Lecture Notes – Classifying Chemical Reactions    

      Ch 10.2 & 10.3                                Std 3g

 

 This first type of classification concentrates on DRIVING FORCES:  

1.   oxidation / reduction – where electrons are taken by one atom and lost by another, and heat is always produced

      The atom that is oxidized is the loser (of electrons).

      The atom that is reduced is the gainer (of electrons).

 

Examples

      a.  thermite reaction           

 

                        Fe2O3 (s)  +  2 Al (s)  ---->  Al2O3 (s)  +  2 Fe (s)  +  lots of heat

 

iron goes from Fe3+ (in Fe2O3 (s) )  to   Fe (s)   (the element, no net charge)  -  REDUCED 

      (gained electrons from Al (s) )

aluminum goes from Al (s) (the element, no net charge) to Al3+ (in Al2O3 (s))  -  OXIDIZED 

      (lost electrons to Fe)

 

b.  methane bubbles

 

                  CH4 (g)  +  2 O2 (g)     ---->   CO2 (g)  +  2 H2O (g)   +   heat

 

oxygen goes from O (the element, no net charge) in the O2 (g)  molecule,  to the O2- (the ion) in the water molecule – REDUCED 

                        (gained electrons from C)

carbon goes from C4-  (in the CH4 (g) ) to  C4+  (in CO2 (g) ) -  OXIDIZED  (lost electrons to oxygen)

 

 

2.   precipitate (solid) forms   

     

      In these reactions, soluble ionic compounds (aqueous solutions) are mixed together and one or more type of precipitate is formed.

 

        Example

      Solubility of Some Ionic Compounds

      In the Solubility of Some Ionic Compounds  lab, we had several examples of precipitates forming.  Use your Solubility Chart to figure out if any ionic compound will be a precipitate. 

 

3.   water is formed – also called     acid / base    reactions

      Whenever you mix acids with bases, you wind up with water and a salt.  Remember that salts are any metal with any non-metal (or non-metal polyatomic ion). 

 

        Example

 

                              HCl (aq)  +   NaOH (aq)  ---->  HOH +  Na+(aq)  +  Cl-(aq) 

 

If we net out this reaction and only look at the ions that form water, we will have the following:

 

                  H+  +  OH-  ---->   HOH

 

      This is the simplest of acid / base reactions.  Notice water is written HOH instead of H2O because oxygen is actually in the middle between two hydrogens.  In this case the salt is common table salt. 

 

Driving Forces for Chemical Reactions

 

One or more of the following must occur for a reaction to take place.  We call these things driving forces.

      1.   Formation of a solid (precipitation)     

(aq) + (aq)  --->  (s)  +  (aq)

      2.   Formation of water(l) & salt(aq) (acid + base reactions)    

(aq)  +  (aq)  ---->  (l)   +  (aq)

      3.   Transfer of electrons (oxidation-reduction)   combustion (burns w O2 ) 

                        synthesis  (2 become 1)   decomposition (1 becomes 2 or more)

 

   If you mix two chemicals and one or more of these things is likely to occur, a chemical reaction is likely to occur.

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Classifying Types of Chemical Reactions Differently:

 

1.  Synthesis (also called Direct Combination) - A synthesis reaction involves two or more substances combining to make a more complex substance.  The reactants may be elements or compounds, and the product will always be a compound.  The general formula for this type of reaction can be shown as;

A            +             B                 ---->                 AB
or
element or compound + element or compound -----> compound

Some examples of synthesis reactions are shown below;

2H2(g) + O2(g) ----> 2H2O(g)

C(s) + O2(g) ----> CO2(g)

CaO(s) + H2O(l) ----> Ca(OH)2(s)

2.  Decomposition (also called Analysis) - In a decomposition reaction, one substance is broken down into two or more, simpler substances.  This type of reaction is the opposite of a synthesis reaction, as shown by the general formula below;

AB       ---->      A       +      B
or
Compound ------> element or compound + element or compound

Some examples of decomposition reactions are shown below;

C12H22O11(s) ----> 12C(s) + 11H2O(g)

Pb(OH)2(cr) ----> PbO(cr) + H2O(g)

2Ag2O(cr) ----> 4Ag(cr) + O2(g)

3.  Single Replacement - In this type of reaction, a neutral element becomes an ion as it replaces another ion in a compound.   The general form of this equation can be written as;

In the case of a positive ion being replaced:  A + BC ----> B + AC
or
In the case of a negative ion being replaced:  A + BC ----> C + BA
in either case we have;

element + compound ----> element + compound

 

Some examples of single displacement reactions are shown below:

Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)

2Al(s) + 3CuCl2(aq) ---> 2AlCl3(aq) + 3Cu(s)

Cl2(g) + KBr(aq) ----> KCl(aq) + Br2(l)

4.  Double Replacement - Like dancing couples, the compounds in this type of reaction exchange partners.  The basic form for this type of reaction is shown below;

AB + CD ----> CB + AD
or
Compound + Compound ----> Compound + Compound

Some examples of double displacement reactions are shown below;

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

ZnBr2(aq) + 2AgNO3(aq) ----> Zn(NO3)2(aq) + 2AgBr(cr)

H2SO4(aq) + 2NaOH(aq) ----> Na2SO4(aq) + 2H2O(l)

5.  Combustion - When organic compounds like propane are burned, they react with the oxygen in the air to form carbon dioxide and water.  The reason why these combustion reactions will stop when all available oxygen is used up is because oxygen is one of the reactants.  The basic form of the combustion reaction is shown below;

hydrocarbon + oxygen ----> carbon dioxide and water

Some examples of combustion reactions are;

CH4(g) + 2O2(g) ----> 2H2O(g) + CO2(g)

2C2H6(g) + 7O2(g) ----> 6H20(g) + 4CO2(g)

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