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Vocabulary1.  Potential  energy  in  the  reactant2.  Potential  energy  in  the  product3.  Heat  of  reaction4.  Catalyst5.  Activation  energy6.  Activated  complex7.  Reactants  8.  Products

Homework  #  10.2Potential  Energy  Diagrams  WS Aim:  How  can  we  represent  the  relationship  

between    the    potential  energy  of  the  reactants  and  product  of  a  chemical  reaction?  

H2O2

Pb(NO3)2 +  KI                                                      PbI2 +  KNO3

How  do  you  prepare  a  chicken  soup?

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Reactants Product

Chicken  Soup

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Heat  of  reaction  is  the  amount  of  heat  given  off  or  absorbed  in  a  chemical  reaction.

Reactants                                                  Products2H2 +    O2 à 2  H2O      +      571.6  KJ

∆H            = HProducts          -­ HReactants

∆H            = PEP -­ PER

Open  your  reference  table  I,  look  at  the  first  reaction    and  what  information    can  you    get  from  

there?

Type  of  chemical  reaction:  Combustion

What  is  an  exothermic  reaction?

• Exothermic  reaction:  release  heat

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Table  I How  do  you  write  a    thermochemical    equation?

CH4(g)  +2O2(g)  à CO2(g)  +  2  H2O  (l)    +  890  KJ

Exothermic  reaction

ΔH  is  written  as  a  product

How  do  you  write  the  thermochemical    equation?

H2(g) +    I2 (g)        +  53.0  KJ  à 2  HI(g)      

ΔH  is  written  as  a  reactant

53.0  KJ        +  H2(g) +    I2 (g)  à 2  HI(g)      

Potential  and  Kinetic  Energy  Changes  during  a  Collision

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KE                        is  Converted  to                      PE

KE PE

KE PE 2.    Potential  Energy  Diagrams

Energy  released  or  absorbed  by  a  chemical  reaction  can  be  represented  by  a  potential  energy  diagram.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf

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3.    Exothermic  Reactions:  See  table  I

CH4(g)  +  2O2(g)    à CO2(g)  +  2H2O(l)  +  890.4  KJ

Products:  • decrease  in  potential  energy.• increase  in  kinetic  energy• released  heat• ∆  H    =  negative  

Where  do  compounds  store  energy?

Potential  and  Kinetic  Energy  Changes  during  a  Collision

Endothermic

Reactants          Activated  Complex   Products

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Reactants

Activated  Complex

ProductsPE

1.  Is  this  an  endothermic  or  an  exothermic  reaction?  

2.  What  observable  evidence  indicates  that  a  chemical  reaction  is  taking  place?

14KMnO4 +  4C3H5(OH)3 à 7K2CO3 +  7Mn2O3 +      5CO2 +  16H2O

H2OKNO3(s)  +  34.89  KJ                                K+(aq)  +  NO3

-­‐ (aq)  

Products:  • increase  in  potential  energy.• decrease  in  kinetic  energy:  cold• absorbed  heat• ∆  H    =  positive

Ammonium  thiocyanate  is  mixed  with  barium  hydroxide.

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A  drop  of  water  is  placed  on  a  block  of  wood. The  bottom  of  the  beaker  becomes  cold  enough  to  freeze  the  water  and  stick  to  the  wood.

1.  Is  this  an  endothermic  or  an  exothermic  reaction?  Support  your  answer.

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Potential  and  Kinetic  Energy  Changes  during  a  Collision

Reactants

Activated  Complex

Products

Many  cold  packs  used  for  treating  sprains  and  aches  are  really  just  a  bag  of  ammonium  nitrate  and  water.  When  an  athlete  or  sportsperson  hurts  themselves  they  mix  the  contents  of  the  bag  together  and  put  it  on  the  painful  part  of  their  body.  The  endothermic  reaction  takes  heat  out  of  their  body  …...  this  'freezes'  the  painful  area.

The  smallest  amount  of  energy  needed  to  start  a  reaction:  spark,  flame,  bunsen  burner,  and  high  temperature. Eac

Eac

Forward  Reaction   Reverse  Reaction

A B

Eac  =  activation  energy

PE

F.R.

R.R.

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PEPPER  • Exothermic

H  =  PEP  -­‐ PER  =  PEP<PERA<B

• EndothermicH  =  PEP  -­‐ PER=  PEP>PER  A>B

6.  Activated  Complex

• The  activated  complex  is  the  short-­‐lived  and  unstable  intermediate  species  located  at  the  highest  of  the  activation  energy.

Reactants

Activated  Complex

Products

7.  Catalyst

• Speed  up  the  reaction  and  lowers  the  activation  energy.

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Eac

Catalyzed  Reaction

Eac

Eac    :    ACTIVATION    ENERGY

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• Ba(OH2)  .  8  H2O  (s)    +    2  NH4SCN  (s)    =      Ba(SCN)  (aq)    +  2  NH3  (aq)    +  10  H2O

• When  the  solids  become  liquid,  set  the  flask  on  a  moist  piece  of  wood.    Hold  until  the  water  freezes  and  flask  sticks  to  wood.    Lift  the  flask  to  show  its  hold  on  the  wood.