Chapter 8 Chemical Reactions - DOC by hcj


									Chapter 8 Chemical Reactions and Ch 13 Section 1 Compounds in
Aqueous Solution

1. Writing Chemical Equations
    A chemical equation is a representation of a chemical reaction; the formulas
   of the reactants (on the left) are connected by an arrow with the formulas of
   the products (on the right).

   Reactant + Reactant  Product + Product

   iron + oxygen  iron(III) oxide

   Hydrogen Peroxide  Water and Oxygen

2. A catalyst is a substance that speeds up the reaction but is not used up in the
reaction. Notice the following symbols and notations in chemical equations.

3. Balancing Chemical Equations
    A chemical reaction is described by a balanced equation in which each side
    of the equation has the same number of atoms of each element and mass is

To write a balanced chemical equation, first write the reactants and products.
Then use coefficients to balance the equation so that it obeys the law of
conservation of mass. Coefficients are whole numbers that are placed in front
of the formulas in an equation in order to balance it. The absence of a
coefficient represents the coefficient 1.

Zn + 2HCl  ZnCl2 + H2

Suggested Order for Balancing:
  1. Metals
  2. Polyatomic ions if present on both sides
  3. Nonmetals other than H and O
  4. H or O, whichever is in fewer places
  5. Remaining H or O

   •   Sometimes it is helpful to write H2O as HOH
   •   When balancing combustion reactions, start by putting a coefficient
       2 in front of fuel
   •   If you have an odd number on one side, try making it even by adding
       a coefficient 2

Aluminum is a good choice for outdoor furniture because it reacts with oxygen in
the air to form a thin protective coat of aluminum oxide. Balance the equation for
this reaction.

4Al    +   3O2     2Al2O3

Write and balance this equation:

calcium hydroxide + sulfuric acid  calcium sulfate + water

Types of Chemical Reactions
   The heat and smoke of burning charcoal are the products of a combustion
   reaction. Combustion is one of the five general types of chemical reactions.
   If you can recognize a reaction as being a particular type, you may be able
   to predict the products of the reaction.

4. Classifying Reactions
     What are the five general types of reactions?
     The five general types of reaction are combination (synthesis),
     decomposition, single-replacement, double-replacement, and combustion.

A combination reaction (synthesis reaction) is a chemical change in which two
or more substances react to form a single new substance.

A decomposition reaction is a chemical change in which a single compound
breaks down into two or more simpler products.

A single-replacement reaction is a chemical change in which one element
replaces a second element in a compound.

  The activity series of metals lists metals in order of decreasing reactivity.

Use the activity series with single replacement series to determine if a reaction will
occur. If a metal is higher than another on the series, it is reactive enough to replace
it. For example, Zn will replace Cu in copper(II) sulfate. Na will replace hydrogen in
HCl or water.

What if the single element is not a metal? Halogens often replace other
halogens in a compound. Fluorine is the most active halogen and will replace
any other halogen (group 17). Any element in group 17 can replace any element
below it, but not any element above it.
Ex: Cl2 + 2KBr  2KCl + Br2

Ex: Br2   + KCl    no reaction

 A double-replacement reaction is a chemical change involving an exchange
 of positive ions between two compounds.

Remember 2 couples dancing, they trade partners. 2 new couples.

A combustion reaction is a chemical change in which an element or a
compound reacts with oxygen, often producing energy in the form of heat and
light. Usually, CO2 and H2O are the products.

Write a balanced equation for the complete combustion of benzene, C6H6 and
ethanol, CH3CH2OH

5. Predicting the Products of a Chemical Reaction

  The number of elements and/or compounds reacting is a good indicator of
  possible reaction type and thus possible products. Figure out the reaction
  type so you can predict products. For example, if you have a single element
  and an ionic compound, the single element will replace part of the compound
  to form a new single element and a new compound. Analogy: a single person
  and a couple are dancing. The single person replaces one of the dancers to
  form a new single person and a new couple dancing. Cations replace cations.
  Anions replace anions. Two compounds? Two couples trade partners.

   What type of reaction is described by the following equation?
     6Li + N2  2Li3N

       combination reaction

   Predict the missing product in the following unbalanced equation.
   Pb(NO3)2 + KOH  KNO3 + ____________


6. Writing Net Ionic Equations
   • Most ionic compounds separate into cations and anions when they
       dissolve in water.

       Ex: NaCl separates into Na+(aq) and Cl-(aq) in water.

   •   A complete ionic equation shows dissolved ionic compounds as
       dissociated free ions.

       Ex: Silver nitrate (aq)+ sodium chloride (aq) silver chloride (s) + sodium
           nitrate (aq)

An ion that appears on both sides of an equation and is not directly involved in
the reaction is called a spectator ion.

The net ionic equation is an equation for a reaction in solution that shows only
those particles that are directly involved in the chemical change. It is balanced
with respect to both mass and charge.

      Mark out the spectator ions in the complete ionic equation and balance it.
       This is the net ionic equation.

Remember that a precipitate is a solid, insoluble salt that forms when two ionic
compounds react. It is written as (s) solid in the equation.

  Aqueous solutions of iron(III) chloride and potassium hydroxide are mixed. A
  precipitate of iron(III) hydroxide forms. Identify the spectator ions and write a
  balanced net ionic equation for the reaction.

7. Predicting the Formation of a Precipitate
   Will a precipitate form when a sodium carbonate solution is mixed with a
   barium nitrate solution?

 Sodium nitrate is soluble but barium carbonate is insoluble (see last rule). The
 net ionic equation is

Real Life Application of Insoluble Salt
Most of the human midsection is invisible to X-rays, which pass right through soft
tissue. However, barium sulfates particles are dense enough to block X-rays.
The film shows in sharp relief all gastrointestinal curves and abnormalities. The
compound is insoluble in water. Even in an intestine designed to digest, barium
sulfate will not dissolve in water therefore it goes in and comes right back out
rather than entering the bloodstream. Other barium compounds aren't so
harmless! Soluble barium compounds are quite poisonous.

8. Solving for Empirical Formula by Combustion Analysis
Ex: We have a 1.05 g sample of an organic vanillin compound, C?H?O?. By
complete combustion it produces 2.43 g of CO2 and 0.50 g of H2O. Find the
empirical formula of the compound.

   •     Write out the equation.
   •     To find the empirical formula you must get a mole ratio.
   •     We use the % composition of CO2 and H2O to determine the mass of C
         and H. Then convert to moles.
   •     To find mass of oxygen in the unknown fuel you must first subtract the
         masses of carbon and hydrogen. Whatever is left must be oxygen.
         Convert to moles.
   •     Find the lowest whole number ration of moles for each element.


C?H?O?      + O2  CO2 + H2O.

1.05 g                .43 g   .50 g

2.43 g CO2        12.011 g C   = 0.663184 g C
                   44.01 g CO2

 0.50 g H2O       2.0158 g H       = 0.05595 g H
                 18.0146 g H2O

1.05 g C?H?O? - 0.663184 g C – 0.05595 g H = 0.330866 g O

Now convert to moles of each element

= 0.0552147 mol C

= 0.055105 mol H

= 0.0206799 mol O

Divide each by the smallest number of moles to get simplest whole number ratio.
Then find empirical formula.



To top