Top2-Ch2-P43-46 Flashcards
Water has a higher (three things) more than most common solvents.
Water has higher melting point, boiling point, and heat of vaporisation than most common solvents
For water. It has a ________ bond with the e- being shared _________ between H and O.
covalent, unequal
Which is more electronegative (tendency to attract electrons) in water molecule O or H?
And is H2O polar or not?
O more electronegative so attracts e-
H2O = polar compound
What is the seawater molecule?
CH2O
What is the angle of a H-O-H bond in water?
104.5 degrees
Hydrogen bonding explain
Hydrogen acceptors: Electronegative atoms (usually O or N, Not C)
Hydrogen donors: H atoms covalently bonded to an electronegative atom
Uneven e- sharing → electrostatic attraction
This allows the formation of H-bonds :
which are longer & weaker than covalent bonds
wikipedia: A hydrogen bond is the electromagnetic attractive interaction of a polar hydrogen atom in a molecule or chemical group and an electronegative atom, such as nitrogen, oxygen or fluorine, from another molecule or chemical group. It is not a covalent chemical bond.
See hydrogen bond in picture below compared to covalent bond.
(S also slightly electronegative)
What is bond dissociation energy? And compare hydrogen bonds to other covalent bonds.
Energy required to break a bond.
Hydrogen bond dissocation energy is: 23kj/mol while O-H covalent bond in water is 470kj/mol or a covalent C-C bond is 348kj/mol
What is “flickering clusters” in context of hydrogen bonding?
When one hydrogen bond breaks another hydrogen bond forms with the same partner or a new one within 0.1ps (1ps = 10-12seconds)
How many hydrogen bonds can each water molecule form with neighbouring water molecules?
Up to four
Explain ice (solid state of water)? Bonds, structure type and density.
- 4 H bonds each water molecule
- Crystal lattice structure
- Less dense than liquid water, floats on water and insulates lower layers
See lattice below of frozen water
In liquid water how many H bonds, on average, create the flickering clusters and what is the range of the H bond lifetime for liquid water.
Average bond 3.4H bonds
Lifetime range of flickering clusters is 1-20ps
(1ps = 10-12 seconds)
Water is almost ______, being held together by substantial internal cohesive forces called _______.
These determine the properties of water.
- solid
- H-bonds
During melting or evaporation the entropy of the aqueous system_______
Recall formula for free energy change
increases
∆G = ∆H - T ∆S
where;
∆G is free energy change the driving force
∆H is the enthalpy change from making and breaking bonds
∆S is the change in randomness
The ∆G must be a negative value for a process to occur spontaneously.
Explain how butanol has a higher boiling point than butane
Butanol (CH3(CH2)2CH2OH) : BP of 117oC.
Butane (CH3(CH2)2CH3) : BP of 0.5oC.
Butanol has a polar hydroxyl group that can form intermolecular hydrogen bonds increasing the energy required to break bonds and get it to boil.
See chart below. In chart below the properties are due to attractions (H bonds) between adjacent water molecules, which give liquid water great internal cohesion.
Note butanol vs butane
Due to just one –OH group and look at water
Are hydrogen bonds unique to water?
What type of compounds do hydrogen bonds form which are of central importance to living cells?
- No
- forms hydrogen bonds with organic compounds
Look at and memorise important biological H bonds. Name the four groups used.
- Between hydroxyl group of alcohol and water
- between carbonyl group of a ketone and water
- between peptide groups in polypeptides
- between complementary bases of DNA
When is hydrogen bond strength greatest?
greatest when 3 atoms involved in H bond in a straight line
