactants and products are present in concentrations which have no further tendency to change over time. It's when forward and reverse reactions occur at equal rates so there's no net change observed.Equivalence Point
: The equivalence point in a titration is when exactly enough titrant has been added to react completely with the substance being analyzed.Homeostasis
: Homeostasis refers to an organism's ability to maintain stable internal conditions despite changes in its environment.Net Ionic Equation
: The net ionic equation represents only those components involved directly in the chemical reaction, excluding spectator ions. It shows what happens chemically when reactants turn into products.pH
: pH is a measure of the acidity or alkalinity of a solution. It's calculated as the negative logarithm (base 10) of the concentration of hydrogen ions in a solution.Stoichiometry
: Stoichiometry involves calculations based on balanced chemical equations regarding quantities involved in chemical reactions - both reactants and products.Strong Acid Strong Base Reactions
: These are reactions where strong acids react with strong bases producing salt and water. They are also known as neutralization reactions because they result in a solution that is neither acidic nor basic.Titration
: Titration is an analytical method used in chemistry to determine the concentration of an unknown solution using a known concentration of another solution.Weak Acid Strong Base Reactions
: These are reactions where a weak acid reacts with a strong base to form a salt and water. The pH at the equivalence point is greater than 7.8.4 Acid-Base Reactions and Buffers
4 min read•january 10, 2023
Dylan Black
Jillian Holbrook
Dylan Black
Jillian Holbrook
Now that we understand with acids and bases, it is time to get into the fun stuff: mixing acids and bases together! In this section, we introduce the idea of reactions between acids and bases, such as the reaction CH3COOH + NaOH → CH3COONa + H2O, and how it relates to a new topic: .
Defining Buffers
are awesome. are a mixture of an acid (HA) and its (A-) in a solution. What makes a buffer interesting is that it is resistant to changes in . When adding some acid or base to a buffer, you will find that the (for a range) will not change much. Why? If you add acid, the will take care of it, and vice versa.
are important both in chemistry and biology. are found everywhere in organisms to maintain , a healthy state. In fact, your own blood is a buffer! 🩸
are incredibly important not only because of their biological ramifications but also in their creation via chemical reactions between acids and bases.
Reactions with Acids and Bases
Strong Acid Strong Base Reactions
A strong acid strong base reaction is one that you are most likely familiar with. This is the type of acid-base reaction students see in Unit 4 when acids and bases are first introduced without the ✨magic✨ of .
An example of a strong acid strong base is the reaction between HCl and NaOH to form NaCl and H2O. However, when we look closer, we find that for this reaction (and any strong acid strong base reaction), the ions are spectators, so the net ionic is just H+ + OH- <=> H2O.
Take a look at an example problem with a strong acid strong base reaction:
What is the after the addition of 10.0 mL of 0.100 M NaOH to 25.0 mL of 0.100 M HCl?
Start by writing out our reaction:
HCl + NaOH <=> NaCl + H2O
Writing the net ionic, we find that Cl- and Na+ are spectators, giving us:
H+ + OH- <=> H2O
By multiplying mL times molarity, we can find the number of millimoles of each species:
10.0 * 0.1 = 1mmol OH-
25.0 * 0.1 = 2.5mmol H+
We use to find how much H+ is left over (we know that all of the OH- will be used up because H+ is in excess).
Finally, we divide 1.5mmol H+ by the volume (35mL) to find [H+] = 4.8 * 10^-2, which corresponds to a of 1.37.
It is worth noting that with (and other reactions we will discuss), that there are three "zones": the zone where the acid is in excess, where the base is in excess, or where the two are equimolar. This thinking will come in handy when we apply these reactions in the next section about titrations.
Weak Acid Strong Base Reactions
are very similar to , except the acid does not fully , meaning our will be slightly different. For the example in this section, we will use the reaction CH3COOH + NaOH <=> CH3COONa + H2O.
Like before, we see that Na+ is a spectator. However, we cannot say the same about CH3COO- because CH3COOH is a weak acid and does not fully . Our for this reaction will be CH3COOH + OH- <=> CH3COO- + H2O, which we will use for calculations.
In general, you usually want to start problems with by writing out the . With these problems, we are going to start seeing , mixtures of an acid and its . We will discuss calculating the of these in detail, but for now, just know that we can use this equation:
Let's take a look at an example!
Calculate the in the of 25.0 mL of 0.100 M acetic acid with 0.100 M NaOH after adding 10.00 mL of 0.100 M NaOH.
We can use the same procedure for solving our strong acid strong base problem for this problem. First, calculate millimoles, and then use to find what's left.
However, what's different here is that instead of having H+ or OH- at the end, we have a mixture of CH3COOH and its CH3COO-. We can use the above equation to solve for :
Note that the equation calls for concentrations of the and the acid. Because we have the same volume of each, the divisions would cancel out.
Again, like the strong acid strong base reaction, you will have points with only acid, a buffer region (this is unique to weak acid/base reactions), an (where mmol acid = mmol base), and a post-equivalence region.
The strategies shown in this example can be applied to weak base strong acid reactions. Just remember to be careful about managing conjugate acids/bases and distinguishing between or pOH. On the AP Exam, a weak acid strong base example is much more likely, but be ready for either.
Key Terms to Review (14)
Acid-Base Reactions
: Also known as neutralization reactions, these involve an acid reacting with a base to produce water plus a salt.Buffers
: Buffers are solutions that resist changes in their pH when small amounts of an acid or a base are added.Conjugate Acid/Base
: A conjugate acid is the particle formed when a base gains a hydrogen ion, and a conjugate base is what remains after an acid has donated a proton during a chemical reaction.Conjugate Base
: The conjugate base is what remains of an acid after it has donated its proton during an acid-base reaction.Dissociate
: Dissociation in chemistry refers to the process where compounds split into smaller particles, usually ions.Equilibrium
: Equilibrium refers to the state in which both reactants and products are present in concentrations which have no further tendency to change over time. It's when forward and reverse reactions occur at equal rates so there's no net change observed.Equivalence Point
: The equivalence point in a titration is when exactly enough titrant has been added to react completely with the substance being analyzed.Homeostasis
: Homeostasis refers to an organism's ability to maintain stable internal conditions despite changes in its environment.Net Ionic Equation
: The net ionic equation represents only those components involved directly in the chemical reaction, excluding spectator ions. It shows what happens chemically when reactants turn into products.pH
: pH is a measure of the acidity or alkalinity of a solution. It's calculated as the negative logarithm (base 10) of the concentration of hydrogen ions in a solution.Stoichiometry
: Stoichiometry involves calculations based on balanced chemical equations regarding quantities involved in chemical reactions - both reactants and products.Strong Acid Strong Base Reactions
: These are reactions where strong acids react with strong bases producing salt and water. They are also known as neutralization reactions because they result in a solution that is neither acidic nor basic.Titration
: Titration is an analytical method used in chemistry to determine the concentration of an unknown solution using a known concentration of another solution.Weak Acid Strong Base Reactions
: These are reactions where a weak acid reacts with a strong base to form a salt and water. The pH at the equivalence point is greater than 7.
Resources
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.