Chapter 20: Chemical Reactions

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Chapter 20 Chemical Reactions

• Section 1 Forming New Substances

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So what is a chemical reaction?

• A chemical reaction is the process by which one
  or more substances undergo change to produce one
  or more different substances.

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Clues to Chemical Reactions

• Gas formation (bubbles)
• Solid formation (precipitate)
• Color change (bleach on your blue jeans)
• Energy change (heat or light)

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Breaking and Making Bonds

• New substances are formed in a chemical reaction
  because chemical bonds in the starting substances
  break, atoms rearrange, and new bonds form to
  make the new substances.

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Chemical Formulas

• A chemical formula is a shorthand notation for a
  compound or a diatomic element using chemical
  symbols and numbers.
• A chemical formula indicates the chemical makeup
  by showing how many of each kind of atom is
  present in a molecule.

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Chemical Formulas Include

• Symbols (letter/s that identify an atom)
• Subscripts (the number written below and to the
  right to tell how many of that atom is present
  if there is no subscript then there is only one
  atom)

CO2
Symbol -gt
lt- subscript
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Prefixes for Covalent Compounds
Prefixes Used in Chemical Names Mono- 1 Di-
2 Tri- 3 Tetra- 4 Penta- 5 Hexa-
6 Hepta- 7 Octa- 8 Nona- 9 Deca- 10

• A prefix is a syllable or syllables attached to
  the beginning of a word.
• Each prefix used in a chemical name represents a
  number.
• Chart on page 533

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Writing Formulas for Ionic Compounds

• Remember that an ionic compound contains a metal
  and a nonmetal.
• To write the formula for an ionic compound, you
  must make sure the compounds overall charge is
  zero by using subscripts that cause the charges
  of the ions to cancel out.

Magnesium chloride MgCl2
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Chemical Equations

• A chemical equation is a shorthand description of
  a chemical reaction using chemical formulas and
  symbols.
• The starting materials in a chemical reaction are
  called reactants.
• The substances formed from reactants are called
  products.

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Example
C O2 CO2
Remember that accuracy is important! If you
use the wrong subscripts you can completely
change the formula. If you leave the 2 off of
this formula you will create carbon monoxide, a
deadly gas, instead of carbon dioxide, the
colorless, odorless gas that you exhale.
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An Equation Must be Balanced

• Writing a balance equation requires the use of
  coefficients. A coefficient is a number placed
  in front of a chemical symbol or formula.
• Coefficients are used when balancing equations
  because subscripts CANNOT be changed. Changing a
  subscript changes the formula.

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Mass is Conserved - Its a Law!

• The total mass of the reactants is equal to the
  total mass of the products.
• Law of Conservation of Mass- mass is neither
  created or destroyed in ordinary chemical or
  physical changes.

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Homework

• Read Chapter 20 Section 1.
• Complete workbook pages (155-156).