Chemistry  I  Notes  Unit  6:  Chemical  Reactions  

 A  chemical  reaction—process  by  which  substances  are  changed  into  different  substances.    

• Reactants—substances  present  at  the  beginning  of    a  chemical  reaction  • Products—substances  present  after  a  chemical  reaction.    

 Law  of  conservation  of  mass—total  mass  of  reactants  must  equal  the  total  mass  of  products  for  a  chemical  reaction.    A  chemical  equation  represents,  with  symbols  and  formulas,  the  identities  and  amounts  reactants  and  products  in  a  chemical  reaction.  

 

   2  Types  of  Chemical  Reactions  

1. Endothermic—chemical  reaction  in  which  energy  is  added  (also  called  an  endergonic  reaction).  2. Exothermic—chemical  reaction  in  which  energy  is  released  (also  called  an  exergonic  reaction).  

 Indications  of  a  Chemical  Reaction    Certain  easily  observed  changes  usually  indicate  that  a  chemical  reaction  has  occurred.    1.   Evolution  of  energy  as  heat  and  light  2.   Production  of  a  gas  3.   Formation  of  a  precipitate.  

• A  solid  that  is  produced  as  a  result  of  a  chemical  reaction  in  solution  and  that  separates  from  the  solution  is  known  as  a  precipitate.  

4.   Color  change    7  Diatomic  Elements  N2  H2  O2  I2  Br2  Cl2  F2    

A  word  equation  is  an  equation  in  which  the  reactants  and  products  in  a  chemical  reaction  are  represented  by  words.      

example:      methane  +  oxygen    à    carbon  dioxide  +  water    A  formula  equation  represents  the  reactants  and  products  of  a  chemical  reaction  by  their  symbols  or  formulas.      example:  The  formula  equation  for  the  reaction  of  methane  and    oxygen  is    

CH4(g)  +  O2(g)      à        CO2(g)  +  H2O(g)          (not  balanced)    Symbols  found  in  a  chemical  equation:    

   Image  Source:  http://image.slidesharecdn.com/unit1ale-­‐140911145133-­‐phpapp02/95/unit-­‐1-­‐chemical-­‐reactions-­‐in-­‐our-­‐surroundings-­‐8-­‐638.jpg?cb=1410447261    Balancing  Chemical  Equations  

1. Write  a  word  equation.    water    à      hydrogen  +  oxygen    

2. Write  a  formula  equation  from  the  word  equation.  H2O(l)  à   H2(g)  +  O2(g)  (not  balanced)  

 3. Balance  the  formula  equation.  

A. Balance  the  different  types  of  atoms  one  at  a  time.  B. First  balance  the  atoms  of  elements  that  are  combined  and  that  appear  only  once  on  each  side  

of  the  equation.  C. Balance  H  atoms  and  O  atoms  last  D. Count  atoms  to  be  sure  that  the  equation  is  balanced.  

 

5  Major  Types  of  Chemical  Reactions:    a. Synthesis  (also  known  as  a  metathesis  or  combination  reaction)  b. Decomposition  c. Single-­‐displacement  (single-­‐replacement)  d. Double-­‐displacement  (double-­‐replacement)  e. Combustion  

 Other  Types  of  Reactions  

f. Oxidation/reduction  (redox)  g. Acid/base  

 *An  equation  may  be  classified  more  than  one  way.  

 Synthesis  (Composition/Metathesis)  Reactions  

• In  a  synthesis  reaction,  also  called  a  composition  reaction,  two  or  more  substances  combine  to  form  a  new  compound.  

• In  a  synthesis  reaction,  simple  compounds  make  more  complex  compounds.    General  equation:  

A  +  X     à      AX  Two  or  more  reactants  à  one  product  

 Predicting  the  Products  of  a  Chemical  Reaction  Synthesis  (Composition)  Reactions  

• element  +  oxygen  à  oxide  • element  +  sulfur  à  sulfide  • element  +  halogen  à  ionic  compound  • metal  oxides  +  water  à  metal  hydroxides  

 Decomposition  Reactions  

• In  a  decomposition  reaction,  a  single  compound  produces  two  or  more  simpler  substances.    • Opposite  of  synthesis  reaction  • More  complex  substances  make  less  complex  ones.  

 General  equation:  

AB  à  A  +  B  One  reactant  à  two  or  more  products  

 The  decomposition  of  a  substance  by  an  electric  current  is  called  electrolysis.    Decomposition  Reaction    

• Binary  compound  à  individual  element  • metal  carbonates  à  metal  oxide  +  carbon  dioxide  • metal  chlorate  à  metal  chloride  +  oxygen.  • Acid  à  nonmetal  oxides  +  water  

         

Single-­‐Displacement  Reactions    

• In  a  single-­‐displacement  reaction,  also  known  as  a  single-­‐replacement  reaction,  one  element  replaces  a  similar  element  in  a  compound.  

 General  Equation:  

 A  +  BX  à  AX  +  B  

Element  +  compound  à  new  compound  +  new  element    Single  Replacement  (Single  Displacement)    

• Replacement  of  a  metal  in  a  compound  by  another  metal  (the  two  metals  switch  places).  • Replacement  of  hydrogen  in  water  by  a  metal  produces  a  metal  hydroxide  and  hydrogen.  • Replacement  of  Hydrogen  in  an  acid  by  a  metal  produces  a  metal  compound  and  hydrogen  • Replacement  of  halogens  by  another  halogen  (the  two  halogens  change  places).  

 Double-­‐Displacement  Reactions    

• In  double-­‐displacement  reactions,  the  ions  of  two  compounds  exchange  places  in  an  aqueous  solution  to  form  two  new  compounds.  

• Two  atoms  trade  places    

General  Equation:  AX  +  BY  àAY  +  BX  

compound  +  compound  à  2  new  compounds    Double  Replacement  (Double  Displacement)    

• Formation  of  a  precipitate  (ions  of  compounds  change  places)  • Formation  of  a  gas  (ions  change  places,  an  insoluble  gas  is  formed  that  bubbles  out)  • Formation  of  water  (also  called  an  acid-­‐base  reaction)  

 Combustion  Reactions    

• In  a  combustion  reaction,  a  substance  combines  with  oxygen,  releasing  a  large  amount  of  energy  in  the  form  of  light  and  heat.  

 example:  combustion  of  propane  

C3H8(g)  +  5O2(g)    à  3CO2(g)  +  4H2O(g)  Substance  +  oxygen  à  CO2  +  H2O  

 Oxidation-­‐Reduction  (Redox)  Reactions    Redox  occurs  in  many  single  displacement  reactions.        The  redox  component  of  these  types  of  reactions  is  the  change  of  charge  on  certain  atoms.      Reduction—gain  of  electrons  Oxidation—loss  of  electrons    

Oxidation  and  reduction  always  occur  in  pairs.    Example:    

Fe  +  CuSO4  →  FeSO4  +  Cu    

• In  this  reaction,  Fe  goes  from  not  having  a  charge  to  having  a  +2  charge  (as  part  of  a  compound)  • Cu  goes  from  having  a  +2  charge  to  not  having  a  charge  

 The  loss  of  1  or  more  electrons  from  a  reactant  is  called  oxidation.    When  a  reactant  is  oxidized,  it  loses  electrons  and,  consequently,  energy.    

• The  gain  of  1  or  more  electrons  is  reduction.    When  a  reactant  is  reduced,  it  gains  electrons  and,  therefore  energy.  

 • The  reactions  of  cellular  respiration  are  oxidation-­‐reduction  (redox)  reactions  because  electrons  

are  transferred  from  one  reactant  to  another  during  the  electron  transport  chain.    REMEMBER—“OILRIG”—Oxidation  is  Loss,  Reduction  is  Gain  

 Acid/Base  Reactions    An  acid–base  reaction  is  a  chemical  reaction  that  occurs  between  an  acid  and  a  base.      Acid-­‐Base  reactions  are  often  double  displacement  reactions.    Some  acid-­‐base  reactions  are  neutralization  reactions.    Most  acid-­‐base  reactions  have  the  general  form:  

Acid  +  Base  à  Ionic  Compound  +  H2O    The  goal  of  any  chemical  reaction  is  to  create  a  more  stable  arrangement  of  matter.        Through  chemical  reactions  elements  and  compounds  become  more  stable  than  they  were  before  a  chemical  reaction.                                      

Summary of Major Reaction Types

        5. Combustion

 

   

Image  Source:  https://lh6.googleusercontent.com/LASySxSAAUVRxeaAuh2vQXp0da1Is88RHnWTV5gX3mCcX7MNYMvnnXJeet_GPmtNPfbRlSt0nlTih-­‐_SF9oyNZMQJ5lhEgyS3krLsIknUe4zBod54C_DBS8fTXiLyejryA  Image  Source:  http://www.iun.edu/~cpanhd/C101webnotes/chemical%20reactions/images/organicb.jpg    Catalysts    Many  reactions  occur  with  the  use  of  a  catalyst.    Catalyst—substance  that  lowers  the  amount  of  activation  energy  required  for  a  reaction  to  occur.        

• A  catalyst  speeds  up  the  chemical  reaction,  but  is  not  a  part  of  the  chemical  reaction.    

   

Enzymes  are  biological  catalysts.              

Chemical  Kinetics        Chemical  Kinetics—branch  of  chemistry  that  deals  with  the  rates  of  chemical  reactions  and  how  the  reactions  occur.    4  Factors  that  Affect  Reaction  Rates  

1. The  number  of  collisions  between  the  substances  that  are  reacting.    Materials  with  more  surface  area  will  have  a  higher  number  of  collisions  (powders  vs.  big  chunks).    

2. Higher  temperature  is  accompanied  by  an  increase  in  the  reaction  rate.    3. Medium  in  which  the  reaction  occurs.  (Does  the  reaction  occur  in  water,  are  the  materials  polar  or  

nonpolar,  are  the  materials  solids,  liquids  or  gases).  4. Presence  of  a  catalyst.  

 Equilibrium  Reactions    Reversible  Reaction—a  chemical  reaction  in  which  the  products  can  react  to  re-­‐form  the  reactants.    Chemical  Equilibrium—occurs  when  the  rate  of  a  forward  reaction  equals  the  rate  of  its  reverse  reaction  and  the  concentrations  of  products  and  reactants  remain  unchanged.  

 

   LeChatelier’s  Principle    If  a  chemical  system  at  equilibrium  experiences  a  change  in  concentration,  temperature,  volume,  or  partial  pressure,  then  the  equilibrium  shifts  to  counteract  the  imposed  change  and  a  new  equilibrium  is  established.      4  Factors  that  affect  Equilibrium     1.  Pressure     2.  Volume     3.  Temperature     4.  Concentration    An  activity  series—list  of  elements  organized  according  to  the  ease  with  which  the  elements  undergo  chemical  reactions.    The  ability  of  an  element  to  react  is  referred  to  as  the  element’s  activity.    

• The  most-­‐active  element  is  placed  at  the  top.  • Used  to  help  predict  whether  certain  chemical  reactions  will  occur.  

 See  Activity  Series  chart  on  next  page.    

 Image  Source:    http://crescentok.com/staff/jaskew/isr/tigerchem/reaction/series.gif