Ch 2: Water

Structure of Water (be able to draw it)

- do not need to know numbers - electronegativity difference of .4

Interactions among permanent dipoles

- ex: carbonyl groups - are much weaker than ionic interactions - permanent dipole also induces a dipole moment in a neighboring group by electrostatically distorting its electron distribution

Qualities of water

- hydrophilic - is a polar molecule - A water molecule consists of two hydrogen atoms covalently bonded to an oxygen atom The unequal sharing of electrons between the atoms and the unsymmetrical shape of the molecule means that a water molecule has two poles - a positive charge on the hydrogen pole (side) and a negative charge on the oxygen pole (side).

Buffers: Titration of a Polyprotic Acid

- no buffer at equivalence points - The first and second equivalence points for titration of H3PO4 occur at the steepest parts of the curve. The pH at the midpoint of each stage provides the pK value of the corresponding ionization.

London dispersion forces

- nonpolar molecules have a small, randomly oriented dipole moment resulting from the rapid fluctuating motion of their electrons - This transient dipole moment can polarize the electrons in a neighboring group - are extremely weak and fall off so rapidly with distance (as r−6) that they are significant only for groups in close contact. They are, nevertheless, extremely important in determining the structures of biological molecules, whose interiors contain many closely packed groups (note that London dispersion forces occur between all atoms, in contrast to the other types of noncovalent interactions, which occur only among atoms with diff ering electronegativities).

General rule for any technique presented in class:

- what it is - how it works - limitations

buffer has to keep pH within 1 of its range

Ch3COOH = HA CH3COO = A-

2.2

Chemical Properties of Water • Water ionizes to H+ (which represents the hydronium ion, H3O+) and OH−. • The concentration of H+ in solutions is expressed as a pH value; in acidic solutions pH < 7, in basic solutions pH > 7, and in neutral solu-tions pH = 7. • Acids can donate protons and bases can accept protons. The strength of an acid is expressed as its pK; the stronger the acid, the lower its pK. • The Henderson-Hasselbalch equation relates the pH of a solution to the pK and concentrations of an acid and its conjugate base. • Buff ered solutions resist changes in pH within about one pH unit of the pK of the buff ering species.

How dialysis works

Dialysis. (a) A concentrated solution is separated from a large volume of solvent by a dialysis membrane (shown here as a tube knotted at both ends). Only small molecules can diffuse through the pores in the membrane. (b) At equilibrium, the concentrations of small molecules are nearly the same on either side of the membrane, whereas the macromolecules remain inside the dialysis bag.

Osmosis

Diffusion of water through a selectively permeable membrane so that the concentration of water is equal

strong acid

HCl

dipole-induced dipole

a weak attraction that results when a polar molecule induces a dipole in an atom or in a nonpolar molecule by disturbing the arrangement of electrons in the nonpolar species - are generally much weaker than dipole-dipole interactions.

SAMPLE QUESTION: Calculate the pH of a 2.0 L solution con-taining 10 mL of 5 M acetic acid and 10 mL of 1 M sodium acetate.

answer: in picture

Van der Waals forces

arise from electrostatic interactions among permanent or induced dipoles (the hydrogen bond is a special kind of dipolar interaction).

dipole-dipole interactions

attractive forces that act between polar molecules - between permanent dipoles - dipole-indued dipole interactions - London dispersion forces know where the electrons are and how it influences interactions with molecules

attractive forces acting on biological molecules

ionic interactions, hydrogen bonds, and van der Waals interactions.

acid

is a compound that can donate a proton, and a base is a compound that can accept a proton

acidity of a solution

is expressed as a pH value, where pH = -log[H+].

canvas: buffer practice problems !!!

know conversion site: chemteam/info?

Micelle

lipid molecules that arrange themselves in a spherical form in aqueous solutions

Equations need to use and know

pH = -log[H+] H+= 10^-pH

Henderson-Hasselbalch Equation

pH = pKa + log [A-]/[HA] *NEED TO BE ABLE TO USE THIS

What is the pH of a solution containing 0.120 mol/L of NH4Cl and 0.030 mol/L of NaOH? (NH3 <-> NH4 + pKa 9.25)

pH = pKa + log [A-]/[HA] answer: 8.77 work is in Copy book concept to master: strong base converts part of weak acid into base

Acetic Acid-Acetate as a Buffer System

pKa = 4.76 buff = 3.76-5.76 (1 unit up/down from pKa) - look at graph as well on next card formula: Ka = (refer to pic)

Many biological molecules

contain ionizable groups so that they are sensitive to changes in pH.

Typical Bond Energies - what is the trend? - what are examples of covalent bonds?

covalent bonds are stronger and higher in energy examples of covalent bonds: O-H C-H C-C - do not need to know numbers but just know trend

Dipole

created by equal but opposite charges that are separated by a short distance

Describe the osmotic challenges facing a cell placed in pure water or in a high-salt solution.

A cell placed in pure water will have water coming into it to try to dilute the contents of the cell -- this could cause it to lyse or burst. A cell placed in a high-salt solution will have the opposite effect -- because the environment has such a higher concentration than the contents of the cell, the water inside the cell will transfer to the salt-water to try to dilute it. This will cause the cell to shrivel.

covalent bond

A chemical bond that involves sharing a pair of electrons between atoms in a molecule

What is the relationship between the strength of an acid and its pK value?

A lower pK value denotes a stronger acid, as in the lower the pK value, the more the acid will dissociate since the pK is a dissociation constant. However, it doesn't determine acid strength, only relative dissociation to the solvent. The pK value is the pH at which a species will donate or accept a proton. * Henderson Hasselbalch Equation

Salvation of Ions

- when a molecule is surrounded by water molecules - ions are good at being solvated in biological systems because they have charges - Solubility depends on the ability of a solvent to interact with a solute more strongly than solute particles interact with each other. Water is said to be the "universal solvent." In particular, the polar character of water makes it an excellent solvent for polar and ionic materials, which are said to be hydrophilic (Greek: hydro, water + philos, loving). On the other hand, nonpolar substances are virtually insoluble in water ("oil and water don't mix") and are consequently described as hydrophobic (Greek: phobos, fear). Nonpolar substances, however, are soluble in nonpolar solvents such as CCl4 and hexane. This information is summarized by another maxim, "like dissolves like."

pH = pKa,

50:50 mixture of acid and anion forms of the compound - •Buffering capacity of acid/anion system is greatest at pH = pKa

pKa

= -log Ka - measures acidity strong acid has large Ka and small pKa (strong acid → large Ka → small pKa)

pH

= pH = pK+ log ([acetate]/[acetic acid])

What must a buffer solution include in order to resist changes in pH on addition of acid or base?

A buffer provides both a weak acid and a conjugate base that will react with a strong base or strong acid added to solution, to minimize the change in pH. It does so by providing a proton to neutralize added base or accepting a proton to neutralize an added acid.

Sample problem: Calculate the pH of a 1 L solution contain-ing 0.100 M formic acid and 0.100 M so-dium formate before and after the addition of 1.00 mL of 5.00 M NaOH. How much would the pH change if the NaOH were added to 1.00 L of pure water?

Answer: pictured

Sample: How many milliliters of a 2.0 M solution of boric acid must be added to 600 mL of a solution of 10 mM sodium borate in order for the pH to be 9.45?

Answer: pictured

Define acid and base.

Bronstead-Lowry definitions state that acids will donate protons while bases will accept protons. Acids increase the H+ concentration of a solution while bases will increase the OH- concentration of a solution.

Why is it important to maintain biological molecules in a buffered solution?

For molecules like blood, bicarbonate serves as the most significant buffer. It's important to remain equilibria between gaseous CO2 dissolved in blood and carbonic acid/bicarbonate formed by H+ dissociation. When the pH falls due to H+ production, the bicarbonate-carbonic equilibrium shifts to carbonic acid. Carbonic acid loses water to become CO2, which is then expired in the lungs as gaseous CO2. But when the pH rises, more HCO3- ions form, therefore increasing the amounts of CO2 that can be reintroduced from the lungs into the blood for the conversion of carbonic acid. Variation in the blood buffer system can lead to acidosis or alkalosis, both of which can fatal.

∆G

Gibbs free energy - negative = favorable - positive = unfavorable, rxn won't occur

Molecules diffuse across membranes which are permeable to them from regions of higher concentration to regions of lower concentration.

In dialysis, solutes diffuse across a semipermeable membrane from regions of higher concentration to regions of lower concentration.

What is the relationship between polarity and hydrophobicity?

Like dissolves in like, meaning polar molecules will dissolve in a polar solution such as water. Water is a polar solution that has the ability to pull apart ionic compounds for dissolving (hydrophilic). Nonpolar molecules instead do not dissolve since the charge of the atoms is not influenced by the charge of water. Therefore, nonpolar molecules are hydrophobic and coagulate to minimize water or polar solution interaction.

Dialysis

What it is: The diffusion of small solutes through a selectively permeable membrane. How it works: diffusion based: in picture w description on next flashcard Limitations: need very large differences in size also refer to note

Explain why amphiphiles form micelles or bilayers in water.

Most biological molecules have both polar (charged) and non-polar segments and are therefore simultaneously hydrophilic and hydrophobic. Micelles are globules of amphiphilic molecules arranged so that the hydrophilic groups are at the surface and the hydrophobic groups associate at the center. They form because the regions are positioned by how they are best suited with the environment.

diffusion

Movement of molecules from an area of higher concentration to an area of lower concentration.

• How does osmosis differ from diffusion? Which process occurs during dialysis?

Osmosis is the flow of a solvent from a lower concentration to a higher concentration while dialysis is the passage of a solute in a permeable membrane. In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration. Both osmosis and diffusion occurs during dialysis.

2.1

Physical Properties of Water • Water is essential for all living organisms. • Water molecules can each form four hydrogen bonds with other molecules because they have two H atoms that can be donated and two unshared electron pairs that can act as acceptors. • Liquid water is an irregular network of water molecules that each form up to four hydrogen bonds with neighboring water molecules. • Hydrophilic substances such as ions and polar molecules dissolve readily in water. • The hydrophobic eff ect is the entropically driven tendency of water to minimize its contacts with nonpolar substances. • Water molecules move through semipermeable membranes from re-gions of high concentration to regions of low concentration by osmosis; solutes move from regions of high concentration to regions of low con-centration by diff usion.

strong base

Sodium hydroxide (NaOH)

Compare the structures of ice and water with respect to the number and geometry of hydrogen bonds.

When water freezes the structure becomes extremely tetrahedral, and each water molecule forms a hydrogen bond with 4 other water molecules. When water is in liquid form this tetrahedral structure collapses, it still forms hydrogen bonds, but it is less structurally sound and is more dense.

What is the role of entropy in the hydrophobic effect?

The hydrophobic effect is the tendency for non-polar molecules to aggregate in an aqueous solution, excluding water molecules. Water molecules form a cage around a nonpolar molecule to minimize the disruption of hydrogen bonding, therefore decreasing entropy. When two non-polar groups associate with each other, water molecules are liberated from the solvation shell, increasing the entropy of the system. The entropy will increase when a nonpolar molecule leaves an aqueous solution. Entropy will decrease when nonpolar molecule enters the aqueous solution.

Describe the nature and relative strength of covalent bonds, ionic interactions, and van der Waals interactions (hydrogen bonds, dipole-dipole interactions, and London dispersion forces).

The order of bond strength follows as: Covalent then non-covalent such as ionic, van der Waals, hydrogen bonds, and dipole-dipole interactions. Covalent bonds are the strongest bonds and are bonds created when two atoms are sharing electrons, making them difficult to break. Non-covalent bonds are substantially weaker and reversible but are greater in number. Ionic bonds are regulated by electrostatic interaction and attraction between a positive and negatively charged molecule (attraction by Coloumbs law). Hydrogen bonds aren't actual bonds but are a type of dipole-dipole interaction that exist due to difference in electronegativity. Dipole-dipole occurs when there is a strong difference in electronegativity with the atoms connected by a covalent bond, making an unequal sharing that results in a partial positive and partial negative charge. Van der Waals/London dispersion is very weak but greater in numbers and exists between molecules momentarily thru electron attraction.

Nonpolar Solutes Aggregate in Water

This figure shows the decrease in surface area that is achieved by aggregation. What force drives this aggregation? Hint: Water in the dark blue region is much more organized than in the light blue region. Figure B has increased entropy because there is less cages of water around the molecule --> lower surface area and therefore more entropy/disorder Force that drives aggregation = ENTROPY (a) The individual hydration of dispersed nonpolar molecules (brown) decreases the entropy of the system because their hydrating water molecules (dark blue) are not as free to interact with other water molecules as they do in the absence of the nonpolar molecules. (b) Aggregation of the nonpolar molecules increases the entropy of the system, since the number of water molecules required to hydrate the aggregated solutes is less than the number of water molecules required to hydrate the dispersed solute molecules. This increase in entropy accounts for the spontaneous aggregation of nonpolar substances in water.

Explain why it is more complicated to calculate the pH of a solution of weak acid or base than to calculate the pH of a solution of strong acid or base.

Unlike strong acids or strong bases, weak acids and bases do not completely dissociate at equilibrium in water, but instead reach equilibrium at a different point.

Which of the functional groups listed in Table 1-2 can function as hydrogen bond donors? As hydrogen bond acceptors?

Water can form hydrogen bonds with hydroxyl groups (door/acceptor), keto groups (acceptor), carboxylate ions (acceptor), and ammonium ions (donor).

What are the products of water's ionization? How are their concentrations related?

Water ionization takes place as: H2O <-> H+ + OH- Pure water dissociates to form hydrogen ions(H+) and hydroxide ions(OH-) Ionic product of water(Kw) which is a dimensionless constant is given as: Kw = [H+][OH-] It is the product of the concentration of hydrogen ions and hydroxide ions. For pure water, Kw = 10-14 Kw is a dimensionless number as the included concentrations are relative to the standard states. Also, for complete ionization, [H+] = [OH-]

Explain why polar substances dissolve in water while nonpolar substances do not.

Water is polar, the negative charge of O will attract the positive charge of an ionic compound while the positive charge of H will be attracted to the negative charge of the ionic molecule, therefore pulling the molecule apart to dissolve. However, a non-polar solution will have no attraction to the charges of water, therefore not dissolving, but instead grouping.

Predict the pH of a sample of water if Kw were 10−10 or 10−20.

We are given: Kw = 10^-10 Kw = [H+][OH-] For complete ionization, let [H+] = [OH-] = x Thus, 10^-10 = x * x x2 = 10^-10 x = 10^-5 Thus, [H+] = 10^-5 pH = -log(H+) = -log(10^-5) = 5 Similarly for Kw = 10^-20 , [H+] = 10^-10 pH = -log[H+] = -log(10^-10) = 10

Describe how to calculate pH from the concentration of H+ or OH−.

We know, Kw = [H+][OH-] Thus, [H+][OH-] = 10^-14 Taking negative logarithm on both sides we get, -log[H+][OH-] = -log(10^-14) -( log[H+] + log[OH-] ) = 14 pH + pOH = 14 If we are given [H+], we can find pH directly with pH = -log[H+] If we are given [OH-], we can find pOH with pOH = -log[OH-] Then, we can use relation pH + pOH = 14 to find pH.

Polar and ionic substances

can dissolve in water.

titration curve

demonstrates that if the concentrations of an acid and its conjugate base are close, the solution is buffered against changes in pH when acid or base is added.

Buffers: Titration Curves of Weak Acids

different buffers have different pKas --> function in different ranges ***buffers and calculating pH in the graph: pKa from top to bottom: 9.25 6.8 4.7 *on test may need to use math or graph like this to solve problems At the starting point, the acid form predominates. As strong base (e.g., NaOH) is added, the acid is converted to its conjugate base. At the midpoint of the titration, where pH = pK, the concentrations of the acid and the conjugate base are equal. At the end point (equivalence point), the conjugate base predominates, and the total amount of OH− that has been added is equivalent to the amount of acid that was present at the starting point. The shaded bands indicate the pH ranges over which the corresponding solution can effectively function as a buffer. Several details about the titration curves in Fig. 2-17 should be noted: 1. The curves have similar shapes but are shifted vertically along the pH axis. 2. The pH at the midpoint of each titration is numerically equivalent to the pK of its corresponding acid; at this point, [HA] = [A−]. 3. The slope of each titration curve is much lower near its midpoint than near its wings. This indicates that when [HA] ≈ [A−], the pH of the solution is relatively insensitive to the addition of strong base or strong acid. Such a solution, which is known as an acid-base buffer, resists pH changes because small amounts of added H+ or OH− react with A− or HA, respectively, without greatly changing the value of log([A−]/[HA]).

Water

dissociates to form H+ and OH- ions, with a dissociation constant of 10-14.

∆H

enthalpy

∆S

entropy = disorder in a system

hydrophobic effect

explains the exclusion of nonpolar groups as a way to maximize the entropy of water molecules

Amphiphiles

fatty acid anions - small polar sections (red) - large non-polar sections (black) - How do amphiphiles interact with an aqueous solvent? Water tends to hydrate the hydrophilic portion of an amphiphile, but it also tends to exclude the hydrophobic portion. Amphiphiles consequently tend to form structurally ordered aggregates.

Amphiphilic substances

form micelles or bilayers that hide their hydrophobic groups while exposing their hydrophilic groups to water. - has both polar and non polar properties

Water: Hydrogen Bonding

hydrogen bonding is a weak interaction between an electronegative atom in one molecule and a hydrogen atom in other; hydrogen bonding plays a crucial role in water - hydrogen bonds = covalent, weaker than covalent and ionic bonds bc they are partial

liquid water

hydrogen bonds rapidly break and re-form in irregular networks

Amphiphiles Form Micelles & Bilayers

polar side faces solvent, non polar side faces away from solvent --> increase in entropy - polar outward - non polar inward - micelles are globules of up to several thousand amphiphilic molecules arranged so that the hydrophilic groups at the globule surface can interact with the aqueous solvent while the hydrophobic groups associate at the center, away from the solvent In aqueous solution, the polar head groups of amphipathic molecules are hydrated while the nonpolar tails aggregate by exclusion from water. (a) A micelle is a spheroidal aggregate. (b) A bilayer is an extended planar aggregate.

Water molecules

polar, can form hydrogen bonds with other molecules water = medium of biochemistry

Henderson-Hasselbalch equation

relates the pH of a solution of a weak acid to the pK and the concentrations of the acid and its conjugate base.

O, C, H, N

the four elements that make up more than 90% of living matter - atoms mostly present in biochem

Proton Jumping

the rapid movement of a proton among hydrogen-bonded water molecules - occurs rapidly - protons are capable of going through water very quickly, extra protons jump around rly fast Proton jumps occur more rapidly than direct molecular migration, accounting for the observed high ionic mobilities of hydronium ions (and hydroxide ions) in aqueous solutions.

pH Values of Common Substances

trend: lower pH = more acidic higher pH = more basic

Dissociation Constants and pK Values of Some Acids

trend??: lower K, higher pK??

dissociation constant

varies with the strength of an acid

Nonpolar Solutes in Water

water forms a cage around solutes - this decreases entropy (∆S) and makes ∆G positive and therefore unfavorable - reason why oil separates from water: water excludes something that does not allow hydrogen bonding hydrophobic effect: The tendency of water to minimize its contacts with hydrophobic molecules

ice

water molecules are hydrogen bonded in a crystalline array - Crystal lattice

Buffers

weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH - keep pH constant, resist change in pH weak acid + conj. base

When is buffering capacity is lost?

when the pH differs from pKa by more than 1 pH unit