Water and Solutions

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Chapter 11

• Water and Solutions

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Water

• The universal solvent. It has the ability to
  dissolve many molecules.
• In living systems these molecules can then be
  transported from one place to another by
  diffusion or by some kind of a circulatory
  system.
• Liquid water has a higher density than ice (solid
  water). Ice thus floats on the surface of liquid
  water. Fish and other organisms can then live
  below the ice in natural bodies of water.
• Water has a very high specific heat. Therefore,
  large bodies of water can moderate temperature by
  absorbing great amounts of heat.
• Water has a very high latent heat. This means
  that a great amount of heat is needed to
  evaporate water. This is what occurs when people
  perspire, we get rid of a lot of heat from our
  bodies, thus enabling us to withstand high
  temperatures.

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Bonds in the Water Molecule

• The H-O bonds in the water molecule are polar
  covalent. The oxygen atom attracts the electrons
  more than the hydrogen atoms because it is more
  electronegative.
• Since the molecule has a V shape
• O Negative end
• H H Positive end
• The oxygen end of the molecule is therefore more
  negative and the hydrogen end of the molecule is
  more positive.
• The molecule is said to be polar and thus
  possesses a dipole (a negative and a positive
  end).

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Hydrogen Bonding

• Due to the dipole moment in the water molecule
  water has many unusual characteristics.
• Hydrogen bonding occurs whenever a H atom is
  bonded to an O, N, or F atom, since these are the
  most electronegative elements. The electrons are
  pulled very close to the O, N, or F atom and
  therefore the O, N, or F atom becomes partially
  negative whereas the H atom bonded to it becomes
  partially positive.
• A dotted line is used to represent a hydrogen
  bond, as opposed to the solid line which is used
  for a single bond.

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Hydrogen Bonding

• The partially negative oxygen of one water
  molecule attracts and is hydrogen bonded to a
  partially positive hydrogen of another water
  molecule. This happens throughout the water
  molecules present in a sample of water.

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Effects of Hydrogen Bonding

• Due to the hydrogen bonding in the water
  molecules water exhibits all the unusual
  properties that it is known for and which
  ultimately permit life on earth to exist.
• Due to the great attraction of water molecules
  for one another the boiling point and melting
  points of water are very high for such a small
  molecule, the molecules like to stay as close to
  one another as possible.
• Other small molecules like water tend to be
  gases at room temperature and do not become
  solids unless the temperature is extremely low,
  much lower than 0oC.

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Hydrogen Bonding in Ice
Hydrogen bonding occurs in solid water (ice), but
it occurs as part of a very organized
crystalline structure in the forms of hexagons.
When the ice melts the molecule collapses from
its highly organized crystal structure to a more
compact structure which permits greater approach
of the hydrogen bonded H and O atoms. This is why
liquid water is more dense than ice.
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Dissolving Process

• There are two types of mixtures
• Heterogeneous mixtures, like oil and vinegar, or
  sand and water, are those where two or more
  distinct layers exist. The two substances are not
  soluble in one another.
• Homogeneous mixtures, like table salt in water or
  sugar in water, are those where there is only one
  layer, since the substances are soluble in one
  another. A homogeneous mixture is a solution.
• A homogeneous mixture that is in the liquid state
  can be recognized because it is clear
  (transparent). It can be colored or colorless.

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Solutions

• A solution is made up of a solute and a solvent.
• The solvent is the substance which is present in
  the higher amount. The solute is present in the
  lesser amount.
• Solutions can be in the gas, liquid or solid
  states, although the most common are in the
  liquid state.
• Solutions in the liquid state can have a gas, a
  liquid, or a solid dissolved in a liquid.
• Air is a gaseous solution made up of 78 N2, 21
  O2, .9 Ar, and other miscellaneous gases.
  Therefore N2 is the solvent.

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Solutions

• Aqueous solutions are those in which the solvent
  is water. Aqueous comes from aqua, the latin word
  for water. The solute can be a solid, a liquid,
  or a gas.
• A solution becomes saturated when the maximum
  amount of solute is dissolved in the solvent.
• A solution which has less than the maximum amount
  of solute dissolved is unsaturated.
• If more than the maximum amount of solute is
  added to a solvent than can dissolve in it at a
  certain temperature, then the excess will settle
  on the bottom of the container (in the case of a
  solid solute) and the solution itself will be
  saturated, since only the maximum that can
  dissolve will be dissolved.
• For example, if the maximum solubility of
  substance A in water is 20 grams of A per 100
  grams of water
• Adding 15 grams of A to 100 grams of
  water-Unsaturated
• Adding 20 grams of A to 100 grams of
  water-Saturated
• Adding 30 grams of A to 100 grams of
  water-Saturated
• Adding 20 grams of A to 150 grams of
  water-Unsaturated
• Adding 10 grams of A to 50 grams of
  water-Saturated

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Degrees of solubility

• Miscible substances
• When there is not limit to how much of a
  solute can dissolve in a solvent.
• Mixtures of gases are always miscible.
• Like dissolves like Two polar substances tend to
  dissolve in one another. Two non polar substances
  tend to dissolve in one another. Water and ethyl
  alcohol are both polar and are miscible in one
  another.

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Degrees of Solubility

• Water is a polar substance, CCl4 (carbon
  tetrachloride) is a non-polar substance, since
  even though the C-Cl bonds are polar the molecule
  has a symmetrical geometry, so it does not have a
  more positive or a more negative end.
• CCl4 will dissolve greases and oils, which are
  non polar.
• Soap consists of a molecule which has a part that
  is polar and a part that is non polar. Oils and
  grease will not dissolve in water alone. However,
  soap can be dissolved in water through its polar
  end and it will in turn dissolve oils and grease
  and is therefore used for cleaning.

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Geometry of CCl4

• C

Cl
Cl
Cl
Cl
The less electronegative atom, the C, is in the
middle symmetrically surrounded by Cl atoms in
the form of a tetrahedron. Therefore the
molecule does not have a more positive or more
negative end and it is nonpolar.
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Dissolving Ionic Compounds in Water

• Many ionic compounds, like NaCl, are soluble in
  water because the Na is attracted to the
  partially negative O of the water molecule. The
  Cl- is attracted to the partially positive H of
  the water molecule.
• This is called hydration, since the ions become
  surrounded by water molecule.

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Dissolving Ionic Compounds in Water

• There is also the effect of the great attraction
  that positive and negative ions in an ionic
  compound have for one another.
• For an ionic compound to be soluble in water the
  attraction of the water molecules and the ions
  has to be greater than that of the ions for one
  another.
• Saturation occurs because there are less water
  molecules available to compete for the ions. It
  is like a tug of war.
• Ionic compounds that have a greater attraction
  for its ions than for water are said to be
  insoluble in water. Usually some miniscule
  amount will dissolve when the compound is
  considered insoluble. Often this will be ionic
  compounds which have ions that have 2 and -2
  charges or higher. Most ionic compounds that
  contain at least one ion with a 1 or -1 charge
  are soluble in water.

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Ionic Compounds dissolved in Water

• NaCl(s) H2O Na(aq) Cl-(aq)
• CaCO3(s) H2O CaCO3(s)

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Concentration of Solutions

• Concentration refers to the amount of solute
  which is dissolved in a given amount of solvent.
• In a qualitative manner when the concentration is
  high the solution is said to be concentrated.
  When the concentration is low the solution is
  said to be dilute.

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Concentration of Solutions

• In a quantitative manner one can express
  concentration of solutions in many ways. One of
  the ways that are used are
• by volume volume of solute x 100
• volume of solution
• An example is rubbing alcohol, which can be, for
  example, 12 by volume. This means that there are
  12 mLof isopropyl alcohol per 100 mL of aqueous
  solution.

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Concentration of Solutions

• by massmass of solute x 100
• mass of solution
• The mass can be in grams or any other mass unit
  in the metric system. It could also be in weight
  units in the English system, like ounces. by
  weight is equivalent to by mass.
• Hydrogen Peroxide, H2O2 is sold as 3 by weight,
  which means that there are 3 ounces of H2O2 per
  100 ounces of aqueous solution.

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2 parts solute x 100 2 100 parts soln
2 mL solute x 100 2 100 mL soln
2 g solute x 100 2 100 g soln
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The by mass of salts in salt water (salinity)
is 35 g salts x 100 3.5 1000 g soln
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Solubility of Ionic Compounds

• Solubility is defined as the maximum amount of a
  solute that will dissolve in a given solvent at a
  specified temperature. When you reach this
  maximum amount of solute the solution is said to
  be saturated.
• For example, adding sugar to tea. If you add too
  much sugar, some will not dissolve because the
  solution becomes saturated.
• For most solutions solubility increases with
  temperature. There are exceptions, however.

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Solubility of Gases

• Gases are the opposite of the majority of ionic
  compounds when it comes to the relationship
  between solubility and temperature.
• The higher the temperature the lower the
  solubility of gases in a solution.
• If you open up a soda can at room temperature
  more of the dissolved carbon dioxide gas will
  escape and the soda will become flat faster than
  if you open it after just taking it out of the
  refrigerator. This is because the carbon dioxide
  gas is less soluble in water at the higher
  temperature.

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Electrolytes

• Electrolytes are substances or solutions which
  conduct electricity. Often these will be aqueous
  solutions.
• In order to conduct electricity there has to be a
  flow of electrons through the solution.
• In order for there to be a flow of electrons
  through a solution there have to be ions in the
  solution.
• The more ions there are the stronger the
  electrolyte will be.

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Only ionic compounds Or acids dissolved in water
will produce ions in solution and electricity
can be then conducted through the Solution.
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Non Electrolytes

• All covalent compounds except for acids are non
  electrolytes, since no ions are present in
  solution.
• Water is a covalent compound, so it is a non
  electrolyte, so are all alcohols and most other
  substances which are liquids or gases at room
  temperature.
• Acids are unique, since they are covalent
  compounds but they ionize when dissolved in
  water. All acids are soluble in water.

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Aqueous Solutions of Acids

• HCl(g) H2O H(aq) Cl-(aq)
  
• hydrogen ion
• This is the same as
• HCl(g) H2O H3O(aq) Cl-(aq)
• hydronium ion
• The hydrogen ion (H) is the same thing as the
  hydronium (H3O) ion.
• The hydrochloric acid, HCl is ionized or
  dissociated in water, so hydrochloric acid is an
  electrolyte. The name hydrochloric acid already
  implies that the acid, HCl or hydrogen chloride,
  is dissolved in water.

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Boiling Point

• Occurs when the pressure of the vapor escaping
  from a liquid is equal to the atmospheric
  pressure.
• The pressure exerted by the vapor is called the
  vapor pressure.
• The normal boiling point of a liquid is the
  boiling point at 1 atm of pressure, which is the
  pressure at sea level.
• The normal boiling point of water is 100oC, or
  212oF.

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Pure Solvent
Solution
The rate of evaporation, and thus the vapor
pressure, is less for a solution than for a
solvent in the pure state. The greater the solute
concentration, the less the vapor pressure.
Therefore, the higher the boiling point.
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Boiling Point of Solutions

• It doesnt matter what the substance is, the more
  particles of solute are present, the more the
  boiling point will be increased, because boiling
  occurs from the surface and the presence of
  solute particles decreases the rate of particles
  escaping to the gas state. They hinder the
  boiling process.
• This is called boiling point elevation

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The more solute particles there are, the higher
the boiling point. For ionic compounds each ion
counts as a particle, so the effect is greater.
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Freezing Point

• Freezing occurs when the kinetic energy of
  molecules has been reduced sufficiently so the
  molecules can come together, forming the crystal
  structure of the solid.
• The freezing point of water at 1 atm is 0oC or
  32oF.
• In aqueous solutions, the presence of solute
  particles interferes with the water molecules as
  they attempt to form the six-sided hexagonal
  crystalline structure.
• The temperature has to be below the freezing
  point of the solvent for the solution to freeze.

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Freezing Point of Seawater

• The freezing point of seawater is lower than that
  of water.
• The ice which forms does not contain any solute
  particles, it is pure water.
• As some ice forms the remaining water contains a
  higher solute concentration, so the freezing
  point is further decreased.
• This is called freezing point depression.

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Practical Uses of Freezing Point Depression and
Boiling Point Elevation

• Salt is spread on snow to lower the freezing
  point of water, which is what snow is made up of.
  CaCl2, calcium chloride, is often used for this
  purpose. This avoids the roads to become slippery
  after the snow melts.
• Ethylene glycol is antifreeze and is added to
  radiator water to lower the freezing point and
  raise the boiling point of water. This is
  protection against temperatures that are too high
  or too low.

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Acids, Bases, and Salts

• Salts are ionic compounds. The hardness of water
  is related to ionic compounds or salts dissolved
  in water.
• The acidity of soils determines how well plants
  grow.
• Acid rain is a by product of industry and
  automobiles. This is harmful for living organisms.

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Acids, bases and salts

• Acids have sour tastes
• Changes litmus paper from blue to red
• Acids react with metals releasing hydrogen gas
• Acids neutralize bases forming water and salt

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Acids, bases and salts

• Bases have bitter taste (caffeine)
• Bases turn litmus paper blue
• Basic solutions feel slippery on skin.
• Bases neutralize acids forming water and salts

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Acids, bases and salts

• Are there any similarities in the lists shown
  before?

Hydronium ion
Acid - any substance that is a proton donor when
dissolved in water.
This is the same as H2O HCl
H(aq) Cl-(aq)
A proton is a hydrogen ion, H
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Bases

• Many bases are ionic compounds which contain
  hydroxide ion (OH-) as the negative ion.
• They produce OH- when dissolved in water
• NaOH Na(aq) OH-(aq)

Proton acceptor A proton is a hydrogen ion (H)
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Acids, bases and salts

• Other bases, like ammonia, NH3, will produce the
  OH- once it is dissolved in water, since a
  hydrogen ion gets transferred from a water
  molecule to the ammonia molecule.

HOH-
Hydroxide ion
Base - Any substance that is a proton acceptor
when dissolved in water.
Proton acceptor
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Acids, bases and salts

• What happens when acids and bases mix?

The H from the acid and the OH- from the base
react to form water
H (aq) OH-(aq) H2O(l)
This is called neutralization
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Strong Acids

• A strong acid is one that ionizes (dissociates)
  100 in aqueous solution.
• HNO3 H2O H(aq) NO3-(aq)
• 100
• Acids can be recognized because the formula
  begins with H.
• Common strong acids include
• HCl hydrochloric acid
• HBr hydrobromic acid
• HI hydroiodic acid
• H2SO4 sulfuric acid
• HNO3 nitric acid

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Weak Acids

• Weak acids only partially ionize (dissociate) in
  aqueous solution. They only produce a small
  percentage of hydrogen ions (H).
• HF H2O H(aq) F-(aq)
• not 100
• A double arrow indicates an equilibrium, so the
  reaction occurs both ways, to the right and to
  the left.
• The arrow to the left is longer because there is
  more of the non ionized acid present than of the
  ions at any given time.

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Weak Acids

• Common weak acids include
• HC2H3O2 acetic acid
• H2CO3 carbonic acid
• HC2H3O2 H2O H(aq) C2H3O2-(aq)
• In the case of acetic acid, only about 1 of
  the acid molecules ionize in aqueous solution.

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Strong Bases

• Strong bases are those that completely (100)
  ionize (dissociate) in aqueous solution.
• NaOH H2O Na(aq) OH-(aq)
• 100
• Common strong bases include
• NaOH sodium hydroxide, common name lye, which
  is used to make soap.
• KOH potassium hydroxide
• Also the other Group IA and Group IIA hydroxides
  except for Be(OH)2 and Mg(OH)2.

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Weak Bases

• Weak bases only partially ionize (dissociate) in
  aqueous solution.
• Mg(OH)2 Mg2(aq) 2OH-(aq)
• not 100
• NH3(aq) H2O NH4(aq)
  OH-(aq)
• not 100

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The pH Scale

• The strength of acids and bases is measured using
  the pH scale.
• A neutral substance, like water, which is neither
  an acid nor a base, has a pH of 7.
• Acidic solutions (acids dissolved in water) have
  a pH lower than 7.
• Basic solutions (bases dissolved in water) have a
  pH higher than 7.

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Salts

• When an acid reacts with a base a salt (ionic
  compound, except bases) is formed. Water is also
  formed
• HCl NaOH NaCl H2O
• acid base salt water
• 2HNO3 Ca(OH)2 Ca(NO3)2 2H2O
• acid base salt
  water
• Neutralization always occurs 100 between an acid
  and a base even if they are weak.

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Importance of Salts

• Salts are important in the diet, since we need
  them as electrolytes and as a source of certain
  elements. These are minerals.
• Plants also require the elements that can be
  provided by certain salts and are added to plants
  as fertilizers. For example, K, N (supplied as
  nitrates, NO3-), and P (supplied as phosphates,
  PO43-).
• Not all salts are soluble in water. Insoluble
  salts is what gives rise to hard water that ends
  up forming rings in your bathtub because when
  mixed with soap insoluble salts are formed.

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Practice Exercises

• p. 294-296 Applying the Concepts
• 1, 2, 3, 4, 6, 7, 9, 10, 12, 13, 14, 15, 16,
  17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27

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Review Chapter 11

• Water-polar molecule, undergoes
• H-bonding.
• Unusual characteristics of water molecule.
• Heterogeneous and Homogeneous (Solutions)
  mixtures.
• Saturated, unsaturated, and miscible solutions.
• Like dissolves like Polar solvents like water
  dissolve polar molecules and ionic compounds. Non
  polar solvents, like Carbon Tetrachloride
  dissolve nonpolar solutes, like grease.
• Soap-Polar and non polar ends, dissolves in water
  and in oils and greases.
• How ionic compounds dissolve in water.
• Concentrated vs. dilute. by mass and by
  volume.

• Solubility-maximum amt. of solute that can
  dissolve in a solvent at a given temperature.
• Solubility of ionic compounds usually increases
  with increasing T, solubility of gases decreases
  with increasing T.
• Electrolytes contain ions in solution. Includes
  ionic compounds dissolved in water and also acids
  dissolved in water. Covalent compounds, including
  water, are non electrolytes.

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Review for Chapter 11 (cont.)

• Ionization of strong and weak acids in aqueous
  solution (100 for strong acids, equilibrium for
  weak acids). (Strong acids are sulfuric, nitric,
  hydrochloric, hydrobromic, and hydroiodic acids.
  The rest are weak acids.)
• Ionization of strong and weak bases in aqueous
  solution.(100 for strong bases, equilibrium for
  weak bases). (Strong bases are the group IA
  (alkali metals) combined with hydroxide ion.
• What is vapor pressure-The pressure exerted by
  the vapor that evaporates from a liquid on the
  surface of the liquid. Vapor Pressure increases
  with temperature.
• What is boiling point. The temperature at which
  the vapor pressure of a liquid is equal to the
  atmospheric pressure.
• Boiling point elevation and freezing point
  depression in solutions. (The boiling point
  increases, the freezing point decreases).

• Properties of acids and bases.
• How ammonia, NH3 is a base because it reacts with
  water to form NH4 and OH- in aqueous solution.
• Neutralization reactions Acid Base react to
  produce salt (ionic compound) plus water.
• The pH scale 0-14. 7 is neutral, like water.
  Less than7 is acidic, higher than 7 is basic.
• The importance of salts They provide
  electrolytes to living organisms, including
  plants, animals and humans and they are a source
  of some essential elements.