Enriched Chemistry

Chapter 8 – Chemical Equations and Reactions

Section 1 – Describing Chemical Reactions

  • chemical reaction - the process by which one or more substances are changed into one or more different substances.
  • The original substances are known as the reactants and the resulting substances are known as the products.
  • According to the law of conservation of mass, matter can be neither created nor destroyed.
  • Therefore, the total mass of the reactants must equal the total mass of the products in a chemical reaction.

Chemical reactions have physical indicators.

1. Evolution of energy as heat and light.

  • Some physical changes involve heat and light, so they are only strong indicators, not conclusive proof.

2. Color change.

  • Same as above.

3. Production of a gas.

4. Formation of a precipitate.

Chemical equations must satisfy the law of conservation of mass.

Please keep the following things in mind when writing and reading chemical equations correctly.

  1. All reactants and products must be identified.
  2. The equation must contain the correct formulas for the reactants and products.

Remember the elements that normally exist as diatomic molecules (p. 249)

  1. The law of conservation of mass must be satisfied.

Atoms are neither created nor destroyed in a reaction.

coefficients are small whole numbers that appear in front of a formula and are used to balance equations.

Writing Word Equations

  • Word equations have only qualitative meanings. They do not give quantities of reactants used or products formed.
  • ex:methane + oxygen carbon dioxide and water
  • The arrow is read as react toyield, yields, produces or forms. How would you read the equation above? Write it out in words.

Writing Formula Equations

  • The next step is to replace the names of reactants and products with the correct symbols and formulas.

CH4 (g) + O2 (g)CO2(g) + H2O(g)

Writing Balanced Equations

  • The equation must now be balanced to satisfy the law of conservation of mass.
  • There must be the same number of atoms of each element on both sides of the equation.

CH4 (g) + O2 (g)CO2(g) + H2O(g)

Additional Symbols Used in Chemical Equations

  • See Figure 1.5 on page 252.

Practice Problems

1. Sodium plus chlorine produces sodium chloride.

2. Zinc II oxide reacts with carbon to produce zinc plus carbon dioxide.

3. Aluminum reacts with zinc II chloride to produce zinc metal plus aluminum chloride.