Chemical Reactions

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					Controlling the Rate of
Chemical Reactions or
 Going Into Solution
A chemical reaction is a process in which
 the physical and chemical properties of
 the reactants change as new substances
  (products) with different physical and
     chemical properties are formed.
Observing a Chemical
     Reaction
 Magnesium and oxygen combine to form magnesium oxide.
  Characteristics of a Chemical
            Reaction
• Chemical reactions always involve a change
  in energy.
• Energy is either absorbed (endothermic
  reaction) or released (exothermic reaction)
  during a chemical reaction.
   Energy & Reactions
• The rate of a reaction is a measure of how
  quickly reactants turn into products.
• Chemical reactions require energy to break
  the chemical bonds of the reactants so that
  the atoms can begin to form the new
  chemical bonds of the products.
• The activation energy is the minimum
  amount of energy needed to start a chemical
  reaction.
   All chemical reactions need a
certain amount of activation energy
          to get started.
Exothermic Reaction

                                          Activation
                                          energy

     E
     n
     e
     r                                                 Heat
     g                                                 released
     y



         Reactants                                        Products
                                 Time

In this type of reaction, the energy of the products is less than the
                       energy of the reactants.
Endothermic Reaction

                                          Activation
                                          energy

     E
     n
                                                Heat absorbed
     e
     r
     g
     y



         Reactants                                      Products
                                Time

In this type of reaction, the energy of the products is greater than
                    the energy of the reactants.
 Factors that Increase the
Rate of a Chemical Reaction
     Causes

 Surface Area
 

                        Effect
 Temperature
 
                    Increased Rate
                     of Reaction
Concentration




Use of a Catalyst
             #1 Surface Area
The surface area of a reactant influences the rate of reaction by -


 • allowing more particles of a substance to
   react
 • The more particles are exposed, the faster the
   reaction will happen.
             Surface Area




To dissolve the particles at the center of the object you must
   first dissolve each of the layers around the center. This
    process will take a long amount of time to complete.
              Surface Area
If your were to break apart the substance into smaller pieces,
              the reaction would take less time.




Less surface area =                     Greater surface area =
slower reaction time                       faster reaction time
           #2 Temperature
When you heat a substance, its particles move faster. Faster moving
particles increase the reaction rate in two ways:


  • particles come in contact more often, which
    increases the chance of a reaction
  • faster moving particles get over the AE
    “hump” easier due to increased energy
  • decreasing the temperature slows down
    reaction rates
Changing Temperature
                 +       -


                                                        +       -
                                 +       -
+       -




                     +       -                      +       -




                                     +       -
    +       -




               Lower                           Higher
            temperature                      temperature
      #3 Concentration
Concentration is the amount of a substance in a given volume.




         Less                                More
     Concentrated                         Concentrated
  Increasing the concentration of
reactants supplies more particles to
               react.
     Other Factors that Affect
         Reaction Rates
• A catalyst is a material that increases the rate
  of a reaction by lowering the activation
  energy.
• An inhibitor is a material used to decrease
  the rate of a chemical reaction.

				
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