Liquids and Aqueous Solutions

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Liquids andAqueous Solutions
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Objectives

• Be able to explain the conditions necessary for a
  substance to be a liquid.
• Understand the concept of vapor pressure.
• Be able to explain the various ways that a
  substance can be made to vaporize.

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Liquids
Liquids are not common because they must meet
two conditions at the same time

• they are molecular compounds with
  intermolecular
• bonds that have just the right strength
• (too weak gas, too strong solid)

• the temperature must be just right
• (too warm gas, too cool solid)

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Changes of State
condensation gas ? liquid
vaporization liquid ? gas
evaporation particles w/ most KE escape liquid
remaining liquid is cooler
vapor pressure (VP) pressure exerted by the
vapor of a liquid
as T ? VP ?
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Boiling
boiling rapid vaporization that occurs when VP
AP (atmospheric pressure)

• two ways to boil
• heat water (increase VP)
• decrease AP

normal boiling point temperature at which VP of
liquid 101.3 kPa or 760 mm Hg
Boiling point decreases by 1oC/1000 feet
elevation
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Vaporization
vaporization (liquid to gas)
evaporation (cools) boiling (AP VP)
heat (increase VP to AP) reduce AP to VP
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Objectives

• Be able to correctly use terminology related to
  aqueous solutions.
• Be able to explain the process of dissolving.
• Be able to use a solubility rules table.

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Aqueous Solutions
aqueous solution homogeneous mixture w/ water
mixture a combination of substances (different
formulas)
solute dissolved material
solvent dissolving liquid (water)
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The Process of Dissolving
solvation/hydration polar water molecules are
attracted to ions they surround both anions and
cations, break the ionic bonds, and separate
the ions by moving them into the solution

-
-
-

-


lower charged ions (1, 1-) usually dissolve
better (due to weaker ionic bonds)


hydrated ions
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Solubility Table
solubility the ability to dissolve
soluble will dissolve
insoluble doesnt dissolve (very much)
aq aqueous (soluble) s solid, (insoluble)
test sodium chloride, barium sulfate,
calcium hydroxide
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Objectives

• Be able to determine if a compound is a strong,
  weak, or non-electrolyte.
• Be able to write a dissociation equation.

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Electrolytes

• electrolyte a compound that conducts an electric
  current when aqueous or molten (forms ions)
• all ionic compounds are electrolytes
• strong electrolyte very soluble and conductive
  (like NaCl or K2SO4)
• weak electrolyte not very soluble, weakly
  conductive (like Fe2O3 or CaCO3)
• non-electrolyte does not conduct (most molecular
  compounds like sugars, alcohols C12H22O11 ,
  CH3CH2OH)
• molecular acids are electrolytes (like HC2H3O2)

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Dissociation Equations

• dissociation the separation of ions during
  hydration (must be soluble)
• NaCl(s) ? Na(aq) Cl-(aq)
• Fe(NO3)3(s) ? Fe3(aq) 3NO3-(aq)
• BaCO3(s) ? n/r (no reaction if insoluble)

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Objectives

• Be able to predict whether or not a precipitate
  will form when two aqueous solutions are
  combined.
• Be able to identify spectator ions.
• Be able to write a net ionic equation for a
  double replacement reaction.

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Double Replacement Reactions
double replacement reaction two aqueous
solutions react to form a precipitate (or water
or a gas)
AX(aq) BY(aq) ? BX(s) AY(aq)
one product must be insoluble (makes a solid
precipitate)
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Double Replacement
BaCO3(s) 2NaCl (aq)

• Na2CO3(aq) BaCl2(aq) ?

• NaCl(aq) K2SO4(aq) ? ?
• No precipitate, no reaction! Ions just mixed
  together.

Sr3(PO4)2 (s) NaNO3 (aq)

• Na3PO4(aq) Sr(NO3)2(aq) ?

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Net Ionic Equations

• Spectator ions dont take part in reactions.
• K2CrO4(aq) Ba(NO3)2(aq) ?
• BaCrO4 is the ppt, K and NO3- are spectator
  ions.
• net ionic equation only shows ions involved in
  reaction.
• Determine the precipitate, ignore the spectator
  ions, and then write the net ionic equation.
• Ba2(aq) CrO42-(aq) ? ?
• NaOH(aq) CaCl2(aq) ? ?
• K3PO4(aq) and Ca(NO3)2(aq) ? ?

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Solutions and Solubility
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Objectives

• Be able to identify the factors that affect the
  rate at which a solute dissolves in a solvent.
• Be able to interpret and use a solubility curve
  graph.

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Solution Formation

• What 3 factors affect how quickly a solute
  dissolves?
• stirring (increases rate)
• temperature (more KE, higher rate)
• particle size (smaller pieces, higher rate)

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Solubility Curve

• solubility the maximum amount of solute that can
  dissolve in a given amount of solvent at a
  certain temperature
• g solute / 100 g H2O
• What is the solubility of NaNO3 at 40oC?
• 106 g NaNO3/100 g H2O (_at_ 40oC)

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Solubility Curve

• saturated solution contains maximum amount of
  solute at that temperature
• If 20 g KClO3 is dissolved in 100 g H2O at 50oC,
  is the solution saturated?
• Yes, it is on the line.

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Solubility Curve

• If 78 g of KNO3 is dissolved in 100 g H2O at
  60oC, is the solution saturated?
• No, it is below the line.
• unsaturated more solute could be dissolved
  (below line)
• About 27 g could still be added or it could be
  cooled by about 13oC.

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Objectives

• Understand how temperature and pressure affect
  the solubility of gases.
• Be able to explain the process of effervescence.

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Solubility of Gases

• most solids increase solubility as T ?
• gases (like NH3 and SO2) decrease solubility as T
  ?
• warmer water dissolves less gas (it evaporates
  away)

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Solubility of Oxygen

• cold water dissolves more O2
• dams can affect water temperature and fisheries

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Solubility and Pressure

• pressure and gas solubility are directly
  proportional (P? S?, P? S?)
• effervescence a rapid decrease in pressure
  reduces the solubility and allows gas (CO2) to
  escape

CO2 pressure equals several atmospheres, CO2
dissolved
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Objectives

• Understand what is meant by the terms
  concentrated and dilute.
• Understand the concept of concentration and be
  able to correctly use the units of molarity.
• Be able to calculate the amount of solute
  required to make a solution of specified
  concentration.
• Be able to make a solution of specified
  concentration.

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Concentration

• dilute small amount of solute
• concentrated large amount of solute
• concentration a measure of the amount of solute
  that is dissolved in a given amount of solution
• molarity moles solute / liters of solution

5.0 M is five molar
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Concentration

• mass of solute Molar mass Volume
  Concentration
• m MVC g (g/mol)(L)(mol/L)
• How many grams of HCl are dissolved in 2.5 L of
  12 M concentrated solution?
• What is the concentration of a solution
  containing 3.66 g NaNO3 dissolved in 125 mL of
  solution?

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Making a Solution

• calculate mass of solute needed (m MVC)
• measure out solute
• rinse, then half-fill the volumetric flask with
  distilled water
• gradually add solute to flask, swirling as needed
• add distilled water to the etched mark (DO NOT GO
  OVER THE LINEuse a squirt bottle)
• Animation

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Diluting a Solution

• A diluted solution has a lower concentration (or
  molarity) due to the addition of water.
• CcVc CdVd
• moles concentrated moles dilute
• How many milliliters of 12.0 M HCl are needed to
  make 100.0 mL of 1.0 M HCl?
• Vc CdVd / Cc

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Diluting a Solution

• Calculate volume of concentrated solution.
• Use a pipet to collect the concentrated amount.
• Dispense concentrated amount into a rinsed and
  half-filled graduated cylinder.
• Add distilled water to the graduation mark.

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Hydrates

• hydrate ionic compound with H2O bonded within
  the crystal
• many hydrates are useful!
• Plaster of Paris water gypsum
• (CaSO4)2H2O 3H2O?2CaSO42H2O

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Hydrates

• anhydrous without water
• desiccants tend to absorb water from air
• deliquescent compounds absorb so much water they
  form a solution