Chemical Equations and Reactions

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Chemical Equations and Reactions
Reactants Zn I2
Product Zn I2
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Chemical Equations and Reactions

• Chemical equations show the relative amounts and
  types of reactants and products in a reaction.
• 4 Al (s) 3 O2 (g) ---gt 2 Al2O3 (s)
• The numbers in the front are called
• stoichiometric coefficients.
• The letters after the molecule (s), (g), (aq) and
  (l) are the physical states of the compounds
  solid, gas, aqueous and liquid.

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Chemical Equations and Reactions

• Elements are pure substances that cannot be
  broken down into other substances. Theses are
  represented by chemical symbols.
• Examples are Ca, Br, H, C, Mg
• Compounds are homogeneous substances made up of
  two or more elements chemically combined.
• Examples are NaCl, KBr, F2, H2O

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Parts of a Reaction Equation

• Chemical equations show the conversion of
  reactants (the molecules shown on the left of the
  arrow) into products (the molecules shown on the
  right of the arrow).
• A sign separates molecules on the same side
• The arrow is read as yields
• Example C O2 ? CO2
• This equation reads carbon plus oxygen react to
  yield carbon dioxide

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Balancing Equations Rules

• When balancing a chemical reaction you may place
  coefficients in front of the compounds to balance
  the reaction, but you may NOT change the
  subscripts.
• Changing the subscripts changes the compound.
  Subscripts are determined by the valence
  electrons (charges for ionic or sharing for
  covalent).

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Subscripts vs. Coefficients

• The subscripts tell you how many atoms of a
  particular element are in a compound. The
  coefficient tells you about the quantity, or
  number, of molecules of the compound.

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Reading Chemical Equations

• Equations can be read in terms of atoms and
  molecules or moles, so
• The equation
• 4 Al(s) 3 O2(g) ---gt 2 Al2O3(s)
• means
• 4 Al atoms 3 O2 molecules ---produces---gt
• 2 molecules of Al2O3
• AND/OR
• 4 moles of Al 3 moles of O2 ---produces---gt
• 2 moles of Al2O3

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Steps to Balancing Equations

• There are four basic steps to balancing a
  chemical equation.
• Write the correct formula for the reactants and
  the products.
• Find the number of atoms for each element on the
  left side. Compare those against the number of
  the atoms of the same element on the right side.
• Determine where to place coefficients in front of
  formulas so that the left side has the same
  number of atoms as the right side for EACH
  element in order to balance the equation.
• Check your answer to see if
• The numbers of atoms on both sides of the
  equation are now balanced.
• The coefficients are in the lowest possible whole
  number ratios. (reduced)

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Example
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Reaction Types

• There are 5 basic reaction types
• Synthesis
• Decomposition
• Single Replacement
• Double Replacement
• Combustion

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1. Synthesis Reactions

• Synthesis reactions occur when two substances
  (generally elements) combine and form a compound.
  
• Reactant A Reactant B? 1 Product AB
• Basically A B ? AB
• Examples 2H2 O2 ? 2H2O
• C O2 ? CO2 2Mg O2 ? 2 MgO

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Synthesis Reaction Example
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2. Decomposition Reactions

• Decomposition reactions occur when a compound is
  broken down into two or more simpler substances.
• 1 Reactant AB ? Product A Product B
• In general AB ? A B
• Example 2 H2O ? 2H2 O2
• 2 HgO ? 2Hg O2
• 2 Al2O3 ? 4 Al 3 O2

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Decomposition Reaction Example
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3. Single Replacement Reactions

• Single Replacement Reactions occur when one
  element replaces another in a compound.
• A BC ? BA C
• Example Zn H2SO4 ? ZnSO4 H2

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Single Replacement Example
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4. Double Replacement Reactions

• Double Replacement Reactions occur when a metal
  replaces a metal in a compound and a nonmetal
  replaces a nonmetal in a compound
• Compound Compound ? Product Product
• AB CD ? AD CB
• Example
• NaCl AgNO3 ? NaNO3 AgCl

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Double Replacement Reactions

• Think about it like foiling in algebra, first
  and last ions go together inside ions go
  together
• Example
• AgNO3(aq) NaCl(s) ? AgCl(s) NaNO3(aq)
• Another example
• K2SO4(aq) Ba(NO3)2(aq) ? 2KNO3(aq) BaSO4(s)

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5. Combustion Reactions

• Combustion reactions occur when a hydrocarbon
  reacts with oxygen gas.
• CxHy O2 ? CO2 H2O
• Products in combustion are ALWAYS carbon dioxide
  and water.
• Example
• CH4 (g) 2O2(g) ? CO2(g) 2H2O(g)