Chemical Equations and Reactions

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Chemical Equations and Reactions
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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

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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

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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?

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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

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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
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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)

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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!

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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)

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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

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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)

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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.

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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)

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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.

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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)

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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!

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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)

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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

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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.