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Chemical Equations

 Equations show:

bulletthe reactants which enter a reaction
bulletthe products which are formed
bulletthe amounts of each substance used and produced

 Two important points to remember:

bulletevery chemical compound has a formula which cannot be altered
bulletchemical reactions must account for every atom used (the Law of Conservation of Mass which states that in a chemical reaction atoms are neither created nor destroyed)

Key points:

bulletthe diatomic elements when standing alone are written: H2, N2, O2, F2, Cl2, Br2, I2
bulletthe sign, à means 'yields' and shows the direction of the action
bulleta "delta  'Δ' " above the arrow shows that heat has been added
bulleta double arrow  ß>  shows that the reaction is reversible and can go in both directions
bulletalways check that each formula is correct (and cannot be changed)
bulletbalancing is done by placing coefficients in front of the formulas to have the same number of atoms of each element on both sides of the arrow
bulletcheck the activity series for replacement reactions (are products soluble or insoluble)
bulletif a reactant or product is a solid, (s) is placed after the formula
bulletif a reactant or product is a gas, (g) is placed after the formula
bulletif a reactant or product is soluble in water solution, (aq) is placed after the formula
bulletsome products are unstable and break down (decompose) as they are produced during the reaction:

             H2CO3 (aq)  à  H2O (l)  +  CO2 (g)

             H2SO3 (aq)  à  H2O (l)  +  SO2 (g)

             NH4OH (aq)  à  NH3 (g)  +  H2O (l)

   Four Basic Type of Chemical Reactions (with examples):

 A.  Synthesis (composition)  - two or more elements or compounds may combine to form a more complex                                                           compound

 examples:

bulletmetal + oxygen à metal oxide                             2 Mg (s)   +  O2 (g)  à  2 MgO (s)
bulletnonmetal + oxygen  à  nonmetallic oxide           C (s)  +  O2 (g)  à  CO2 (g)
bulletmetal oxide + water  à metallic hydroxide          MgO (s)  +  H2O (l)  à  Mg(OH)2 (s)
bulletnonmetallic oxide + water  à acid                        CO2 (g)  +  H2O (l)  à  H2CO3 (aq)
bulletmetal + nonmetal  à salt                                        2 Na (s)  +  Cl2 (g)  à  2 NaCl (s)
bulletnonmetals with other nonmetals                             2P (s)  +  3 Cl2 (g)  à  2PCl3 (g)

 two important reactions to remember:

bulletN2 (g)  +  3H2 (g)  à  2NH3 (g)
bulletNH3 (g)  +  H2O (l)  à  NH4OH (aq)

 B.  Decomposition  -  a single compound breaks down into its component parts or simpler compounds

 examples:

bulletmetallic carbonate, when heated, form metallic oxides and carbon dioxide gas

             CaCO3 (s)  à  CaO (s)  +  CO2 (g)

bulletmost metallic hydroxides, when heated, decompose into metallic oxides and water

             Ca(OH)2 (s)  à  CaO (s)  +  H2O (l)

bulletmetallic chlorates, when heated, decompose into metallic chlorides and oxygen

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

bulletsome acids, when heated, decompose into nonmetallic oxides and water

             H2SO4  à  H2O (l)  +  SO3 (g)

bulletsome oxides, when heated, decompose

            2HgO (s)  à  2Hg (l)  +  O2 (g)

bulletsome decomposition reactions are produced by electricity

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

            2 NaCl (l)  à  2 Na (s)  +  Cl2 (g)

C. Replacement – a more active element takes the place of another element in a compound and sets the less active one free

examples:

bulletreplacement of a metal in a compound by a more active metal

Fe (s)  +  CuSO4 (aq)  à  FeSO4 (aq)  +  Cu (s)

bulletreplacement of hydrogen in water by an active metal

2 Na (s)  +  2 H2O (l)  à  2 NaOH (aq)  +  H2 (g)

Mg (s)  +  H2O (l)  à  MgO (s)  +  H2 (g)

bulletreplacement of hydrogen in acids by active metals

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

bulletreplacement of nonmetals by more active nonmetals

Cl2 (g)  +  2 NaBr (aq)  à  2 NaCl (aq)  +  Br2 (l)

NOTE:  refer to the activity series for metals and nonmetals to predict products of replacement reactions.  If the free element is above the element to be replaced in the compound, then the reaction will occur.  If it is below, then no reaction occurs. 

D.  Ionic  -  occurs between ions in aqueous solution  -   a reaction will occur when a pair of ions come together to produce at least one of the following:

1.  a precipitate

2.  a gas

3.  water or some other non-ionized substance

examples:

bulletformation of precipitate

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

BaCl2 (aq)  +  Na2SO4 (aq)  à  2 NaCl (aq)  +  BaSO4  (s)

bulletformation of a gas

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

bulletformation of water (reaction between acid and base is called neutralization)

HCl (aq)  +  NaOH (aq)  à  NaCl (aq)  +  H2O (l)

bulletformation of a product which decomposes

CaCO3 (s) +  HCl (aq)  à  CaCl2 (aq)  +  CO2 (g)  +  H2O (l)

NOTE:  Use the solubility rules to decide whether a product of an ionic reaction is insoluble in water and will thus form a precipitate.  If a compound is soluble in water then it should be shown as being in aqueous solution, or left as separate ions.

E.  Combustion of Hydrocarbons

Another type is that of the combustion of a hydrocarbon.  When a hydrocarbon is burned with sufficient oxygen supply, the products are always carbon dioxide and water vapor.  If the supply of oxygen is low or restricted, then carbon monoxide will be produced.  This is why it is dangerous to have an automobile engine running inside a closed garage or to use a charcoal grill indoors.

examples:

CH4 (g)  +  2O2 (g)  à  CO2 (g)  +  H2O (g)

2 C4H10 (g)  +  13 O2  à  8 CO2 (g)  +  10 H2O (g) 

Thanks to Gwen Sibert of the Roanoke Valley Governor’s School for many of these examples.

 

 

Page Last Updated: Friday March 02, 2007           Webmaster: Larry Jones                 Pickens County School District