Chemistry: Water and Hydrogen Bonding
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Questions and Answers

What is the enthalpy change associated with melting ice?

  • 0 kJ/mol
  • +6 kJ/mol (correct)
  • +12 kJ/mol
  • -6 kJ/mol

Why does water melt easily at 25°C?

  • The melting point of water is above 25°C.
  • The solid is more disordered than the liquid.
  • The liquid is more disordered than the solid. (correct)
  • The liquid has lower enthalpy than the solid.

What is the value of TΔS for water melting at 25°C?

  • 6.6 kJ/mol (correct)
  • 6.0 kJ/mol
  • 12.6 kJ/mol
  • 0 kJ/mol

Which statement about breaking hydrogen bonds in water is true?

<p>It requires energy input in the form of enthalpy. (B)</p> Signup and view all the answers

Which relationship is true regarding TΔS and ΔH for water melting at 25°C?

<p>TΔS &gt; ΔH (A)</p> Signup and view all the answers

How many hydrogen bonds does ice possess on average per water molecule?

<p>4.0 (B)</p> Signup and view all the answers

What is the average number of hydrogen bonds in liquid water per molecule?

<p>3.4 (A)</p> Signup and view all the answers

Which of the following atoms can form hydrogen bonds?

<p>Oxygen (A)</p> Signup and view all the answers

Hydrogen bonds are considered...

<p>Weak interactions compared to covalent bonds. (C)</p> Signup and view all the answers

What is the strength range of a hydrogen bond in kJ/mol?

<p>8-21 (A)</p> Signup and view all the answers

When are hydrogen bonds considered strongest?

<p>When linear arrangement is maintained. (B)</p> Signup and view all the answers

What type of interaction is hydrogen bonding classified as?

<p>Electrostatic interaction (D)</p> Signup and view all the answers

Which molecule structure is mentioned in the context of water and ice?

<p>Network structure (C)</p> Signup and view all the answers

What is the pH level of a solution with complete dissociation of NaOH?

<p>pH ~ 14 (B)</p> Signup and view all the answers

Which of the following describes a weak acid?

<p>A donor of protons (D)</p> Signup and view all the answers

What is true about most biological acids and bases?

<p>They undergo incomplete dissociation. (D)</p> Signup and view all the answers

What is formed when a weak base reacts with water?

<p>Conjugate acid and hydroxide ion (A)</p> Signup and view all the answers

Which option correctly describes a weak base?

<p>An acceptor of protons (A)</p> Signup and view all the answers

What is the maximum energy associated with electrostatic interactions?

<p>200 kJ / mol (B)</p> Signup and view all the answers

Which of the following correctly describes electrostatic interactions?

<p>Attraction between opposite charges or repulsion between similar charges (D)</p> Signup and view all the answers

Which of the following elements has the highest enthalpy as given in the content?

<p>Oxygen (O) (D)</p> Signup and view all the answers

What is required to break hydrogen bonds?

<p>Addition of enthalpy (D)</p> Signup and view all the answers

Which of the following is NOT an important hydrogen bond donor or acceptor in cells?

<p>Phosphorus (P) (D)</p> Signup and view all the answers

What type of chemical interactions do hydrogen bonds represent?

<p>Only attractive interactions (A)</p> Signup and view all the answers

How do electrostatic interactions impact cellular function?

<p>They provide structural stability and influence reactivity (A)</p> Signup and view all the answers

Which pair of elements typically forms a hydrogen bond in cells?

<p>N and H (A)</p> Signup and view all the answers

What is the primary reason hydrocarbons disfavor dissolution in water?

<p>Reduced entropy of water. (D)</p> Signup and view all the answers

Which statement best describes amphipathic molecules?

<p>They contain both polar and non-polar groups. (D)</p> Signup and view all the answers

What is the effect of amphipathic molecules on water entropy?

<p>They help raise the water entropy by clustering hydrophobic groups. (A)</p> Signup and view all the answers

What do detergents, like sodium dodecylsulphate, primarily form in aqueous solutions?

<p>Micelles. (B)</p> Signup and view all the answers

How do van der Waals interactions contribute to the structure of micelles?

<p>They facilitate interactions among the hydrocarbons. (B)</p> Signup and view all the answers

What defines the 'Hydrophobic Effect' in terms of molecular interactions?

<p>The clustering of non-polar molecules to minimize interaction with water. (B)</p> Signup and view all the answers

What happens to the free energy of amphipathic molecules when they reach their lowest energy state?

<p>It decreases, leading to stability. (A)</p> Signup and view all the answers

In a detergent micelle, where do the hydrophilic groups typically orient themselves?

<p>Towards water molecules. (B)</p> Signup and view all the answers

Which of the following substances is an example of an amphipathic molecule?

<p>Proteins. (A)</p> Signup and view all the answers

What role do amphipathic molecules play in organizing cellular structures?

<p>They help organize micelles and membranes. (D)</p> Signup and view all the answers

What is the pH when the concentration of the weak acid [HA] is equal to that of its conjugate base [A-]?

<p>pH = pKa (C)</p> Signup and view all the answers

What happens to the pH in a titration curve near the end when there is no more HA to ionize?

<p>The pH rises sharply. (C)</p> Signup and view all the answers

In the buffering region of a titration curve, what happens when small amounts of strong acid or base are added?

<p>There is little to no change in pH. (C)</p> Signup and view all the answers

What is the main buffer system found in cells?

<p>H2PO4-/HPO4^2- (B)</p> Signup and view all the answers

Which equation relates the concentrations of a weak acid and its conjugate base to the pH?

<p>pH = pK_a - log10 ([HA]/[A-]) (C)</p> Signup and view all the answers

What is the pKa value of H2PO4-?

<p>7.2 (C)</p> Signup and view all the answers

In a weak acid and conjugate base system, what does a high concentration of H+ indicate?

<p>The weak acid is dominant. (A)</p> Signup and view all the answers

What results from adding a small amount of strong acid to a well-buffered solution?

<p>Minimal change in pH. (C)</p> Signup and view all the answers

Flashcards

Hydrogen Bonds

H-bonds (hydrogen bonds) are electrostatic attractions between polarized molecules, like those containing O-H, N-H, or F-H.

H-bonding in Water

Water forms networks through hydrogen bonding, with an average of 4 H-bonds per molecule in ice and 3.4 H-bonds per molecule in liquid water.

Strength of H-bonds

The strength of a hydrogen bond is determined by the linearity of the bond. Linear bonds, where the H, the shared hydrogen, and the two heavy atoms are in a line, are the strongest.

Importance of H-bonds

Hydrogen bonds play a crucial role in many biological processes, such as protein folding, DNA replication, and the properties of water.

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Weak Interaction

Hydrogen bonds are weaker than covalent bonds, with typical energies of 8-21 kJ/mol.

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Electrostatic Attraction

H-bonds form electrostatic interactions between polarized molecules. Polar molecules have positive and negative regions, just like magnets.

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Water Properties and H-bonds

Hydrogen bonding influences the properties of water, impacting its boiling and melting points, surface tension, and ability to act as a solvent.

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Water States and H-bonds

Water molecules form a network, impacting its ability to change states from solid (ice), to liquid (water), and even into gas (water vapor).

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Enthalpy of Fusion (∆Hmelt)

The energy required to break the bonds in a substance, leading to a change in its state.

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Entropy of Fusion (∆Smelt)

The change in entropy (disorder) during a phase change like melting.

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T∆Smelt

The energy associated with the increase in entropy during melting.

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Why does water melt easily at 25°C?

Water melts easily at 25°C because the increase in entropy (T∆Smelt) outweighs the energy required to break hydrogen bonds (∆Hmelt).

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Liquid water vs. Ice

The liquid state of water is more disordered than the solid state (ice).

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Ionic Bond

A type of electrostatic interaction that occurs between oppositely charged ions, such as Na+ and Cl-

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Repulsive Interaction

A type of electrostatic interaction that occurs between similarly charged ions, such as two Na+ ions.

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Hydrogen Bond Donor

A molecule that can donate an electron to form a hydrogen bond.

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Hydrogen Bond Acceptor

A molecule that can accept an electron to form a hydrogen bond.

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Enthalpy of H-Bond Breaking

The amount of energy needed to break a hydrogen bond.

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Importance of Hydrogen Bonds

Hydrogen bonds are important in biological systems, such as holding DNA strands together and stabilizing proteins.

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Strong Base

A strong base completely dissociates in solution, releasing all its hydroxide ions (OH-) into the solution. This results in a high concentration of OH-, leading to a very basic solution with a high pH value (close to 14).

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Weak Acid

A weak acid only partially dissociates in solution, meaning only a small fraction of its molecules release protons (H+). This results in a lower concentration of H+ compared to a strong acid, making it a weaker acid.

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Weak Base

A weak base only partially accepts protons (H+) from the solution. Unlike strong bases, they don't readily capture all available protons, making them weaker bases.

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Conjugate Base

The conjugate base of a weak acid remains after the acid has donated a proton. For example, when acetic acid (CH3COOH) donates a proton (H+), it forms the acetate ion (CH3COO-), which acts as its conjugate base.

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Conjugate Acid

The conjugate acid of a weak base is formed when the base accepts a proton. For example, when ammonia (NH3) accepts a proton, it forms the ammonium ion (NH4+), which is its conjugate acid.

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Half-Equivalence Point

The point in a titration curve where the concentration of the weak acid (HA) equals the concentration of its conjugate base (A-), resulting in a pH equal to the acid's pKa.

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Buffer

A solution containing a weak acid (WA) and its conjugate base (CB) that resists changes in pH upon addition of small amounts of acid or base.

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Buffering Region

The pH range where a buffer solution effectively resists changes in pH. It is centered around the pKa of the weak acid in the buffer.

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Henderson-Hasselbalch Equation

The equation that relates the pH of a solution containing a weak acid (WA) and its conjugate base (CB) to the acid's pKa and the ratio of their concentrations.

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pKa

The negative logarithm of the acid dissociation constant (Ka). It quantifies the strength of a weak acid.

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Ka (Acid Dissociation Constant)

The ionization constant of a weak acid. It measures the extent to which the acid dissociates in water.

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Titration

The process of adding a solution of known concentration (titrant) to a solution of unknown concentration (analyte) to determine the concentration of the analyte.

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Titration Curve

A graph plotting the pH of a solution against the volume of titrant added.

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Hydrophobic Effect

The tendency of non-polar molecules, like hydrocarbons, to cluster together in water, minimizing their contact with water molecules.

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Amphipathic Molecules

Molecules containing both polar and non-polar regions. Examples include detergents, lipids, proteins, and nucleic acids.

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Self-Assembly of Amphipathic Molecules

The process where amphipathic molecules arrange themselves in water, forming structures like micelles or membranes, with non-polar parts clustered together and away from water.

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Micelle

A spherical structure formed by amphipathic molecules in water, with non-polar tails pointing inward and polar heads facing the surrounding water.

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Van der Waals Interactions

The attractive forces between non-polar molecules, important for maintaining the hydrophobic core of structures like micelles.

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Detergent

A type of amphipathic molecule used for cleaning, with a polar head (hydrophilic) and a non-polar tail (hydrophobic).

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Hydrophilic Group

A component of detergents that interacts with water due to its polar nature.

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Hydrophobic Group

A component of detergents that avoids water due to its non-polar nature. This part typically forms the core of a micelle.

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Lipid

A type of amphipathic molecule crucial for cell membranes, with a polar head and two long non-polar tails.

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Study Notes

Water

  • Water exists as a hydrogen-bonded network
  • Ice has an average of 4 hydrogen bonds per water molecule
  • Liquid water has an average of 3.4 hydrogen bonds per water molecule

Hydrogen Bonding

  • Hydrogen bonds are electrostatic attractions between polarized molecules containing O-H, N-H, or F-H
  • Hydrogen bonds are strongest when linear (two heavy atoms and the shared hydrogen are in a line)
  • Hydrogen bonding is a weak interaction compared to covalent bonding (8-21 kJ/mol)

Electrostatic Interactions

  • Electrostatic interactions are attractions between oppositely charged ions or repulsions between like-charged ions
  • These interactions can be up to 200 kJ/mol
  • Important hydrogen bond donors and acceptors in cells include: -C-O-H, -N-H

Hydrogen Bonds (in relation to Water)

  • Breaking hydrogen bonds requires the addition of enthalpy
  • For ice, enthalpy of melting (ΔHmelting) = +6 kJ/mol
  • Liquid water has more disorder than solid water, and the change in entropy during melting (TASmelt) is > 0 and is greater than the enthalpy change (ΔHmelt)
  • At 25°C: TASmelt = 6.6 kJ/mol and ΔGmelt = ΔHmelt - TASmelt = -0.6 kJ/mol
  • Melting is an entropy-driven process

Biomolecules and Water Interactions

  • Biomolecules interact with water through hydrogen bonding... and electrostatic interactions
  • When NaCl dissolves in water, enthalpy is required to break Na+ and Cl- bonds (+ΔH)
  • Enthalpy is also required to disrupt hydrogen bonding of water (+ΔH)
  • Enthalpy is released when new water-ion interactions form (-ΔH). This is called solvation.
  • The net enthalpy change for dissolving NaCl in water is small and slightly positive.
  • Solid NaCl is highly ordered. NaCl in solution is highly disordered. A large entropy increase favors dissolution.

van der Waals Interactions

  • van der Waals interactions are short-range, very weak attractions (~4 kJ/mol)
  • Non-polar helium atoms form a liquid at 4K due to induced dipole attractions

Non-polar Hydrocarbons in Water

  • Non-polar hydrocarbons interact with each other via van der Waals interactions
  • When a non-polar hydrocarbon is dissolved in water:
    • Hydrocarbon van der Waals interactions are broken (+ΔH)
    • Water hydrogen bonds are broken (+ΔH)
    • New water hydrogen bonds are formed in an organized cage around the hydrocarbon (-ΔH)
    • This optimizes van der Waals interactions between the hydrocarbon and water, and optimizes the hydrogen bonding among the water molecules

Water Entropy and Hydrophobic Effect

  • The entropy of water is reduced, disfavoring the dissolution of hydrocarbons in water (-ΔS)
  • This is called the hydrophobic effect
  • Amphipathic molecules contain both polar and nonpolar groups (e.g., detergents, lipids, proteins, nucleic acids)
  • Their lowest free-energy states have hydrophobic groups clustered together away from water, which increases water entropy.
  • They help organize detergent micelles, membranes, proteins, and DNA

Detergent Sodium Dodecylsulfate

  • A detergent, sodium dodecylsulphate, forms a micelle
  • In the micelle, the hydrocarbons interact with each other via van der Waals forces to form a hydrophobic core.
  • The hydrophilic groups associate with water.

Ionized Water

  • Water can ionize (H2O ↔ H+ + OH-)
  • The equilibrium constant (Keq) for this reaction is: [H+] [OH-] = 1.8 x 10-16 M at 25°C
  • [H2O] = 55.5 M
  • So (Keq x 55.5) = [H+] [OH-] = 10-14 M2 = Kw
  • pH = -log10[H+]
  • pOH = -log10[OH-]
  • In pure water, [H+] = 10-7 Molar and [OH-] = 10-7 Molar
  • When pH = pOH, water is neutral (pH = 7)

pH

  • pH can range from 0 to 14
  • Below pH 7, water is acidic (e.g., gastric juice, pH = 1)
  • Above pH 7, water is basic (e.g., egg white, pH = 8)
  • Strong acids (like HCl) dissociate completely in solution

Acids and Bases

  • Strong bases (like NaOH) dissociate completely in solution
  • Most biological acids and bases are weak
  • A weak acid is a proton donor (e.g., -COOH)
  • A weak base is a proton acceptor
  • Weak acids and bases in solution undergo incomplete dissociation

Weak Acids and Bases

  • Often biochemical reactions release H+ or OH-.

  • The cell uses buffers

  • Titration curves are used to illustrate pH values of mixtures of a weak acid and its conjugate base.

  • Buffers are mixtures of a weak acid and its conjugate base that resist changes in pH when small amounts of strong acid or base are added.

  • The buffering region is around the pKa

  • Ka = [H+] [A−] / [HA] where pKa = -log10Ka

  • The main buffer found in cells is H2PO4− and HPO42− with a pKa of 7.2.

  • The Henderson-Hasselbalch equation determines the pH of a weak acid-conjugate base pair

  • pH = pKa + log10([A−]/[HA]).

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This quiz explores the fascinating properties of water, specifically focusing on melting ice, enthalpy changes, and hydrogen bonding. Questions assess understanding of key concepts such as TΔS, pH levels, and the behavior of weak acids and bases. Test your knowledge of the unique interactions that make water essential for life.

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