Chapter 11 What is a chemical reaction?

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Chapter 11 What is a chemical reaction?
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Reactions involve rearrangements of atoms.

• Reactants are converted into products.
• The law of conservation of mass states that mass
  cannot be created nor destroyed.
• This means that the number of atoms in the
  product must be equal to the original reactants.

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• Atoms do not become other kinds of atoms, nor do
  they appear or disappear.

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Chemical reactions release or absorb energy

• Endothermic reaction- reaction in which energy is
  absorbed.
• - Bond formation always requires energy.
• Exothermic reaction- reaction in which energy is
  released.
• Bond breaking always releases energy.

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• Particles must collide for a chemical reaction to
  occur.
• Example, when a safety match is lit, the reaction
  begins when the two substances are brought
  together by striking the match head across the
  striking surface.
• If this collision happens with enough energy, the
  bonds in the reactants are broken, allowing new
  bonds to form between the atoms.

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Balancing Equations
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Balancing requires patience

• To satisfy the law of conservation of mass, you
  should insert coefficients into the chemical
  equation in order to balance the number of
  elements.
• There must be equal numbers of atoms for each
  element on each side of the equation when an
  equation is balanced.

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation CH3CH2OH O2 CO2
  H2O
• Carbon atoms 2
  1 No
• Hydrogen atoms 6
  2 No
• Oxygen atoms 3
  3 Yes

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• To balance this equation, there are two carbon
  atoms in the reactant column and only one in the
  product column.
• To balance the number of carbon atoms you need to
  double the number of carbon dioxide molecules in
  the products.

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• CH3CH2OH ? O2 2CO2 ?
  H2O
• The 2 is called a coefficient and indicates
  that there are two carbon dioxide molecules.
• Coefficients are normal whole numbers, when a
  coefficient is equal to 1, the 1 is not written
  for simplicity.
• Coefficients are written in front of the
  formula and multiply the entire formula.

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation CH3CH2OH O2 2CO2
  H2O
• Carbon atoms 2
  2 Yes
• Hydrogen atoms 6
  2 No
• Oxygen atoms 3
  5 No

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• The carbon atoms are now balanced by the hydrogen
  atoms are not. In addition the oxygen atoms have
  been thrown out of balance.
• Lets start with hydrogen, there are six in the
  reactant column, so we must add three to the
  product column.

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation CH3CH2OH O2 2CO2
  3H2O
• Carbon atoms 2
  2 Yes
• Hydrogen atoms 6
  6 Yes
• Oxygen atoms 3
  7 No

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• The oxygen atoms are still unbalanced.
• Multiplying the number of oxygen molecules by
  three results in an equal number of oxygen atoms.

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation CH3CH2OH 3O2 2CO2
  3H2O
• Carbon atoms 2
  2 Yes
• Hydrogen atoms 6
  6 Yes
• Oxygen atoms 7
  7 Yes

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• Now you have written a balanced chemical
  equation.
• Replacing the question marks in the formula
  equation with the right coefficients now give a
  correct chemical equation.

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Tips for balancing equations

1. Save the elements that appear in several
   reactants or products for last. (usually oxygen
   and hydrogen )
2. Balance lone elements last.
3. Balance the equation from left to right.
4. For ionic equations, be sure that charges are
   balanced.

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation NH3 O2
  NO H2O
• Nitrogen atoms 1
  1 Yes
• Oxygen atoms 2
  2 Yes
• Hydrogen atoms 3
  2 No

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation NH3 O2
  NO H2O
• Nitrogen atoms
• Oxygen atoms
• Hydrogen atoms

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Testing an equation for balance

• Reactants Products
  Balance?
• Unbalanced
• Formula equation 4NH3 5O2 4NO
  6H2O
• Nitrogen atoms 4
  4 Yes
• Oxygen atoms 10
  10 Yes
• Hydrogen atoms 12
  12 Yes