5.1: Introduction to Chemical Reactions

Learning Objectives

• Define terms used to represent chemical reactions including reactants, products, states, coefficients, and stoichiometry, reaction conditions.
• Apply the law of conservation of matter to balanced chemical reactions
• Differentiate between the liquid phase and the aqueous phase

What are Chemical Equations?

Chemical equations are how chemists describe chemical reactions, the process by which one form of matter (reactants) turn into another form of matter (products). Below is the unbalanced chemical reaction for the combustion of methane (CH4) with oxygen (the reactants) to produce carbon dioxide and water (the products).

CH4(g) + O2(g) ® CO2(g) + H2O(g)
(unbalanced equation)

Law of Conservation of Mass: Balancing Reactions

Antoine Lavoisier (1743-1794) is attributed to introducing the Law of Conversation of Matter, which states that during a chemical process mass is neither created or destroyed, and thus the total mass of the products equals the total mass of the reactants. This was first published in his 1789 "Traité Élémentaire de Chimie" and in France this was known as Lavoisier's Law, "Rien ne se perd, rien ne se crée, tout se transforme. (Nothing is lost, nothing is created, everything is transformed."). The Gutenberg Project offers a free English translation of the book (Elements of Chemistry)..

As matter is made of compounds and elements, and the compounds may change, this means the total number of atoms of each element must be conserved. In the above equation only atoms of carbon are conserved, and so the equation is not balanced. In the equation below, that atoms of each element are conserved, and so it is balanced.

CH4(g) + 2O2(g) ® CO2(g) + 2H2O(g)
(balanced equation)

Stoichiometric Coefficients:

Once the reactants and products are written, you are not able to change the subscripts of the compound to balance the chemical reaction without changing the identity of the reactants and products. However, you may use stoichiometric coefficients in front of each chemical entity (molecule or element) to balance the equations. The word "stoichiometry" come from the Greek stoicheion (element) and metrein (to measure).

What other information can be included in a chemical equation?

• Phases
• (s) - Solid
• (l) - liquid
• (g),(v) - gas, vapor
• (aq) - aqueous
• Reaction Conditions (often indicated above the arrow)
• ($$\Delta$$) - Heat
• Catalyst

Aqueous vs. Liquid Phase:

Liquid is a phase, the condensed fluid phase (in contrast to a gas, which is the dispersed fluid phase). Aqueous is a solution where something is dissolved in water, the solvent. NaCl(l) is molten sodium chloride, that is obtained by heating it above the melting point. NaCl(aq) is aqueous sodium chloride, which is made by dissolving the salt into water. At one atm pressure and normal temperatures, you can have NaCl(aq) (salt water), but you can't have molten NaCl (l), because the melting point is 1,474oF.

Catalysts:

Catalysts can be understood by looking at the mechanism of a reaction, that is, how it actually proceeds, and can be considered to be chemicals that function as both a reactant and a product. That is, the catalyst reacts with something and form a new chemical, an intermediate, and that intermediate also reacts, and reproduces the catalyst. Catalysts can speed up reactions, or make reactions happen which without the catalyst are so slow that they don't really happen.