Chemistry  I  Notes  Unit  9  Notes:  Solutions,  Molarity,  and  Net  Ionic  Equations  

 Two  Types  of  Mixtures  

1. Homogeneous  Mixtures  2. Heterogeneous  Mixtures  

 A  solution  is  a  homogeneous  mixture  of  two  or  more  substances  in  a  single  phase.        Soluble  is  capable  of  being  dissolved.    In  a  solution,  one  substance  is  soluble  in  another.    For  example,  in  a  solution  composed  of  salt  and  water,  salt  is  soluble  in  water.    Two  parts  of  a  solution:  

1. Solvent—the  dissolving  medium  in  a  solution.  2. Solute—the  substance  dissolved  in  a  solution.  

 In  the  saltwater  solution,  water  is  the  solvent  and  salt  is  the  solute.    Because  it  will  dissolve  so  many  different  types  of  things  and  different  types  of  materials,  water  is  known  as  the  “universal  solvent.”    Solutions  may  exist  as  gases,  liquids,  or  solids.    

 State  of  Solvent   State  of  Solute   State  of  

Solution  Example  

Gas   Gas   Gas   Air    

Liquid   Liquid   Liquid   Antifreeze    

Liquid   Solid   Liquid   Sea  water,  Kool-­‐Aid    

Liquid   Gas   Liquid   Carbonated  Water    

Solid   Solid   Solid   Alloys  (brass,  bronze,  stainless  steel)    

Solid   Gas   Solid   Colored  Glass    

 Heterogeneous  Mixtures—mixtures  that  are  not  the  same  throughout.    Heterogeneous  mixtures  contain  particles  that  are  larger  than  those  of  a  solution.  

1. Suspensions  2. Colloids  

 If  the  particles  in  a  solvent  are  so  large  that  they  settle  out  unless  the  mixture  is  constantly  stirred  or  agitated,  the  mixture  is  called  a  suspension.    Particles  that  are  intermediate  in  size  between  those  in  solutions  and  suspensions  form  mixtures  known  as  colloidal  dispersions,  or  simply  colloids.    

The  Tyndall  effect  occurs  when  light  is  scattered  by  particles  in  a  transparent  medium.    The  Tyndall  effect  can  be  used  to  distinguish  between  a  solution  and  a  colloid.  

 Image  Source:  http://images.slideplayer.com/26/8717050/slides/slide_2.jpg    Types  of  Colloids  

Name   Solvent   Solute   Example  Foam   Water   Gas   Whipped  Cream  

 Emulsion   Liquid   Liquid   Mayonnaise  

 Gel   Liquid   Solid   Gelatin  (like  Jell-­‐o)  

 Aerosol   Gas  or  Liquid   Solid  or  Liquid   Smoke,  Fog  

   Electrolytes  and  Nonelectrolytes    A  substance  that  dissolves  in  water  to  give  a  solution  that  does  not  conduct  electric  current  is    called  a  nonelectrolyte.    This  solution  does  not  conduct  electricity  because  there  are  no  ions  in  the  solution.    

 Image  Source:  http://solutions4.weebly.com/uploads/2/6/3/0/26307730/5450231.jpg?358  

 

A  substance  that  dissolves  in  water  to  give  a  solution  that  conducts  electric  current  is  called  an  electrolyte.    In  an  electrolyte,  the  positive  and  negative  ions  separate  from  each  other  in  solution  and  are  free  to  move,  making  it  possible  for  an  electric  current  to  pass  through  the  solution.    The  strength  with  which  substances  conduct  an  electric  current  is  related  to  their  ability  to  form  ions  in  solution.    Electrolytes  can  be  classified  strong  or  weak  electrolytes.    A  strong  electrolyte  is  any  compound  whose  dilute  aqueous  solutions  conduct  electricity  well.  

•  HCl,  HBr,  HI  •  All  soluble  ionic  compounds  

 A  weak  electrolyte  is  any  compound  whose  dilute  aqueous  solutions  conduct  electricity  poorly  

• Vinegar  • Citric  acid  

 

 Image  Source:  http://scottscience.weebly.com/uploads/5/6/2/8/5628695/395607932.jpg    3  Types  of  Solutions  

1. Unsaturated  Solution  2. Saturated  Solution  3. Supersaturated  Solution  

 Saturated  Versus  Unsaturated  Solutions    

• A  solution  that  contains  the  maximum  amount  of  dissolved  solute  is  described  as  a  saturated  solution.  

• A  solution  that  contains  less  solute  than  a  saturated  solution  under  the  same  conditions  is  an  unsaturated  solution.  

 Supersaturated  Solutions    Supersaturated  solution-­‐-­‐a  solution  that  contains  more  dissolved  solute  than  a  saturated  solution  contains  under  the  same  conditions.    Supersaturated  solutions  of  sugar  and  water  are  commonly  used  to  make  rock  candy.    Solubility  Values    

The  solubility  of  a  substance  is  the  amount  of  that  substance  required  to  form  a  saturated  solution  with  a  specific  amount  of  solvent  at  a  specified  temperature.  

Solute-­‐Solvent  Interactions    

• Solubility  varies  greatly  with  the  type  of  compounds  involved.  • “Like  dissolves  like”  is  a  useful  rule  for  predicting  whether  one  substance  will  dissolve  in  another.  

• Polar  compounds  dissolve  other  polar  compounds.  • Non-­‐polar  compounds  dissolve  other  non-­‐polar  compounds.  

 Water  is  often  referred  to  as  the  “universal  solvent”  because  it  will  dissolve  a  wide  variety  of  substances.    Solute-­‐Solvent  Interactions    Liquids  that  are  not  soluble  in  each  other  are  immiscible.      Liquids  that  dissolve  freely  in  one  another  in  any  proportion  are  miscible.    Materials  that  will  dissolve  in  water  are  hydrophillic.    Materials  that  will  not  dissolve  in  water  are  hydrophobic.    Factors  Affecting  the  Rate  of  Dissolution  (Solution  Formation)  

• Surface  area—larger  surface  area,  faster  rate  of  dissolution.  • Stirring  or  shaking—speeds  up  the  dissolving  process.  • Temperature—higher  temperature,  faster  rate  of  dissolution  

 Henry’s  Law    

• Henry’s  law  states  that  the  solubility  of  a  gas  in  a  liquid  is  directly  proportional  to  the  partial  pressure  of  that  gas  on  the  surface  of  the  liquid.  

 • The  rapid  escape  of  a  gas  from  a  liquid  in  which  it  is  dissolved  is  known  as  effervescence.  

 Effects  of  Temperature  on  Solubility    

• Increasing  the  temperature  usually  decreases  gas  solubility.  • Increasing  the  temperature  usually  increases  solubility  of  solids  in  liquids.  

 Concentration    

• The  concentration  of  a  solution  is  a  measure  of  the  amount  of  solute  in  a  given  amount  of  solvent  or  solution.  

• Concentrated—containing  a  large  amount  of  solute  dissolved  in  solvent.  • The  opposite  of  concentrated  is  dilute.  

 Calculating  Concentration    Molarity    

Molarity  is  the  number  of  moles  of  solute  in  one  liter  of  solution.    The  symbol  for  molarity  is  M.      

𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦   𝑀 =  𝑎𝑚𝑜𝑢𝑛𝑡  𝑜𝑓  𝑠𝑜𝑙𝑢𝑡𝑒  (𝑚𝑜𝑙)𝑣𝑜𝑙𝑢𝑚𝑒  𝑜𝑓  𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛  (𝐿)  

Colligative  Properties  of  Solutions    Properties  of  a  solution  that  depend  on  the  concentration  of  solute  particles  but  not  on  their  identities  are  called  colligative  properties.  

• Freezing  point  depression  • Boiling  point  elevation  • Osmotic  pressure  • Vapor  pressure  lowering  

 Freezing  point  depression,  Δtf,  is  the  difference  between  the  freezing  points  of  a  pure  solvent  and  a  solution  that  is  directly  proportional  to  the  concentration  of  the  solution.    Boiling  point  elevation,  Δtb,  is  the  difference  between  the  boiling  points  of  the  pure  solvent  and  a  solution  that  is  directly  proportional  to  the  concentration  of  the  solution.    Dissociation    

Dissociation  is  separation  of  ions  that  occurs  when  an  ionic  compound  dissolves.    Whether  or  not  a  material  dissociates  when  placed  in  water  can  be  determined  using  the  solubility  rules.  

 

   

General  Solubility  Rules    

 Image  Source:  http://www.chem.umass.edu/~whelan/genchem/whelan/exam_questions_images/Solubility_Rules.gif          

Net  Ionic  Equations    A  net  ionic  equation  includes  only  those  compounds  and  ions  that  undergo  a  chemical  change  in  a  reaction.    Ions  that  do  not  take  part  in  a  chemical  reaction  and  are  unchanged  by  the  reaction  are  spectator  ions.  

 Steps  for  Writing  Net  Ionic  Equations.    

1. Balance  equation.  2. Use  solubility  rules  to  break  soluble  compounds  into  ions.  3. Cross  out  spectator  ions.  4. Rewrite  equation.