Chapter Three: Chemical Reactions and Reaction Stoichiometry

3.1 Chemical Equations

Chemical equations are the primary method chemists use to represent chemical reactions on paper. They provide a concise way to describe the transformation of reactants into products, using symbols and formulas to denote substances and their quantities.

• Chemical equations show the starting materials (on the left), called reactants, and the ending materials (on the right), called products.

• Arrows separate reactants from products, indicating the direction of the reaction.

• The + sign separates multiple starting or ending materials.

Example: The reaction between hydrogen and oxygen to form water is written as:

• Two reactants: hydrogen (H2) and oxygen (O2).

• One product: water (H2O).

In space-filling models, atoms are depicted as colored spheres: O atoms as red spheres, H atoms as white spheres. H2 is two white spheres fused together, O2 is two red spheres fused together, and H2O is a red sphere fused to two white spheres. Two molecules of H2O are shown.

Balancing Equations

Balancing chemical equations is essential to ensure the Law of Conservation of Mass is followed. This law states that matter cannot be created or destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides of the equation.

• Follow the Law of Conservation of Mass.

• Start with an element that is only in one reactant and one product.

• Balance by changing coefficients, not subscripts. (Like in math, a "1" is not written, but it is assumed.)

• Use trial and error: move on to other elements, without changing coefficients that are set, until complete. Check all elements at the end (H, then O).

Example: Combustion of Methane

The combustion of methane is a classic example of a balanced chemical equation. Methane reacts with oxygen to produce carbon dioxide and water.

• Reactants: methane (CH4) and oxygen (O2).

• Products: carbon dioxide (CO2) and water (H2O).

• Each side has 1 C, 4 H, and 4 O atoms.

In space-filling models: C atoms are black spheres, H atoms are white spheres, O atoms are red spheres. CH4 is a central black sphere with four white spheres. O2 is two red spheres fused together. CO2 is a black sphere fused to two red spheres. H2O is a red sphere fused to two white spheres.

Additional info:

Balancing equations is a fundamental skill in chemistry, necessary for stoichiometric calculations and understanding reaction mechanisms. Always check your work by counting atoms of each element on both sides of the equation.