Chapter 7 Intro to Chemical Reactions

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Chapter 7 Intro to Chemical Reactions

• 7.3 Balancing Chemical Equations

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Objective

• To learn to write a balanced chemical equation
  for a chemical reaction

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Balancing Chemical Equations

• In chemical reactions, atoms are neither created
  or destroyed
• Atoms are conserved in a chemical reaction
• Must have the same number and type of atoms on
  both the reactant AND product side of the
  equation
• The formulas of the compounds must never be
  changed when balancing an equation!
• You CANNOT change the subscripts

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Balancing Chemical Equations

• Balance by Trial and Error
• Ex H2 (g) O2 (g) ? H2O (g)
• Step 1 Count the atoms on the reactant and
  product side (make an atom inventory)
• Step 2 Choose the most complicated molecule,
  and balance by using coefficients
• Step 3 Continue adding coefficients until all
  atoms are balanced

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Balancing Chemical Equations

• Example 2
• C2H5OH (l) O2 (g) ? CO2 (g) H2O (g)

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Balancing Chemical Equations

• If you have the same polyatomic ion on both sides
  of the reaction, keep the polyatomic ion together
  when counting!
• FePO4 (s) Cu (s) ? Fe (s) Cu3(PO4)2 (s)

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Balancing Chemical Equations

• Now you try some!
• C2H6 O2 ? CO2 H2O
• 2C2H6 7O2 ? 4CO2 6H2O
• NH3 O2 ? NO H2O
• 4NH3 5O2 ? 4NO 6H2O

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Balancing Chemical Equations

• How to Write and Balance Equations
• Step 1
• Read the description of the chemical reaction.
  What are the reactants, products and their
  states? Write the appropriate formulas
• Step 2
• Write the unbalanced equation that summarizes the
  information from step 1.
• Step 3
• Balance the equation by trial and error, starting
  with the most complicated atom
• Step 4
• Check to see that the coefficients give the same
  number of each type of atom on both sides of the
  arrow. Also check to make sure that the
  coefficients used are the smallest integers that
  give the balanced equation.

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Balancing Chemical Equations

• Solid potassium reacts with liquid water to form
  gaseous hydrogen and potassium hydroxide that
  dissolves in water
• Reactants?
• Products?
• State of matter?
• Unbalanced equation?
• K (s) H2O (l) ? H2 (g) KOH (aq)
• Now balance the equation!

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Balancing Chemical Equation

• Glass is sometimes decorated by etching patterns
  on its surface. Etching occurs when hydrofluoric
  acid, (an aqueous solution of HF) reacts with the
  silicon dioxide in the glass to form gaseous
  silicon tetrafluoride and liquid water. Write
  and balance the equation for this reaction.

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Balancing Chemical Equations

• Gaseous nitrogen monoxide decomposes to produce
  dinitrogen monoxide gas and nitrogen dioxide gas.