Chemistry: Buffer Solutions and Entropy

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Questions and Answers

How does the dissociation constant (Ka) influence the buffering capacity of a solution?

A higher Ka value indicates a stronger buffer action, allowing the solution to better resist changes in pH.

What happens to the equilibrium of an acidic buffer when an acid is added to the solution?

The equilibrium shifts to the left to decrease the concentration of H+ ions by forming more undissociated acid molecules.

Describe the effect on a basic buffer when H+ ions are introduced.

H+ ions react with OH- ions released by ammonia, causing the equilibrium to shift to the right to replace the consumed OH- ions.

In the context of buffering solutions, explain the role of conjugate bases and weak acids.

Conjugate bases react with added acids, while weak acids neutralize added bases, maintaining the stability of the solution's pH. Signup and view all the answers

Why are buffers considered essential in chemical reactions and biological systems?

Buffers maintain stable pH levels, which are crucial for optimal reaction conditions and proper functioning of biological processes. Signup and view all the answers

What is entropy a measure of in a system?

Entropy measures the degree of disorder in a system. Signup and view all the answers

How does the dissolution of a solid affect the entropy of a system?

Dissolution increases the entropy as particles become more spread out. Signup and view all the answers

What does an increase in entropy imply about the stability of a system?

An increase in entropy implies that the system becomes more energetically stable. Signup and view all the answers

What information does Gibbs free energy provide about a reaction?

Gibbs free energy indicates the energy change considering both entropy and enthalpy. Signup and view all the answers

Define a conjugate acid-base pair.

A conjugate acid-base pair consists of two compounds that differ by a single proton. Signup and view all the answers

Why can the formula for pH only be applied to weak acids?

The pH formula applies to weak acids because they do not dissociate fully. Signup and view all the answers

What does the acid dissociation constant (Ka) indicate about an acid?

Ka measures the extent to which an acid dissociates in solution, indicating its strength. Signup and view all the answers

How does the equilibrium position of weak acids generally behave?

The equilibrium for weak acids lies far to the left with few dissociated ions. Signup and view all the answers

Why is the lattice enthalpy of ionic compounds always exothermic when forming a solid from gaseous ions?

The formation of a solid ionic lattice releases a large amount of energy due to the strong electrostatic forces of attraction between oppositely charged ions. Signup and view all the answers

How does ionic charge affect the lattice formation enthalpy?

Increasing the ionic charge enhances the attraction between ions, resulting in a more negative lattice formation enthalpy. Signup and view all the answers

What role does ionic radius play in lattice enthalpy?

Decreasing the ionic radius brings ions closer together, increasing the attraction and leading to a larger, more negative lattice formation enthalpy. Signup and view all the answers

Explain the significance of an endothermic ΔH atomization.

An endothermic ΔH atomization indicates that energy is required to break bonds between atoms, converting elements to gaseous atoms. Signup and view all the answers

What is the first electron affinity and how is it represented?

The first electron affinity (EA1) is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form gaseous ions with a single negative charge. Signup and view all the answers

What factors influence the strength of electron affinity?

Factors include nuclear charge, distance between the nucleus and electron, and shielding effect, affecting the attractive forces exerted on the incoming electron. Signup and view all the answers

Why is only the first electron affinity exothermic?

The first electron affinity is exothermic because the addition of an electron to a neutral atom stabilizes it, releasing energy. Signup and view all the answers

What is the difference between standard conditions and standard temperature and pressure (STP)?

Standard conditions are defined as 101 kPa and 298 K (25 degrees Celsius), while STP is 273 K and 101 kPa. Signup and view all the answers

Why are the second and third electron affinities considered endothermic?

They are endothermic because energy is absorbed to overcome the repulsive forces between the incoming electron and the already negative ion. Signup and view all the answers

What trend occurs in the electron affinity (EA1) of Group 16 elements as you move down the group?

EA1 becomes less exothermic as you go down the group due to increased shielding and the outermost electrons being held less tightly to the nucleus. Signup and view all the answers

Why does fluorine have a lower EA1 than chlorine despite being higher in the periodic table?

Fluorine has a lower EA1 due to its smaller atomic radius which causes higher electron repulsion with the incoming electron. Signup and view all the answers

How does hydration enthalpy relate to the size of ions?

Hydration enthalpy is higher for smaller ions due to stronger ion-dipole attractions with water molecules. Signup and view all the answers

Explain the relationship between the charge of an ion and its hydration enthalpy.

The hydration enthalpy increases with the charge of the ion; highly charged ions create stronger attractions with water molecules. Signup and view all the answers

What is the primary reason for the trend of decreasing hydration enthalpy down a group in the Periodic Table?

The primary reason is that larger ions experience weaker attractions with water molecules due to increased ionic radius. Signup and view all the answers

Describe the role of water molecules in the hydration of ions.

Water molecules form ion-dipole attractions with the ions, stabilizing them as they dissolve in solution. Signup and view all the answers

What is the significance of the term 'exothermic' in the context of electron affinity and hydration enthalpy?

Exothermic refers to the release of energy; in electron affinity, it indicates energy release upon adding an electron, and in hydration, the energy released when ions interact with water. Signup and view all the answers

What is the Henderson-Hasselbalch equation used for?

It is used to calculate the pH of buffer solutions and weak acids. Signup and view all the answers

Explain how an amphoteric buffer behaves in the presence of an acid.

An amphoteric buffer reacts with an acid by allowing the proton to join the amine group, forming H3NCH(R)COOH+. Signup and view all the answers

What keeps human blood pH between 7.35 and 7.45?

The buffer action of carbonic acid (H₂CO₃) and bicarbonate anions (HCO₃⁻) maintains blood pH. Signup and view all the answers

What is the significance of Ksp in solubility product calculations?

Ksp helps determine the solubility of sparingly soluble salts and indicates whether a precipitate will form. Signup and view all the answers

How does the common ion effect influence the solubility of a salt?

The common ion effect reduces the solubility of a salt when a second compound with a common ion is introduced. Signup and view all the answers

Outline the steps to determine if a precipitate will form in a saturated solution.

Identify the equilibrium reaction and calculate the product of ion concentrations; compare this with Ksp. Signup and view all the answers

What reaction occurs when a base reacts with an amphoteric buffer?

The base reacts by taking a hydrogen ion from the carboxyl group, forming H2NCH(R)COO- and water. Signup and view all the answers

What does a Ksp value of 1x10^-14 indicate regarding the pH of water?

It signifies that the highest possible pH in pure water is 14. Signup and view all the answers

Study Notes

Chemical Energetics

Atomisation enthalpy (ΔH_at): The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state. ΔH_at is always positive (endothermic).
Lattice dissociation enthalpy: The enthalpy change needed to convert one mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive (endothermic).
Lattice formation enthalpy: The enthalpy change when one mole of solid crystal is formed from its scattered gaseous ions. Lattice formation enthalpies are always negative (exothermic).
Ionic bonds are strong: When ions combine to form an ionic solid lattice, a large amount of energy is released.
Ionic compounds are stable. The strong electrostatic forces of attraction between oppositely charged ions make ionic compounds more stable than their gaseous ions.

Factors Affecting Lattice Enthalpy

Ionic charge: Increasing the ionic charge increases the attraction between the ions, leading to a larger, more negative lattice enthalpy.
Ionic radius: Decreasing the ionic radius brings the ions closer together, increasing the attraction and leading to a larger, more negative lattice enthalpy.
Charge density: Both ionic charge and ionic radius affect charge density. A higher charge density results in a greater lattice enthalpy.

Electron Affinity

First electron affinity (EA₁): The enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous ions each with a single negative charge. The first electron affinity is typically exothermic (negative) for most elements.
Factors affecting EA:
- Nuclear charge: An increase in nuclear charge increases the attraction between the nucleus and the incoming electron, resulting in larger, more negative affinities.
- Distance: A greater distance between the nucleus and the outermost electron shell/orbital weakens the attraction, resulting in lower, less negative affinities.
- Shielding: More electron shells increase the shielding effect, reducing the attraction and subsequently decreasing the electron affinity.

Trends in Electron Affinity of Group 16 & 17 Elements

General trend: Moving down a group, EA₁ becomes less exothermic due to increased atomic size, greater distance between the nucleus and the outermost electron, and greater shielding.
Fluorine exception: Fluorine has a lower EA₁ than chlorine due to its very small atomic radius resulting in high electron density, and therefore greater repulsion between the incoming electron and existing electrons.

Enthalpy Changes (Table of Definitions)

A table including various enthalpy changes, such as formation, neutralisation, combustion, enthalpy of atomisation, ionisation enthalpy, hydration enthalpy, etc. along with their respective thermodynamic signs (positive or negative).

Enthalpy of Solution and Hydration

Enthalpy of solution: The enthalpy change when one mole of an ionic substance dissolves in water to give a solution of infinite dilution.
Hydration enthalpy: The enthalpy change when one mole of gaseous ions dissolves in sufficient water to give an infinitely dilute solution. Hydration enthalpy is always negative.
Attraction between ions and water: When an ionic solid dissolves, positive and negative ions are formed in water. Water molecules form ion-dipole attractions with these ions.

Entropy

Entropy (S): A measure of disorder in a system. Increasing disorder results in increased entropy, making the system energetically more stable.
Gas > Liquid > Solid: Gases have the highest entropy, followed by liquids and then solids.
Temperature change: Entropy generally increases with increasing temperature as the particles gain more energy and disorder increases.
Change in state: Phase transitions (melting, vaporization, etc.) generally increase entropy. Dissolving a solid into a liquid, increases the entropy.

Gibbs Free Energy

Gibbs free energy (ΔG): A measure of spontaneity. ΔG = ΔH - TΔS.
- If ΔG is negative, the reaction is spontaneous at that particular temperature.
- If ΔG is positive, the reaction is nonspontaneous at that particular temperature.
Temperature change and feasibility: A reaction that is not feasible at low temperatures may become feasible at higher temperatures, or vice versa, depending on the relative enthalpy and entropy changes.
Units of ΔG, ΔH, and ΔS are crucial.

Equilibria

Conjugate acid-base pair: Compounds that differ by one proton.
pH: The negative logarithm of the hydrogen ion concentration.
pOH: The negative logarithm of the hydroxide ion concentration.

Acid Dissociation Constant (K_a)

Ka: measure of the extent of dissociation of a weak acid; the smaller the Ka the weaker the acid.

Acidic Buffers

Buffer solutions: Resist changes in pH when small quantities of acid or alkali are added.
How Ka relates to buffer action: A buffer with a higher K_a value has greater resistance to pH change than a buffer with a lower K_a value.

Basic Buffers

Buffer solutions: Resist changes in pH when small quantities of acid or alkali are added.
How Ka relates to buffer action: A buffer with a higher K_a value has greater resistance to pH change than a buffer with a lower K_a value.

Amphoteric Buffers

Amphoteric substances: Substancdes that can act as both acids and bases.
Amino acids in blood: The amino and carboxyl groups in amino acids often act as acids and bases to maintain the blood pH.
Carbonic acid/ bicarbonate buffer in blood: These two components buffer changes in blood pH.

Electrochemistry

Methods for predicting substances liberated during electrolysis from various factors such as electrolyte state (molten or aqueous), redox series (electrode potential), and concentration.
Balancing Redox half equations Details of steps in balancing atoms, water molecules, protons and electrons when balancing redox reactions in different scenarios.

Solubility Product (K_sp)

Solubility Product (Ksp): A measure of the solubility of a sparingly soluble salt in water. A large Ksp means that the salt is more soluble.
Sparingly Soluble Salts: Salts that dissolve only slightly in water.
Common Ion Effect: Addition of a common ion to a saturated solution decreases the solubility of the salt.

Partition Coefficients

Partition coefficient (K_pc): The ratio of concentrations of a solute in two immiscible solvents at equilibrium. The higher the K_pc, the greater the solute's solubility in the organic solvent.
Factors affecting solubility.

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Description

This quiz explores key concepts related to buffer solutions, dissociation constants, and entropy in chemical systems. Understand how added acids affect equilibrium in buffers and the significance of entropy in measuring stability. Dive into the interactions between weak acids and their conjugate bases.