Chapter 11 – Chemical Reactions

  1. Definition of a chemical equation
  1. Chemical reaction definition: any interaction among chemical substances that brings about some change.
  2. How do we know that a chemical change has occurred?
  1. Gas production.
  2. Heat production/absorption.
  3. Permanent change in color.
  4. Appearance of a precipitate. A precipitate is an insoluble substance, this means you see a powder form on the bottom of the beaker or flask.
  1. Chemical Equation definition: a method for expressing a chemical reaction in symbols and formulas.
  1. We must know how the substances react and what they form to write a chemical equation.
  2. Every chemical equation must be balanced. The number of atoms on the left side must equal the number of atoms right side.
  1. Law of Conservation of Mass. The concentration of mass says that matter is neither created nor destroyed in a chemical reaction. In other words, you cannot create or destroy matter in a chemical reaction.
  1. They can be written as complete equations or ionic equations.
  1. Terms, Symbols and their meanings.
  1. Reactants: substances which interact, written on the left side.
  2. Products: substances that are formed, written on the right side.
  3. Single arrow : goes in one direction (direction of the arrow).
  4. Double arrow: goes in both directions – reversible.
  5. (g), (l), (s), (aq): gas, liquid, solid, solution is dissolved in water, respectively.
  6. : heat.
  7. Catalyst: any substance that increases the rate of a chemical reaction, but is recovered unchanged at the end of the reaction.
  1. Usually written above or below the arrow.

III.Guidelines for balancing equations

  1. There are various ways to balance equations. These guidelines are generally applicable for most of the chemical equations you will encounter in this class.
  1. Write the correct formulas for the reactants and the products.
  1. Once you have the correct formula, you will not change it during the balancing process. Instead, you will add coefficients in front of the formula to obtain a balanced equation.
  1. Begin the process by selecting a specific element to balance.
  1. Generally, you select an element from the compound that has the greatest number of atoms. Also, you select the element present in the largest number, excluding hydrogen and oxygen and polyatomic ions that remain the same on both sides of the equation.
  1. Balance the polyatomic ions that remain the same on both sides of the equation.
  2. Balance the H and then the O atoms, which were not balanced in Step 3.
  3. Check all the coefficients to make certain that they are whole numbers and in the lowest possible ratio.
  4. Check to make certain that it is balanced.

Example:

Calcium hydroxide + phosphoric acid  calcium phosphate + water

Ca(OH)2(aq) + H3PO4(aq) Ca3(PO4)2(s) + H2O(l)

3 Ca(OH)2(aq) + 2 H3PO4(aq) Ca3(PO4)2(s) + 6 HOH(l)

  1. Examples

a): Fe(s) + HCl  FeCl2(aq) + H2(g)

Fe(s) + 2 HCl  FeCl2(aq) + H2(g)

b): C4H10(g) + O2 CO2 + H2O(fraction coefficient)

C4H10(g) + 13/2O24 CO2 + 5H2O

2 C4H10(g) + 13 O28CO2 + 10 H2O

  1. Five types of sample chemical equations
  1. Combination:

A + Z  AZ

  1. Decomposition:

AZ  A + Z

  1. Single Replacement reactions:

A + BZ  AZ + B

X + BZ  BX + Z

  1. Double displacement reactions:

AX + BZ  AZ + BX

  1. Neutralization reactions:

HX + MOH  MX + HOH