Chapter 8 � Chemical Equations and Reactions by rz4zjFQ

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									Chapters 9 – Chemical Reactions and
             Equations
              Goals for Chapter 9

• Describe, compose, and balance chemical equations

• Recognize the main types of chemical reactions – predict products

• Use Activity series to predict reactions

• Understand the solubility chart
   Describing Chemical Reactions
• OK, so what is a chemical reaction?
   – What are the main parts of a chemical reaction?
                     Reactants and products
• Define chemical equation:
 Chemical equations use symbols and formulas that represent the
   identities and relative amounts (moles and molecules) of the
           reactants and products in a chemical reaction

   C6H14 (l) +   O2 (g)            CO2 (g) + H2O (l) (not balanced)
                          What type of reaction is this?
• Law of Conservation of mass tells us:
        The total mass of reactants MUST equal the total mass of
                products for any given chemical reaction

                   What can’t a chemical equation tell us?
LCM
   How can we tell a reaction has taken place?

• heat or light
   – can also happen with
     physical changes
• gas bubbles
   – means a gas is being
     created as product
• precipitate
   – solid is being created
• color change
        Chemical Equations

    The equation must represent known facts

        It is vital you know what you are working with!


 The equation must contain the correct formulas!

  Chapter 7 is quite important – naming and writing formulas


The law of conservation of mass must be satisfied
                         “balancing act”
     Word and Formula Equations
The coefficient:


  C6H14 (l) + O2 (g)            CO2 (g) + H2O (l) (not balanced)

  C6H14 (l) + 9.5O2 (g)            6CO2 (g) + 7H2O (l) (almost)

 2C6H14 (l) + 19O2 (g)            12CO2 (g) + 14H2O (l) (balanced)

                   What’s the significance of the arrow?
      Lettuce get the formulas right first!

solid zinc sulfide + oxygen gas yields solid zinc oxide and sulfur dioxide gas


              ZnS (s) + O2 (g)                 ZnO (s) + SO2 (g)

                              The reverse approach

              Zn (s) + HCl (aq)                   ZnCl2 (aq) + H2 (g)


                     solid zinc + hydrochloric acid yields
                   aqueous zinc chloride and hydrogen gas

           Hydrochloric acid? We need to go back to page 248-249 for a moment.
                 You need to know all of the common acids in Figure 7-27.
Symbols used in Chemical Equations

                                    “Yields”

                                   Reversible

     (s), (g), (aq)        Solid, gas, aqueous solution


          or
                           precipitate or gas formation

           D                           heat

   x atm or pressure   Reaction proceeds at defined pressure
                          or above atmospheric pressure
        x oC                A temperature at which the
                               reaction is carried out
          cat
                               A catalyst is employed
Balancing Chemical Equations
What is the point? What are the rules?

    1. Identify the names of the reactants and products

Potassium iodide + lead (II) nitrate yields lead (II) iodide + potassium nitrate

               What can you tell about this reaction?

                  2. Write the formula equation

             Pb(NO3)2 (aq) + KI (aq)      PbI2 (s) + KNO3 (aq)


                                EXAMPLES
                     Let’s try some
  Given the following word equations suggest the chemical equation

                  Water yields hydrogen and oxygen

                   H2O   (l)     H2   (g)   +     O2 (g)


     Zinc + hydrochloric acid yields zinc chloride and hydrogen

             Zn (s) + HCl (aq)              ZnCl2 (aq)      + H2 (g)

Nitric acid + magnesium hydroxide yields magnesium nitrate and water

       HNO3 (aq) + Mg(OH)2 (s)                  Mg(NO3)2 (aq)     + H2O (l)


 Silver nitrate + sodium chloride yields silver chloride and sodium nitrate

             AgNO3 (aq) + NaCl (s)                 AgCl    (s)   + NaNO3 (aq)
    Balancing the equations – Rules Continued
3. Balance the formulas according to the law of conservation of mass
         Use the coefficients only! DO NOT CHANGE SUBSCRIPTS


   1. Balance the atoms one at a time
   2. Balance the atoms that are combined and appear once on each side
      of the equation
   3. Balance polyatomic ions that appear on both sides of the
      equation as single units
   4. Lastly, balance H and O atoms
   5. Simplify if you can – do not leave fractions.

4. Count the atoms and be sure the equation is balanced

               Yes – some of these will be trial and error – but if you
         follow the rules you will be able to quickly balance the equations.
                Try some!
               H2O   (l)   H2   (g)     +       O2 (g)


              2H2O   (l)   2H2    (g)       +    O2 (g)


       Zn (s) + HCl (aq)              ZnCl2 (aq)         + H2 (g)

      Zn (s) + 2HCl (aq)              ZnCl2 (aq)         + H2 (g)


 HNO3 (aq) + Mg(OH)2 (s)                Mg(NO3)2 (aq)         + H2O (l)

2HNO3 (aq) + Mg(OH)2 (s)                Mg(NO3)2 (aq)         + 2H2O (l)


  AgNO3 (aq) + NaCl (s)                 AgCl    (s)   + NaNO3 (aq)

  AgNO3 (aq) + NaCl (s)                 AgCl    (s)   + NaNO3 (aq)
                                     More?
  Al2(SO4)3    (aq)       + Ca(OH)2      (aq)       Al(OH)3   (s)          +   CaSO4 (s)

 Al2(SO4)3   (aq)   + 3Ca(OH)2          (aq)       2Al(OH)3     (s)        +   3CaSO4 (s)



                          Fe (s) + Cl2 (g)            FeCl3    (s)


                      2Fe (s) + 3Cl2 (g)               2FeCl3        (s)




 NH4Cl (aq) + Ca(OH)2 (aq)                      CaCl2 (aq)    + NH3 (g) + H2O (l)

2NH4Cl (aq) + Ca(OH)2 (aq)                      CaCl2 (aq)    + 2NH3 (g) + 2H2O (l)



             C23H48        (l)   + O2 (g)          CO2 (g)    + H2O (l)

        C23H48      (l)   + 35O2 (g)             23CO2 (g)      + 24H2O (l)

                                    Let’s try a lot more
       Types of Chemical Reactions
• Recognize the 5 main types of reactions:
   – Example: Synthesis reaction between an element and
     oxygen:


                  Mg (s) +    O2 (g)     MgO (s)


                             Balanced?


                 2Mg (s) +    O2 (g)      2MgO (s)



                     Let’s go through the rest.
         #1 - Synthesis Reactions
• Two or more substance combine to form a new
  compound: A + X     AX
                 Reactions of elements with O2 and S8
            Rb (s) + S8 (s)           Rb2S (s)    (balanced?)
           16Rb (s) + S8 (s)           8Rb2S (s)    (balanced)
                General formula for Group 1 and Group 2?
             M2O and MO, same with sulfur (M2S and MS)

                 Nonmetals also react with oxygen
          S8 (s) + O2 (g)           SO2 (g)     (balanced?)

          S8 (s) + 8O2 (g)           8SO2 (g)    (balanced)

          Keep in mind that some metals can react with oxygen and
           form two different oxides – we have learned about these.
   Synthesis reactions continued
– Metals reacting with Halogens


        Rb (s) + Cl2 (g)          RbCl (s)    (balanced?)


       2Rb (s) + Cl2 (g)          2RbCl (s)    (balanced)



– Reactions with oxides

       CaO (s) + H2O (l)           Ca(OH)2 (s)    (balanced?)


                           Already balanced
        #2 - Decomposition Reactions
• A single compound (reactant) undergoes a process where two
  or more simpler substances are produced: AX      A + X

                               electricity
                    2H2O (l)                   2H2 (g)     + O2 (g)

                  Name this type of decomposition reaction

                                   electrolysis

  • Oxides of less active metals undergo decomposition reactions when heated:


                                   D
                  2HgO (s)                   2Hg (l)     + O2 (g)
Other types of Decomposition Reactions

          • Decomposition of metal carbonates:
                                 D
           CaCO3 (s)                       CaO (s)     + CO2 (g)

          • Decomposition of metal hydroxides:

                             D
         Ca(OH)2 (s)                      CaO (s)      + H2O (g)

            • Decomposition of metal chlorates:

                                  D
             2KClO3 (s)                     2KCl (s)      + 3O2 (g)

                    • Decomposition of acids:

                                  D
             H2SO4 (aq)                      SO3 (g)     + H2O (l)


        What would you predict for the gas product of decomposing carbonic acid?
    #3 - Single Replacement Reaction
• AKA “displacement reactions”, is where a single element
  replaces a similar element in a compound. General equations:

          A + BX                    AX + B
          Y + BX                    BY + X
                  What is the difference?


  • Replacement of a metal in a compound:


            Al    (s)   + Pb(NO3)2 (aq)          Pb (s) + Al(NO3)3 (aq)

                                           Balanced?

            3Al   (s)   + 2Pb(NO3)2 (aq)           2Pb (s) + 3Al(NO3)3 (aq)
Single Replacements Continued

    • Replacement of hydrogen in water by a metal:


       2K (s) + 2H2O (aq)       2KOH (aq) + H2 (g)


    • Replacement of hydrogen in acid by a metal:


      Mg (s) + 2HCl (aq)       MgCl2 (aq) + H2 (g)


        • Replacement of Group 17 elements:


      Cl2 (g) + 2KBr (aq)      2KCl (aq) + Br2 (g)
How to predict SRR




      How about G17?
  #4 - Double Replacement Reactions
• Ions of two compounds exchange places in solution to form
  two new compounds:
                    AX + BY                 AY + BX

               Characteristics of this type of reaction?


                       • Formation of a precipitate:

                Pb(NO3)2 (aq) + 2KI (aq)     PbI2 (s) + 2KNO3 (aq)


                            • Formation of a gas:

                  FeS (s) + 2HCl (aq)      H2S (g) + FeCl2 (aq)


                 How do you know if a compound is soluble?
Solubility Table
Lastly, #5 - Combustion Reactions
• Substances combining with oxygen releasing energy in
  the form of heat and light, and water, and mainly CO2:
  Write and balance the combustion reaction of propane in excess oxygen


           C3H8(g)   + 5 O2(g)        3CO2(g)+ 4H2O(g)


            Let me show you an easy way to balance these!
Name some reactions
Mg (s) + 2HCl (aq)             MgCl2 (aq) + H2 (g)

              Single replacement

Cl2 (g) + 2KBr (aq)            2KCl (aq) + Br2 (g)
              Single replacement

    CaO (s) + H2O (l)                   Ca(OH)2 (s)
                      Synthesis
                      D
  CaCO3 (s)                   CaO (s)     + CO2 (g)
                 Decomposition

 C3H8(g)     + 5O2(g)             3CO2(g)+ 4H2O(g)
                   Combustion

   Pb(NO3)2 (aq) + 2KI (aq)       PbI2 (s) + 2KNO3 (aq)

             Double Replacement
 What’s Next? Chapters 10 and 11 – The Mole and
    The Mathematics of Chemical Equations
• Please R&O Chapters 10 and 11 – print off all
  materials listed for these chapter.

								
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