CHEMISTRY YEAR 910

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CHEMISTRY YEAR 9-10

• Writing and Balancing Equations

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Writing and Balancing Equations

• A chemical equation is a shorthand expression for
  a chemical change or reaction.
• word - equation states in words
• mercury (II) oxide heat mercury oxygen
• bulky and cumbersome
• 2 HgO 2Hg O2

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Writing and Balancing Equations

• General Format for Writing Equations
• 1. The reactants are separated from the products
  by an arrow that indicates the direction of
  the reaction.
• 2. The reactants are placed to the left and the
  products to the right of the arrow. A plus sign
  () is placed between reactants and between
  products when needed.

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Writing and Balancing Equations

• 3. Conditions required to carry out the
  reaction may be placed above or below the arrow.
  A delta indicates heat is applied.
• 4. Coefficients 2 H2O are placed in front to
  balance the equation. One is never placed there,
  it is just understood.

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Writing and Balancing Equations

• 5. The physical state of a substance is indicated
  by the following symbols
• (s) solid (l) liquid (g) gaseous (aq)
  aqueous means water solution
• yields reversible
• reaction
• H2 gas evolved solid precipitate

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Writing and Balancing Equations

• Balancing Equations
• 1. First you must have the equation in symbol
  format, you cannot balance a word equation.
• 2. Balancing
• a. Count and compare the number of atoms of
  each element on each side of the equation and
  determine those that must be balanced.

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Writing and Balancing Equations

• Balancing Equations
• b. Balance each element one at a time, by
  placing whole numbers (coefficients) in front of
  the formulas containing the unbalanced element.
• It is usually better to balance in this order
  metals, nonmetals, hydrogen, oxygen.

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Writing and Balancing Equations

• A coefficient placed before a formula multiplies
  every atom in the formula by that number!
• 2H2SO4 4Hs 2Ss 8Os
• c. re-check each time you balance an element
  to see if anything else has become unbalanced.
  Make adjustments as needed.

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Writing and Balancing Equations

• d. Do a final check making sure that each
  element is balanced and that the smallest
  possible set of whole number coefficients has
  been used
• 4HgO 4Hg 2O2 incorrect
• 2HgO 2Hg O2 correct

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Writing and Balancing Equations

• Equations for practice
• Magnesium oxygen Magnesium Oxide
• Potassium Chlorate Potassium Choride
  Oxygen
• Aluminum hydroxide sulfuric acid
• Aluminum sulfate water
• C4H10 O2 CO2 H2O

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Writing and Balancing Equations

• Types of Chemical Reactions
• 1. Combination or Synthesis Reaction
• Two reactants combine to give one product.
• A B AB
• 2Mg O2 2MgO

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Writing and Balancing Equations

• Types of Chemical Reactions
• 2. Decomposition
• A single substance is decomposed or broken
  down to give two or more different substances
• AB A B
• 2PbO2 2PbO O2
• 2 Na HCO3 Na2CO3 H2O CO2

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Writing and Balancing Equations

• Types of Chemical Reactions
• 3. Single - Displacement Reaction
• One element reacts with a compound to take the
  place of one of the elements of that compound.
• A BC B AC
• Zn 2HCl H2 ZnCl2
• Note Activity series table helps to predict
  which substances (elements) will be able to
  replace various other substances.

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Writing and Balancing Equations

• Types of Chemical Reactions
• 4. Double - Displacement or Metathesis
• Two compounds exchange partners with each
  other to produce two different compounds.
• AB CD AD CB
• NaCl KNO3 NaNo3 KCl

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Writing and Balancing Equations

• Heat in Chemical Reactions
• Energy changes always accompany chemical
  reactions. One reason why reactions occur is
  that the products attain a lower, more stable
  energy state than the reactants.

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Writing and Balancing Equations

• Heat in Chemical Reactions
• Reactions are either exothermic or endothermic.
  
• Exothermic reactions liberate heat
• H2 Cl2 2HCl 185kJ (exothermic)
• Endothermic reactions absorb heat
• N2 O2 181kJ 2NO (endothermic)