Lecture 2 - Water and Weak Bonds

Why is water considered the medium for life?

Water is essential for biological reactions, molecular structure, temperature regulation, and transport.

Why did life evolve in water?

Water provided:

• A medium for chemical reactions

• Protection from harmful UV radiation

What percentage of most organisms is water?

Approximately 70–90% water.

Why is water called an excellent solvent?

It dissolves many ionic and polar substances, allowing biochemical reactions to occur.

Where do most cellular chemical reactions occur?

In aqueous (water-based) solutions.

Why is water important for metabolism?

It dissolves reactants, allowing molecules to move and interact efficiently.

Give an example of a reaction involving water.

Hydrolysis, where water breaks chemical bonds (e.g., ATP hydrolysis).

What does it mean that water has a high heat capacity?

Water can absorb or release large amounts of heat with only a small change in temperature.

Why is water's high heat capacity important for living organisms?

It helps maintain stable internal temperatures despite environmental changes.

Water acts as what type of temperature regulator?

A heat buffer.

Is water or ice more dense?

Liquid water is more dense than ice.

Why is ice less dense than liquid water?

Hydrogen bonding forms an open crystal lattice that spaces water molecules farther apart.

Why is it biologically important that ice floats?

Floating ice insulates the water below, allowing aquatic life to survive during cold weather.

Why is water essential for proteins?

Water influences protein folding, stability, and function.

Why is water important for nucleic acids (DNA and RNA)?

It stabilizes their structure and supports their interactions.

How does water affect cell membranes?

Water drives the hydrophobic effect, helping phospholipids form lipid bilayers.

Most organisms are approximately ________ water.

70–90%.

Water is an excellent ________.

Solvent.

Most biochemical reactions occur in ________ solutions.

Aqueous.

Water has a high ________ capacity.

Heat.

Water acts as a ________ buffer.

Heat.

Water is more ________ than ice.

Dense.

Water is essential for the structure of proteins, nucleic acids, and ________.

Membranes.

Why is water considered indispensable for life?

Water:

1. Served as the environment where life originated.

2. Is an excellent solvent for biochemical reactions.

3. Participates directly in many reactions.

4. Buffers temperature due to its high heat capacity.

5. Is denser than ice, allowing aquatic ecosystems to survive freezing conditions.

What is the geometry of the electron pairs around the oxygen atom in water?

Tetrahedral (sp³ hybridized).

How many electron pairs surround the oxygen atom in water?

Four electron pairs.

What are the four electron pairs around oxygen composed of?

• 2 bonding pairs (forming O–H covalent bonds)

• 2 lone (nonbonding) pairs

What type of hybrid orbitals does oxygen use in water?

sp³ hybrid orbitals.

Why is oxygen in water sp³ hybridized?

Because it has four regions of electron density (2 bonds + 2 lone pairs).

What is the H–O–H bond angle in water?

104.5°.

What is the bond angle of a perfect tetrahedron?

109.5°.

Why is water's bond angle (104.5°) smaller than a perfect tetrahedral angle (109.5°)?

The lone pairs repel more strongly than bonding pairs, pushing the hydrogen atoms closer together.

How many lone pairs are on the oxygen atom in water?

Two.

Why do lone pairs occupy more space than bonding pairs?

Their electrons are localized around oxygen and are not shared between atoms, leading to stronger repulsion.

What effect do lone pairs have on water's shape?

They create a slightly distorted tetrahedral (bent) molecular shape.

What is the molecular shape of water?

Bent (angular or V-shaped).

Is water a perfect tetrahedron?

No. The electron geometry is tetrahedral, but the molecular shape is bent due to the two lone pairs.

Oxygen in water is ________ hybridized.

sp³.

Water has ________ bonding pairs and ________ lone pairs.

2; 2.

The electron geometry of water is ________.

Tetrahedral.

The molecular geometry of water is ________.

Bent.

The H–O–H bond angle is ________.

104.5°.

Lone pairs ________ the H–O–H bond angle.

Decrease.

Explain why water has a bent shape instead of a perfect tetrahedral shape.

• Oxygen has four electron pairs arranged tetrahedrally.

• Two are bonding pairs and two are lone pairs.

• Lone pairs repel more strongly than bonding pairs.

• This stronger repulsion compresses the H–O–H bond angle from 109.5° to 104.5°, giving water its bent molecular shape.

Why is water a polar molecule?

Because oxygen is more electronegative than hydrogen, pulling shared electrons closer to itself and creating unequal charge distribution.

What is electronegativity?

The ability of an atom to attract shared electrons in a chemical bond.

Which atom in water is more electronegative?

Oxygen.

What partial charge does the oxygen atom carry in water?

Partial negative charge (δ−).

What partial charge do the hydrogen atoms carry in water?

Partial positive charges (δ+).

Why does oxygen have a partial negative charge?

It attracts the shared electrons more strongly than hydrogen.

What is a dipole?

A separation of positive and negative charges within a molecule.

How many electric dipoles are present in one water molecule?

Two, one along each O–H bond.

What creates the net dipole moment in water?

The combination of the polar O–H bonds and the bent molecular shape.

What is the net dipole moment?

The overall separation of charge that makes water a polar molecule.

Why can water act as a hydrogen bond donor?

Because its hydrogen atoms have partial positive charges (δ+).

Why can water act as a hydrogen bond acceptor?

Because oxygen has two lone pairs and a partial negative charge (δ−).

Can a single water molecule both donate and accept hydrogen bonds?

Yes.

Why is water's polarity important biologically?

It allows water to:

• Form hydrogen bonds

• Dissolve ionic and polar substances

• Stabilize proteins, DNA, and membranes

Which property of water allows it to dissolve salts and other polar molecules?

Its polarity (dipole moment).

Oxygen is more ________ than hydrogen.

Electronegative.

Oxygen carries a partial ________ charge.

Negative (δ−).

Hydrogen carries a partial ________ charge.

Positive (δ+).

Water contains ________ electric dipoles.

Two.

Water is both a hydrogen bond ________ and ________.

Donor; acceptor.

Water's bent shape creates a net ________.

Dipole moment.

Explain why water is polar.

• Oxygen is more electronegative than hydrogen.

• Electrons spend more time near oxygen.

• Oxygen becomes δ−, while each hydrogen becomes δ+.

• Because water has a bent geometry (104.5°), the two bond dipoles do not cancel.

• This creates a net dipole moment, making water a polar molecule capable of hydrogen bonding.

What type of crystal structure does ice form?

A hexagonal crystal lattice.

Why is hexagonal ice highly ordered?

Why is hexagonal ice highly ordered?

Does ice have high or low entropy?

Low entropy, because it is highly ordered.

How many hydrogen bonds does each water molecule form in ice?

4 hydrogen bonds.

How many hydrogen bonds does each water molecule form in liquid water on average?

Approximately 3.4 hydrogen bonds.

Why does ice have more hydrogen bonds than liquid water?

The rigid crystal lattice allows each water molecule to form the maximum number of stable hydrogen bonds.

Why is ice less dense than liquid water?

The hydrogen-bonded lattice spaces water molecules farther apart than in liquid water.

Why does ice float on water?

Ice is less dense than liquid water.

Why is the lower density of ice biologically important?

Floating ice insulates the water below, preventing lakes and oceans from freezing solid and allowing aquatic life to survive.

Is melting ice an endothermic or exothermic process?

Endothermic—it absorbs heat.

What is the sign of ΔH (enthalpy) during melting?

Positive (ΔH > 0).

What happens to entropy (ΔS) when ice melts?

Entropy increases (ΔS > 0) because water molecules become more disordered.

Why can melting occur even though ΔH is positive?

The increase in entropy is large enough that the −TΔS term outweighs the positive ΔH, making ΔG negative at temperatures above the melting point.

What equation relates free energy, enthalpy, and entropy?

ΔG = ΔH − TΔS

During melting, which term favors the reaction?

The increase in entropy (ΔS).

Which has greater entropy: ice or liquid water?

Liquid water.

Ice forms a ________ lattice.

Hexagonal.

Ice has ________ entropy than liquid water.

Lower.

Each water molecule forms ________ hydrogen bonds in ice.

Four.

Each water molecule forms about ________ hydrogen bonds in liquid water.

3.4.

Ice is ________ dense than liquid water.

Less.

Melting absorbs heat, so ΔH is ________.

Positive.

Water has ________ entropy than ice.

Greater.

Explain why ice floats on water.

• Ice forms a hexagonal hydrogen-bonded lattice.

• This lattice holds water molecules farther apart.

• As a result, ice has a lower density than liquid water.

• Because it is less dense, ice floats, helping insulate bodies of water and protect aquatic organisms.

Why is water called the "universal solvent"?

Because it dissolves many charged (ionic) and polar substances.

Why is water a good solvent for charged and polar molecules?

Because it is polar and can form hydrogen bonds and electrostatic interactions with them.

What types of substances dissolve well in water?

Charged (ionic) and polar substances.

Why do amino acids and peptides dissolve in water?

They contain charged and polar groups that form hydrogen bonds with water.

Why are alcohols generally soluble in water?

Their hydroxyl (-OH) groups form hydrogen bonds with water.

Why are carbohydrates soluble in water?

They contain many hydroxyl (-OH) groups that hydrogen bond with water.

Why are nucleic acids soluble in water?

Their charged phosphate backbone and polar groups interact strongly with water.

Can polar molecules have limited solubility in water?

Yes.

Why don't all polar molecules dissolve infinitely in water?

They still disrupt water's hydrogen-bonding network (water lattice), limiting their solubility.

Why is water a poor solvent for nonpolar substances?

Nonpolar molecules cannot form hydrogen bonds and disrupt the hydrogen-bonding network of water.