Chapter 3: Water

Question 1

What are the properties of molecules based on?

Answer 1

Composition

Covalent Structure

Bonding and Molecular Geometry

Question 2

What is electronegativity?

Answer 2

The ability of an atom within a molecule to draw electrons towards itself

Question 3

Which atom in water is partially negative? why?

Answer 3

Oxygen because it has a larger electronegative pull than the Hydrogens so the electrons reside slightly closer to the oxygen

Question 4

Are Hydrogen Bonds polar or nonpolar?

Answer 4

Hydrogen Bonds are Polar.

Question 5

In water, are the hydrogen bonds static or are they broken and reformed?

Answer 5

Hydrogen Bonds break and reform constantly and typically only last mere moments

Question 6

What kind of molecules is water a good solvent for?

Answer 6

Water is a good solvent for polar/charged molecules, but it is a poor solvent for nonpolar molecules.

Question 7

How does water dissolve crystalline salts?

Answer 7

Water does this by hydrating their component ions and breaking up ionic interactions. The water molecules cluster around the ions with positive ions having the oxygen facing them and negative ions having the hydrogens face towards it.

Question 8

What are the types of Noncovalent Interactions?

Answer 8

Ionic (Coulombic) interactions

Dipole interactions

Van der Waals interactions

Hydrophobic Effect

Question 9

What are Ionic (Coulombic) interactions?

Answer 9

electrostatic interactions between permanently charged species, or between the ion and a permanent dipole

Question 10

What are Dipole Interactions?

Answer 10

electrostatic interactions between uncharged but polar molecules

Question 11

What are Van der Waals interactions?

Answer 11

• weak interactions between all atoms, regardless of polarity
• attractive (dispersion) and repulsive (steric) component

Question 12

What is the Hydrophobic Effect?

Answer 12

complex phenomenon associated with the ordering of water molecules around nonpolar substances

Question 13

Does water surrounding nonpolar solutes have low or high entropy?

Answer 13

It has low entropy and therefore is thermodynamically unfavorable.

Question 14

Know the Functional Groups

Answer 14

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Question 15

What do amines act as?

Answer 15

They act as bases

Question 16

Which functional group can act as either a base or an acid?

Answer 16

Imidazole

Question 17

How do you find pH from the concentration of protons?

Answer 17

pH = -log[H+]

Question 18

What is the definition of an acid?

Answer 18

An Acid is a substance that contains hydrogen and dissociates in water to yield H+

Examples: HCl, HNO3, HCOOH

Question 19

What is the definition of a base?

Answer 19

A Base is a substance that contains a hydroxyl group (OH-) that dissociates in water to yield OH-

Examples: NaOH, KOH, Ba(OH)2

Question 20

What is the difference between a weak and a strong acid?

Answer 20

Strong acids dissociate from their proton to completion while weak acids reach an equilibrium with their conjugated base.

Question 21

Explain why water has a dipole moment while methane or carbon dioxide does not.

Answer 21

Water has a dipole moment due to the electronegative differences between O and H causing the electrons to move more towards the oxygen molecule and causing it to be partially negative. Methane does not have a large enough difference in electronegativity to cause a dipole moment, and carbon dioxide is in a linear geometry which means that the pulls of the Oxygens are against each other and do not cause a dipole moment.

Question 22

What is an organic acid?

Answer 22

An organic acid is an acid with at least one carboxylic group

Question 23

What is an oxoacid?

Answer 23

An acid where the number of Oxygens equals or exceeds by one the number of Hydrogens. Examples: HNO2 and H3PO4

Question 24

How do you calculate Ka?

Answer 24

ka = ([H+]eq[A-]eq)/[HA]eq

Question 25

If an acid’s Ka is higher, does that mean it is a stronger or weaker acid?

Answer 25

It means it is a stronger acid because more of the acid dissociates and forms its conjugate base.

Question 26

What is pKa relative to ka?

Answer 26

pKa = -logka

Question 27

If an acid’s pKa is higher does that mean it is a stronger or weaker acid?

Answer 27

It means that it is a weaker acid

Question 28

What is the Henderson-Hasselbach Equation?

Answer 28

pH = pKa + log([H+]/[HA])

Question 29

When is buffering capacity greatest?

Answer 29

It is greatest when pH = pKa (This means there are equivalent amount of acid and conjugate base in solution)

Question 30

In vivo, what are buffer systems based on?

Answer 30

• Phosphate, concentration in millimolar ranges
• bicarbonate, important for blood plasma
• histidine, efficient buffer at neutral pH

Question 31

Do hydrogen bonds only occur between separate molecules? Explain

Answer 31

Hydrogen bonds can occur within the same molecule if there is a large enough molecule with multiple areas capable of forming hydrogen bonds.

Question 32

Explain the differences between covalent bonds and hydrogen bonds.

Answer 32

Covalent bonds are actual chemical bonds in which electrons are shared and are quite strong. Hydrogen bonds are electrostatic interactions that are individually weak.