Chapter 8 Chemical Equations and Reactions

							Chapter 8:
Chemical Equations and Reactions

          Section 1:
          Describing Chemical Reactions
Overview
• We will list three observations that a chemical
  reaction has taken place.
• We will state three requirements for a correctly
  written chemical equation.
• We will write a word equation and a formula
  equation for a given chemical reaction.
• We will balance a formula equation.
Indications of a Chemical Reaction
•    There are three indicators a chemical reaction
     is taking place or has taken place.
1.   Heat or light is produced. This indicates a
     release of energy.
2.   A gas is formed. Usually seen through
     bubbles.
3.   A precipitate is created. A precipitate is a solid
     formed from a solution of liquids that react.
4.   ?4? The color changes. This is not always a
     perfect indicator. Most of the time it is.
A Chemical Equation
• Using symbols and formulas, it represents the
  identities and relative amounts of the reactants
  and products in a chemical reaction.
• Reactants – The chemicals you start with in a
  reaction.
• Products – The chemicals you get in a reaction.
Characteristics of a Chemical Equation
1. All reactants and products must be identified.
2. The formulas for the reactants and products
   must be correct.
3. The Law of Conservation of Mass must be
   satisfied. Must have the same of number of
   atoms of each element on both the reactant
   and product sides.
• Coefficients – A small whole number that
   appears in front of a formula in a chemical
   equation which is used to accomplish #3.
Symbols We Use
•   +      •   Added to
•   →      •   Yields or Produces
•   (s)    •   Substance is a solid
•   (l)    •   Substance is a liquid
•   (g)    •   Substance is a gas
•   (aq)   •   Substance is aqueous
•   ↔      •   Reaction can go both ways (Reversible)
•   Δ      •   Heat if above arrows
•   ↑      •   Subs for (g) to show a gas is produced
•   ↓      •   Subs for (s) to show a precipitate is
               formed
Word vs. Formula Equations
• Word Equation - An equation in which the
  reactants and products are represented by
  words.
• zinc + hydrochloric acid → hydrogen + zinc (II)
  chloride
• Formula Equation - An equation in which the
  reactants and products are represented by
  symbols and chemical formulas.
• Zn (s) + HCl (l) → H2 (g) + ZnCl2 (aq)
Balancing Chemical Equations
1. Check that the formula equation is correct.
2. Balance the reactant atoms with the product
   atoms. Count both sides.
  •   Balance elements one at a time.
  •   Balance first the atoms that occur once on both
      sides.
  •   Balance polyatomic ions that appear on both
      sides.
  •   Balance H and O last.
3. Recount reactants and products to make sure
   equation is balanced.
Balancing Example
•   Zn (s) + HCl (l) → H2 (g) + ZnCl2 (aq)
•   1 Zn, 1 H, 1 Cl         1 Zn, 2 H, 2 Cl
•   Need one more Cl and H on reactant side.
•   Put a 2 in front of HCl
•   Zn (s) + 2HCl (l) → H2 (g) + ZnCl2 (aq)
•   1 Zn, 2 H, 2 Cl         1 Zn, 2 H, 2 Cl
•   Balanced!