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Chemical Equations and Reactions

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Title: Chemical Equations and Reactions


1
Chemical Equations and Reactions
2
What is a Chem. Rxn.?
  • Chemical Reaction
  • Process of one or more substances converting to
    form new substances with different properties.
  • Some examples
  • Iron rusting
  • Baking soda and vinegar mixed
  • A campfire
  • Plants converting CO2 and H2O to sugars

3
Indications of Chemical Rxns.
  • What are the five signs of a chemical reaction?
  • Change in heat or light (energy)
  • Production of a gas
  • Production of a precipitate
  • Precipitate an ionic solid formed as the
    product of a chemical rxn.
  • Change in odor
  • Change in color

4
Traits of Chem. Rxns.
  • Reactants
  • Substances that enter a chemical reaction.
  • Ex. Vinegar (acetic acid) and baking soda
    (sodium bicarbonate)
  • Products
  • Substances that are produced by a chemical
    reaction.
  • Ex. Carbon dioxide, sodium acetate, and water
  • Do all combinations result in a chemical
    reaction? Yes or No? Example?

5
Chemical Equations (Eqns.)
  • Chemical Equation
  • Expression using chemical symbols to represent a
    chemical reaction
  • A chemical equation represents the identities and
    relative amounts of reactants and products in a
    chemical reaction
  • Word Equations
  • Ex. Acetic acid Sodium Bicarbonate ? Carbon
    Dioxide Sodium Acetate Water
  • Formula Equations

6
Symbols for use in Chemical Equations
Symbol Meaning
(s) Solid
(l) Liquid
(g) Gas
(aq) In aqueous solution, dissolved in water
? Gas forms
? Solid precipitate forms
Symbol Meaning
? produces or yields, indicates result of reaction
? Reaction in which products can reform into reactants, can go both ways
Heat is applied, may have word heat instead of triangle
Other conditions (temperature, pressure, etc.) may be indicated here, or below the arrow
7
Writing Equations
  • Example
  • Sodium metal is added to water. Hydrogen gas
    bubbles off, and sodium hydroxide is left behind,
    in solution.
  • Step 1
  • Put text into word equation
  • Sodium water ? hydrogen sodium hydroxide
  • Step 2
  • Translate words in word equation to formulas
  • Na H2O ? H2 NaOH
  • Step 3
  • Add symbols to tell states of reactants and
    products.
  • Na(s) H2O(l) ? H2(g) NaOH(aq)

8
Law of Conservation of Mass
  • Law of Conservation of Mass
  • Mass cannot be lost or gained in a chemical
    reaction
  • ALSO, the total mass of the reactants must equal
    total mass of the products
  • Therefore, the NUMBER of each type of atom on
    each side of the equation MUST be the same!

9
Types of Chemical Rxns.
  • Synthesis (Direct Combination) Reaction
  • Occurs when two or more simpler substances
    combine to form a more complex substance.
  • General Form
  • A B ? AB
  • Eqn.
  • Fe(s) O2(g) ? FeO2(s)

10
Synthesis Reactions
  • Signs of a synthesis reaction
  • Start with elements and end with a single
    compound
  • OR start with simple compounds and end with a
    single compound.
  • What is always true of a synthesis reaction?
  • At the end, there is a SINGLE product, a
    compound.
  • Examples
  • Iron metal and oxygen gas combine to form rust
  • Sodium metal and chlorine gas combine to form salt

11
Decomposition Reaction
  • Occurs when a substance breaks down into two or
    more simpler substances.
  • Opposite of a synthesis reaction.
  • General Form
  • AB ? A B
  • Eqn.
  • 2H2O(l) ? 2H2(g) O2(g)

12
Decomposition Reaction
  • Signs of a decomposition reaction
  • A single, complex substance breaks into the
    elements that make it up
  • OR A single, complex substance breaks into
    simpler substances
  • What is always true of a decomposition reaction?
  • The reactant is ALWAYS a single, complex
    compound.
  • Example
  • Liquid hydrogen peroxide, in the presence of
    light, breaks down to form water and oxygen gas.

13
Single Replacement Rxn.
  • Atoms of one element replace atoms of another
    element in a compound.
  • General Form
  • A BC ? AC B
  • Eqn.
  • Cu(s) AgNO2(aq) ? CuNO2(aq) Ag(s)

14
Single Replacement Rxn.
  • Signs of a Single Replacement Rxn.
  • One element reacts with a compound, and this
    produces another free element and a new compound.
  • What is always true of a single replacement
    reaction?
  • The reactants include an element and the products
    include a different element.
  • Example
  • Zinc is put into acid, and eaten away,
    producing hydrogen bubbles.

15
Double Replacement Rxn.
  • Atoms in two different compounds trade places
    with each other.
  • General Form
  • AB CD ? AD CB
  • Eqn.
  • AgNO3(aq) KCl(aq) ? AgCl(?) KNO3(aq)

16
Double Replacement Rxn.
  • What are the signs of a Double Replacement
    Reaction?
  • Two compounds react and product two NEW
    compounds.
  • There is USUALLY a gas or precipitate formed
  • What is always true of a DR Reaction?
  • There are two compounds at the beginning, and two
    compounds at the end of the reaction.
  • Example
  • When vinegar and baking soda are combined, carbon
    dioxide gas bubbles off, vigorously!

17
Combustion Reaction
  • Any hydrocarbon (compound made of only C, H, and
    occasionally O) reacts with oxygen to produce
    carbon dioxide and water.
  • A large amount of energy is released as light and
    heat.
  • General Form
  • CxHx O2 ? CO2 H2O
  • Eqn.
  • CH4(g) 2O2(g) ? CO2(g) 2H2O(g)

18
Combustion Reaction
  • What are the signs of a combustion reaction?
  • There is always light and heat, as well as the
    production of gases (smoke). Something is
    BURNING!
  • What is always true of combustion reactions?
  • They always take place in oxygen (in air), with a
    source of heat (like a match), and start with a
    hydrocarbon (like oil, paper, or anything
    organic)
  • Example
  • When paper burns, it produces light, heat, and
    gases.
  • Anything else flammable ethanol, gasoline,
    marshmallows

19
Exothermic and Endothermic
  • Exothermic reaction
  • A reaction that releases energy and gives off
    heat
  • An exothermic reaction may also release energy in
    the form of light or sound.
  • Endothermic reaction
  • A reaction that absorbs energy resulting in
    lowering of temperature.
  • Endothermic reactions also include any reactions
    that need light to proceed, such as
    photosynthesis.
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