lecture 4 - water

8/18/2019 Lecture 4 - Water

1/23

"Sea Monsters on Medieval and Renaissance Maps"

(British Library, 2013) Chet Van Duzer

The Structure and Function of Water


2/23

Water on the Planet

http://ga.water.usgs.gov/edu/earthhowmuch.html

Large sphere – all the wateron and in earth

Middle sphere – all the fresh

water on and in earth

Smallest sphere – freshwater

available to humans

(diameter= 56 km)

H2O is an unusual chemical

and is critical to the chemistry

of life


3/23

What are the properties of water that make it critical for life?

• being a covalent compound, it is a very stable molecule

• has remarkable solvent properties

• has unusual thermal properties

• plays a direct role in the stability and behavior of other molecules in the cell

• plays a direct role in chemical reaction mechanisms

www.chemistrypictures.org


4/23

Covalent Bonding in H2O

• note that both H and O atoms adopt noble gas configuration (He and Ne)

• two pairs of e- on O atom do not participate in covalent bond formation

these are called non-bonding pairs or lone pairs

cnx.org

Note: O (atomic number 8) also has 2 inner shell electrons (not shown)

!

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=yZlU1kC_fIugeM&tbnid=-6L5KdvBwr0a9M:&ved=0CAUQjRw&url=http://cnx.org/content/m44390/latest/?collection=col11448/latest&ei=ZNEsUtPKIYLE2wWi4oF4&psig=AFQjCNEblp-gspTT2qn1uDe__zblAHIEig&ust=1378755238165486http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=yZlU1kC_fIugeM&tbnid=-6L5KdvBwr0a9M:&ved=0CAUQjRw&url=http://cnx.org/content/m44390/latest/?collection=col11448/latest&ei=ZNEsUtPKIYLE2wWi4oF4&psig=AFQjCNEblp-gspTT2qn1uDe__zblAHIEig&ust=1378755238165486


5/23

The geometry of the H2O molecule

• like methane, water forms a tetrahedral shape in terms of electron density

• because of repulsion from lone pair electrons in H2O, it is a distorted tetrahedron

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=4jpbhi6zWbnPtM&tbnid=y-swkq5hy_af7M:&ved=0CAUQjRw&url=http://www.elmhurst.edu/~chm/vchembook/206bent.html&ei=yJgtUurYNcb62AXo9IHoDg&bvm=bv.51773540,d.aWM&psig=AFQjCNHiiFEpKp13zLqNiq4U6Zo77NLdOg&ust=1378806334807076http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=4UeWgrF99rgxAM&tbnid=5ycaAAi8jqJ6OM:&ved=0CAUQjRw&url=http://www.elmhurst.edu/~chm/vchembook/204tetrahedral.html&ei=-NwsUpnkAobj2wW92ID4DA&psig=AFQjCNGywOystyOG9NbwvZvbW1oJ2dhDqg&ust=1378758231933979


6/23

methane

water

All of these molecules adopt a tetrahedral geometry with respect

to electron density

CH4 NH3 H2O HF

more properly, the water molecule is said

to have a BENT geometry since VESPR

theory accounts only for atomic positions.

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=hoXf2fU3mqI3yM&tbnid=tkh3zDC1fVqNaM:&ved=0CAUQjRw&url=http://www.elmhurst.edu/~chm/vchembook/207epgeom.html&ei=fNcsUuS8OaXA2AXxqYGoDw&psig=AFQjCNHNrrDZu-rISiD7OoEvQC29-gAGXw&ust=1378756835631810http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=4jpbhi6zWbnPtM&tbnid=y-swkq5hy_af7M:&ved=0CAUQjRw&url=http://www.elmhurst.edu/~chm/vchembook/206bent.html&ei=yJgtUurYNcb62AXo9IHoDg&bvm=bv.51773540,d.aWM&psig=AFQjCNHiiFEpKp13zLqNiq4U6Zo77NLdOg&ust=1378806334807076http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=4UeWgrF99rgxAM&tbnid=5ycaAAi8jqJ6OM:&ved=0CAUQjRw&url=http://www.elmhurst.edu/~chm/vchembook/204tetrahedral.html&ei=-NwsUpnkAobj2wW92ID4DA&psig=AFQjCNGywOystyOG9NbwvZvbW1oJ2dhDqg&ust=1378758231933979


7/23

Water has a dipole moment - it is therefore a POLAR molecule

• remember what happens when H atom binds to a strongly

electronegative atom like Oxygen (dipole is formed)

• Polar molecules are molecules with an unequal distribution of charge where one end

of molecule is more negative and other end is more positive.

•

it is the polar nature of water that allows it to interact with other charged orpolar molecules and solvate (dissolve) them (more on this in a bit).

H2O NH3 CO2

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799


8/23

Water molecules interact with each other through

Hydrogen Bonding

• each H2O molecule can interact with up to 4 other H2O molecules

• hydrogen bonding explains why water is a cohesive liquid at moderate temperatures

Go back to section on Hydrogen bonds

if you can’t tell which is the donor and

acceptor in the above H bond


9/23

Hydrogen Bonding in

Liquid Water

• in liquid water, less than the maximum of

4 hydrogen bonds occur between molecules

• hydrogen bond formation in liquid water isa dynamic process, each H bond has half-life

of about 10 picoseconds

• despite this dynamic switching of H bonding

partners amongst H2O molecules, most of themolecules are H bonded by 1 or 2 bonds to

other H2O molecules at any given time.

• this is why water is liquid between 0 and 100oC

and is a cohesive liquid.


10/23

The surface of water - one molecule thick - is held in tension by the innumerable

Hydrogen bonds below holding the surface water molecules in place.

Their attraction to each other is greater than their attraction to molecules on insect

leg.

water striders mating

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=hLazGFryvLrQ5M&tbnid=aK-0Ws3zrxO_FM:&ved=0CAUQjRw&url=http://commons.wikimedia.org/wiki/File:Wasserl%C3%A4ufer_bei_der_Paarung_crop.jpg&ei=lHAuUqXwFcji2wXnxYGICQ&bvm=bv.51773540,d.aWc&psig=AFQjCNGY4bP63jejrejmlOaBw7YFrcQcKg&ust=1378861500746922


11/23

Hydrogen Bonding and its effect on surface tension is also why water will bead

while something with less of an ability to form hydrogen bonds will not.

Water 100% Ethanol


12/23

•Water evaporates because by chance H2O

molecules at the surface fleetingly lose all H bonds

to other water molecules, and float off.

•If the air around the water was 100% saturated

with water vapour, this would not happen (no net

change).

•Liquid water changes to gaseous water at 100oC.

the high thermal energy actively breaks the H bonds.

•Sweating is a vital thermoregulatory mechanism used

by mammals.

The thermal energy in your skin breaks the hydrogen

bonds between water molecules in your sweat.

The heat energy is thereby dissipated and the water

molecules fly off into the air (evaporate).

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799


13/23

Solid Water forms a cross-linked lattice

•hydrogen bonding in solid water (ice) is at a maximum

•molecules are orientated so that H bonds form in a straight line (H atom is in

straight line between O atoms) giving maximum strength to each H bond.

•The result is that ice forms an open lattice-like

arrangement.

•Water molecules pack only about 57% of the

density that other molecules would pack.

•Therefore, solid H2O is less dense than liquid H2O.

•The transition from solid to liquid results from

the breaking of 15-25% of the H bonds in ice.


14/23

IV V VI VII

Hydrides of Group VI elements

Thermal Properties of Water

• an odd property of water is that it is liquid at room temperature.

• compared with related molecules, H2O should have a melting pt. of -100oCand boiling pt. of -91oC (see table below).

• water also has a high heat capacity. That is, the amount of

thermal energy required to raise its temperature. The high

heat capacity of water modulates climatic temperatures.

All of these

properties due

to H bonding


15/23

Solvent Properties of Water

Solid NaCl is held together by ionic

interactions between Na+ and Cl- ions

When placed in water, the polar

H2O molecules strip away the ions

by electrostatically interacting withthe charged ions.

Each ion is surrounded by a HYDRATION

shell of water molecules which shields

the ions from interacting with one another.

The ability of water to form a hydration shell via dipole interactions is a measure of

its DIELECTRIC CONSTANT or RELATIVE PERMITTIVITY.


16/23

Water has a very high dielectric constant

and thus solvates ionic substances

(like salts) very well.

The lower the dielectric constant of aliquid, the less able it is to hydrate ions.

Try this at home:

Mix tablespoon of salt into glass of water

and glass of vegetable oil.

Why doesn’t the salt dissolve in oil.

Because oil is APOLAR (it has no dipole)

therefore cannot interact with the saltions and form hydration shell around each.

The interaction between the Na+ and Cl-

ions is stronger than interaction of oil

molecules with ions.

Dielectric Constants of some solvents at 25oC

Solvent Dielectric Constant (D)

Formamide 109

Water 80

Methanol 33

Ethanol 24

Acetone 21

Acetic acid 6

Chloroform 5

Vegetable Oil 3

Benzene 2Hexane 2


17/23

•Water dissolves ionic solids like salt because of the polar nature of H2O molecule and

the charge on the ion.

•Remember, polar means it has a dipole, or separation of charge (one end is positive,

the other end is negative).

•The positive Hydrogen ends of water molecule interact with negative charge of Cl ions.

•The negative Oxygen end of molecules interacts with positive charge of Na ions.


18/23

Water can also dissolve other molecules that are not ionic so long as they are also polar

(e.g. like sugar molecules)

by forming Hydrogen bonds with the molecules.

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=E9p8oFlC_c3K9M&tbnid=3t_B6-ZhP9DHhM:&ved=0CAUQjRw&url=http://cellbiologyolm.stevegallik.org/node/84&ei=nGMuUpKQJeiU2AX0-4CQAw&psig=AFQjCNE99fVoNiogKTEbziffWXvsduykqg&ust=1378858254686478


19/23

Hydrophobic molecules (such as oils) cannot dissolve in water because oils are

neither ionic or polar and water molecules therefore cannot electrostatically

interact with them.

http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=2MFhQN806kiN_M&tbnid=IXMfzjaxNzo-NM:&ved=0CAUQjRw&url=http://www.reasons.org/articles/water-designed-for-life-part-1-of-7&ei=o9QsUoK9Hoae2gWB_YGwCg&psig=AFQjCNFN2AqpSGE0APZFkbcUXprp3_4xcg&ust=1378755837923799http://www.google.ca/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=n73gWNmqZHzDMM&tbnid=grEiUqUqXXRZvM:&ved=0CAUQjRw&url=http://www.sciencenorth.ca/coolscience/science-post.aspx?id=1623&ei=N2YuUuLUKeiy2wW8nYHoAg&psig=AFQjCNF8wxbC2pb25j8ciCn1mGUI1V97hA&ust=1378858913639068


20/23

•Instead, water molecules interact with themselves but the presence of apolar

molecules alter the pattern of H bond interaction.

•A cage-like arrangement of H2O molecules (called a clathrate) forms around the

hydrophobic molecules.

•A clathrate represents a highly ordered arrangement. This is thermodynamically

unfavorable and the hydrophobic molecules coalesce spontaneously

which reduces surface area exposure to the water.

•This is therefore an entropy driven process.

ddi l l l i l bili i


21/23

alkane Sol (g/l) alcohol Sol (g/l)

methane 1C


22/23

Summary of the Unusual properties of water

• Unusually high -melting point-boiling point

-heat capacity

-surface tension

-dielectric constant

• its solid form is less dense than its liquid form

• extensive solvent properties

•

drive coalescence of hydrophobic molecules

All of these unusual properties are due substantially to water’s ability

to form intermolecular Hydrogen Bonds.

heat energy required to

raise 1 g 1oC

Water 4.184 Joules

Copper 0.385 Joules


23/23

Why all this talk about water?

• water is the medium within which life exists

• it is said life is Carbon-based…one might also claim life is Water-based

• it is the preceding qualities of water that determine the structure and

function of all other important cell constituents (DNA, proteins, carbohydrates,lipids, ions, etc.)

• in addition, we will find out that water has additional important properties

and that it also directly participates as a reactant in many chemical reactions

lecture 4 - water

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