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					Chemical Reactions & the
 Law of Conservation of
         Mass
                             Review

• Chemical bonds are the ‘glue’ that holds the atoms
  of elements together in compounds.

• Chemical bonds form when the electrons in the
  electron cloud around the atoms interact.
  Specifically, it is the ‘valence electrons’ of atoms that
  interact.

• We learned about two types of chemical bonds –
  Ionic and Covalent bonds.
           Chemical Reaction


•   A chemical reaction produces
    new substances by changing the
    way in which atoms are arranged.

•   In a chemical reaction, bonds
    between atoms are broken and
    new bonds form between different
    atoms. Bonds broken  new
    bonds formed.
              Reactants and Products
•   Reactants are the substances present at the beginning of a chemical reaction.

•   Products are the substances formed by a chemical reaction.

•   So chemical reactions turn reactants into products by rearranging atoms.

•   Reactants and products can be elements or compounds depending on the reaction taking
    place.

•   Example – the burning of natural gas

•   Methane (CH4) and oxygen (O2) are the reactants. Carbon dioxide (CO2) and water (H2O)
    are the products formed by the reaction.

•   Reactants – bonds broken           Products- new bonds formed
     CH4 + O2                         CO2 + H2O
        Does matter disappear in a chemical
                    reaction?

• Atoms are not changed by chemical reactions, but
  merely rearranged into different compounds.

• Atoms are neither created nor destroyed by chemical
  reactions.

• So if atoms are neither created nor destroyed, matter
  is neither created nor destroyed; it merely changes
  form.

• At one time scientists thought that chemical reactions
  could create or destroy matter.
        Law of Conservation of Mass
• In the late 18th century, Antoine Lavoisier proved that matter
  can never be created or destroyed in a chemical reaction.

• His research and conclusion is called the law of conservation
  of mass.

• It states – ‘in a chemical reaction atoms are neither created
  or destroyed. All atoms present in the reactants are also
  present in the products.
    Chemical Reactions can be described
          by chemical equations
•   The law of conservation of mass states that in a chemical reaction the total mass of
    reactants is equal to the total mass of products.

•   For example, the mass of Na plus the mass of Cl that reacts equals the mass of the
    product NaCl (sodium chloride).

•   A chemical equation is the ‘shorthand’ way to represent how atoms are rearranged in
    a chemical reaction.

•   The atoms in the reactants are shown on the left side of the equation and the atoms in
    the products are shown on the right side of the equation.

•   Because atoms are rearranged and not created or destroyed, the number of atoms of
    each different element must be the same on each side of the equation.
Chemical Equation
             Types of Chemical Reactions
• Synthesis – a chemical reaction when two or more simple substances
  combine to form a new complex substance.
  Basic form :            A + X  AX
  Example:            Na +Cl2  NaCl

• Decomposition – a chemical reaction when a complex substance breaks
  down into two or more simpler substance.
  Basic form:             AX  A + X
  Example:            Al2O3  Al + O2
• Single Replacement – a chemical reaction when a single uncombined
  element replaces an element that is part of a compound.
  Basic form:   A + BX  AX + B            or         AX + Y  AY + X
  Ex:       Zn + CuCl2  ZnCl2 + Cu                 HgO + Cl2 HgCl+O2

• Double Replacement – a chemical reaction when different atoms in two
  different compounds replace each other.
  Basic form:       AX + BY  AY + BX
  Example:      MgCO3 + HCl  MgCl2 +H2CO3
Chemical Equations must be
         balanced
     Chemical Equations must be balanced
•   The same number of atoms of each element must appear on
    both sides of a chemical equation.

•   However, simply writing down the chemical formulas of reactants
    and products does not always result in equal number of atoms.

•   Therefore, you have to balance the equation to make sure the
    number of atoms are equal on each side of an equation - the law
    of conservation of mass.

•   Also, equations must be balanced in order to determine ‘how
    much’ of each compound is required.
   Usefulness of Conservation of Mass
        and balancing equations

• A compound called sodium azide (NaN3).

• It is a solid and takes up a very small amount of space -
  130 grams( the size of a large tablespoon).


• The reason can be illustrated by the balanced chemical
  reaction for this reaction.
      2NaN3  2Na + 3N2 (a decomp. Reaction)

• According to this balanced equation, three
  molecules of N gas are formed for every two
  molecules of sodium azide that reacts.

• In fact, 67 liters of N gas are produced for every 130
  grams of sodium azide in the reaction.

• This is the compound used for air bags in
  automobiles.
2 H2 + O2  2 H2O
                   Rules for Balancing Equations
•   Never add or change subscripts.    You can only add coefficients to balance an equation.

•   Only add coefficients in front of the chemical formula for the compound ( they indicate
    how many molecules take part in the reaction). Make sure all coefficients are reduced to
    the lowest common denominator.

•   Chemical formulas can have both coefficients and subscripts. In these cases, multiply the
    two numbers together to find the total number of atoms involved in the reaction.

•   For example, 3O2 means there are 6 atoms of oxygen; i.e. 3 molecules containing
    2 atoms each.

•   Never ‘break’ a chemical compound (chemical bond). i.e., never put a coefficient or
    subscript within a compound – for example, if NaCl is the compound you cannot do
    this Na3Cl or        this  Na3Cl

•   Never add reactants and never add products.

•   Your goal is to make sure the number of atoms for each element are equal on both
    sides of an equation.
Example of a Balanced Equation


     reactant       products
     2HgO           2Hg + O2

       Hg =2               Hg=2
       O =2                 O =2

				
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posted:1/24/2012
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