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Questions and Answers
What is the ion product constant ($K_w$) for water at 25°C?
What is the ion product constant ($K_w$) for water at 25°C?
A solution is considered acidic when the concentration of $[H_3O^+]$ is greater than that of $[OH^-]$.
A solution is considered acidic when the concentration of $[H_3O^+]$ is greater than that of $[OH^-]$.
True
What happens to the hydroxide ion concentration ($[OH^-]$) when the hydronium ion concentration ($[H_3O^+]$) increases?
What happens to the hydroxide ion concentration ($[OH^-]$) when the hydronium ion concentration ($[H_3O^+]$) increases?
It decreases.
The formula to calculate pH is pH = −log [$H_3O^+$]. This is useful because it measures quantities that change by _____ of magnitude.
The formula to calculate pH is pH = −log [$H_3O^+$]. This is useful because it measures quantities that change by _____ of magnitude.
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Match the following terms with their correct definitions:
Match the following terms with their correct definitions:
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What does pKa represent in the context of acid-base chemistry?
What does pKa represent in the context of acid-base chemistry?
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The equation pH = pKa when the concentrations of an acid and its conjugate base are equal.
The equation pH = pKa when the concentrations of an acid and its conjugate base are equal.
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What is the formula for calculating pKa?
What is the formula for calculating pKa?
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In the equilibrium reaction HA ↔ A- + H+, the species HA is known as a __________.
In the equilibrium reaction HA ↔ A- + H+, the species HA is known as a __________.
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Match the following terms with their definitions:
Match the following terms with their definitions:
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What is the pH of a neutral solution at 25°C?
What is the pH of a neutral solution at 25°C?
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Brønsted-Lowry acids are defined as substances that accept protons.
Brønsted-Lowry acids are defined as substances that accept protons.
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Which of the following definitions best describes an acid according to Arrhenius?
Which of the following definitions best describes an acid according to Arrhenius?
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Sodium hydroxide (NaOH) is classified as a strong acid.
Sodium hydroxide (NaOH) is classified as a strong acid.
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What is produced when an acid donates a proton to water?
What is produced when an acid donates a proton to water?
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A solution with a pH less than 7 is considered ______.
A solution with a pH less than 7 is considered ______.
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What is the primary ion produced when hydrochloric acid (HCl) is dissolved in water?
What is the primary ion produced when hydrochloric acid (HCl) is dissolved in water?
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A base increases the concentration of ______ when dissolved in water.
A base increases the concentration of ______ when dissolved in water.
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Which of the following substances acts as a strong acid?
Which of the following substances acts as a strong acid?
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Which equation represents the autoionization of water?
Which equation represents the autoionization of water?
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Increasing the concentration of H3O+ in a solution will raise the pH.
Increasing the concentration of H3O+ in a solution will raise the pH.
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Match the following acids and bases with their characteristics:
Match the following acids and bases with their characteristics:
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What is the conjugate base of the weak acid HA?
What is the conjugate base of the weak acid HA?
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The definition of acids and bases is limited to aqueous solutions according to the Arrhenius theory.
The definition of acids and bases is limited to aqueous solutions according to the Arrhenius theory.
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Describe what happens to a proton (H+) when it is placed in water.
Describe what happens to a proton (H+) when it is placed in water.
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In the reaction NH3 + H2O ⇌ NH4+ + OH−, NH3 acts as a ______.
In the reaction NH3 + H2O ⇌ NH4+ + OH−, NH3 acts as a ______.
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What happens to the concentration of OH− when [H3O+] is increased by $10^4$?
What happens to the concentration of OH− when [H3O+] is increased by $10^4$?
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What occurs when the pH is equal to the pKa of a weak acid?
What occurs when the pH is equal to the pKa of a weak acid?
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When pH is greater than pKa, the acid exists mostly as the unprotonated form A−.
When pH is greater than pKa, the acid exists mostly as the unprotonated form A−.
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What is the common-ion effect in acid-base equilibria?
What is the common-ion effect in acid-base equilibria?
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Buffers protect against __________ disturbances and help maintain pH.
Buffers protect against __________ disturbances and help maintain pH.
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Match the terms with their definitions:
Match the terms with their definitions:
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What happens when sodium acetate (CH₃COONa) is added to acetic acid?
What happens when sodium acetate (CH₃COONa) is added to acetic acid?
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The equilibrium constant changes when a common ion is added to a weak acid solution.
The equilibrium constant changes when a common ion is added to a weak acid solution.
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Why are buffers important in biological processes?
Why are buffers important in biological processes?
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What is the characteristic of a strong acid?
What is the characteristic of a strong acid?
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A weak acid only partially ionizes in solution.
A weak acid only partially ionizes in solution.
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What ions are produced when a strong base reacts with water?
What ions are produced when a strong base reacts with water?
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In the equation HA (aq) + H2O (ℓ) ⇌ A− (aq) + H3O+ (aq), HA is a _____ and A− is its _____
In the equation HA (aq) + H2O (ℓ) ⇌ A− (aq) + H3O+ (aq), HA is a _____ and A− is its _____
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Which statement best summarizes a strong base?
Which statement best summarizes a strong base?
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Conjugate bases of strong acids are strong bases.
Conjugate bases of strong acids are strong bases.
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What is formed when a weak base partially accepts protons in solution?
What is formed when a weak base partially accepts protons in solution?
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The strongest possible acid at equilibrium in aqueous solution is _____ and the strongest possible base is _____.
The strongest possible acid at equilibrium in aqueous solution is _____ and the strongest possible base is _____.
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What describes the 'leveling effect' in acid-base chemistry?
What describes the 'leveling effect' in acid-base chemistry?
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In an aqueous solution, weak acids only produce H3O+ ions.
In an aqueous solution, weak acids only produce H3O+ ions.
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What happens to a strong acid in water?
What happens to a strong acid in water?
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A _____ base is characterized by its ability to partially accept protons, while a _____ base fully accepts all protons.
A _____ base is characterized by its ability to partially accept protons, while a _____ base fully accepts all protons.
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What happens to the conjugate base of a weak acid?
What happens to the conjugate base of a weak acid?
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The ions H3O+ and OH− can be considered the strongest acid and base that can exist in water.
The ions H3O+ and OH− can be considered the strongest acid and base that can exist in water.
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Study Notes
Lecture 13 Announcements
- Lecture 13 covers acid-base equilibria, including acid-base equilibria, the autoionization of water, the pH scale, strong acids and bases, weak acids, weak bases, the common-ion effect, and buffers.
- Required reading for the lecture is Brown Chapter 16 and 17.
- Problem Set 12 is due the day before Exercise 13, and must be uploaded to Moodle.
- Exercise 13 is the last exercise of the session.
- Problem Set 13 is due Friday, December 22, 2024, at 2:00 PM and Problem Set 14 is posted on Moodle with solutions. Students are not required to submit Problem Set 14.
- The Study Center will be held next Wednesday from 6:00 PM to 8:00 PM in ETA F5 (last session!).
- There are no office hours today.
Lecture 14
- Brown Chapter 20 will be covered in Lecture 14.
- Topics will include electrochemistry, Oxidation States and Oxidation-Reduction Reactions, Balancing Redox Equations, Voltaic Cells, Cell Potentials under Standard Conditions, and Free Energy and Redox Reactions.
Red Thread
- The last four weeks have covered Properties, Kinetics, Equilibrium, Acid-Base, and Batteries.
- This is in a cyclical format.
Review
- Lecture 12 covered chemical equilibria.
- Chemical reactions display dynamic equilibria.
- Forward and reverse reaction rates balance at equilibrium.
- Equilibrium quantified by the equilibrium constant (Kc or Kp).
- Equilibrium constant defined by the Law of Mass Action.
- Equilibrium constants are unitless.
- Reaction quotient (Q) is used when the system is not at equilibrium.
- Le Chatelier's Principle describes how equilibrium shifts with changes in concentration, temperature, and pressure.
- Catalysts alter the speed of equilibrium attainment, but not the equilibrium constant.
- The relationship between Gibbs Free energy (ΔG) change and the equilibrium constant (K) is important.
Limescale (Kalk) Revisited
- Precipitation reactions involve the formation of solid CaCO3 from dissolved Ca²⁺ and CO₃²⁻ ions.
- Addition of vinegar neutralizes CaCO3 to remove limescale.
- CaCO3 is more soluble in water at higher temperatures; this is counterintuitive.
Why does scale deposit more in a tea kettle?
- The precipitation reaction is more complicated than just a simple precipitation reaction.
- It involves the reaction of Ca²⁺ with 2HCO₃⁻ to produce Ca²⁺, CO₃²⁻, CO₂, and H₂O.
- Heating the water leads to scale deposits because increased temperature reduces CaCO3 solubility.
Today: Acid-Base Equilibria
- Acid = substance that ionizes in water to form protons (H⁺).
- Example: HCl → H⁺ + Cl⁻.
- Base = substance that accepts protons (H⁺).
- Example: NaOH → Na⁺ + OH⁻.
- Acid-base chemistry is essential for engineers.
Definition of Acids and Bases (Arrhenius)
- Acids increase [H⁺] in water, and bases increase [OH⁻] in water.
- The Arrhenius definition is restricted to aqueous solutions.
H⁺ in H₂O?
- Bare proton (H⁺) is small, positive, and seeks electrons.
- When placed in water, it reacts with a lone pair on a water molecule to form a hydronium ion (H₃O⁺).
- H⁺ and H₃O⁺ are often used interchangeably, but H₃O⁺ is more accurate.
Autoionization of Water
- Water molecules can act as both acids and bases, self-ionizing to produce hydronium and hydroxide ions.
- Kc, the ion-product constant (Kw), is a constant for this equilibrium and is equal to 1.0 x 10⁻¹⁴ (at 25°C).
- Only a tiny fraction of water molecules are ionized.
Why Do We Care?
- Kw is important because even though [H₃O⁺] and [OH⁻] are small, changes are significant to the system.
- We can change [H₃O⁺] over many orders of magnitude. Kw is the product of [H3O+] and [OH-] = 10-14.
How Do We Quantify?
- pH is a logarithmic scale for quantifying [H₃O⁺]. pH = -log₁₀[H₃O⁺].
- pH of neutral water is 7.0.
- A change of one pH unit corresponds to a tenfold change in [H₃O⁺].
How Do We Manipulate?
- Acids increase [H₃O⁺] by donating H⁺ to water.
- Bases increase [OH⁻] by accepting H⁺ from water.
- These principles apply beyond water.
Brønsted-Lowry Acids and Bases
- Brønsted-Lowry acid: Donates a proton
- Brønsted-Lowry base: Accepts a proton
Acid-Base Pairs
- Acid-base pairs involve proton transfer.
- H₂O can act as both an acid and a base
- Generic acid-base reaction of HA with H₂O.
- Ka is the acid-dissociation constant.
- Kb is the base-dissociation constant.
Strength of Acids
- Strong acids: Fully ionize in aqueous solution, resulting in a strong conjugate base that does not accept protons.
- Weak acids: Partially ionize, forming equilibrium with conjugate base that does accept protons.
- HA without acidity means HA is not an acid.
Strength of Bases
- Strong bases: Completely ionize in water, resulting in a weak conjugate acid.
- Weak bases: Partially ionize, forming an equilibrium with conjugate acid.
- B without basicity means B is not a base.
Strength of Acids and Bases
- A chart summarizes the relative strengths of common acids and bases.
- Strong acids (e.g., HCl, H₂SO₄, HNO₃) have strong conjugate bases.
Strength of Acids
- Strong acids completely give up their protons.
- Weak acids partially give up their protons.
Strength of Bases
- Strong bases completely accept protons.
- Weak bases partially accept protons.
Common-Ion Effect
- Adding a common ion to a weak electrolyte causes a decrease in ionization.
- The common ion is an ion that is also formed by the ionization of the electrolyte.
Buffers
- Buffers resist changes in pH.
- Buffers are made of weak acid and its conjugate base. The buffer's pH is close to the acid's pKa.
Notes
- We use common-ion effect to manipulate/tune buffer pH.
- To calculate the pH of a buffer, we use the Henderson-Hasselbalch equation.
Carbonate System
- The carbonate system is a buffer system in many natural systems (like blood and oceans).
Engineers Can Use This!
- Capture carbon from the atmosphere (CO2) to reduce greenhouse gas emissions using chemical reactions.
What We Learned
- Key concepts from Lecture 13 include definitions of acids and bases, autoionization of water, proton transfer, equilibrium constants, p scales, and buffers.
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Description
Test your knowledge on acid-base chemistry concepts, including the ion product constant, pH calculations, and definitions of acids and bases. This quiz will help reinforce your understanding of key principles related to hydronium and hydroxide ion concentrations, as well as the concept of pKa.
