Reactions and Reaction Rates

Due to the overwhelming number of chemical reactions it’s helpful to classify them. Chemical reactions can be classified by the type of chemical change that takes place. Most chemical reactions fall into one of five categories: synthesis (combination), decomposition, single replacement (substitution), double replacement, and combustion. Each type of chemical reaction has a specific pattern of how reactants become products.

In a synthesis reaction, two or more elements or compounds combine to form a single product. This type of reaction is represented by the following general equation. A + B -> AB, a single substance as the product is the key characteristic of a synthesis reaction.  One example of this kind of reaction is the formation of sodium chloride, or table salt, by the combination of sodium metal and chlorine gas. Another example of a synthesis reaction is the formation of water via the fusion of hydrogen gas and oxygen gas.

In a decomposition reaction, a single reactant, a compound, breaks down into two or more parts. This type of reaction is represented by the general equation AB -> A+B. There is one substance as a reactant and more than one substance as the product. This is the key characteristic of a decomposition reaction. One example of a decomposition reaction is the electrolysis of water (passing an electrical current through water) which forms hydrogen gas and oxygen gas.

In a single replacement-reaction, also called single-displacement, a more active element replaces a less active element in a compound.  Such a reaction is represented by the general equation A+BC -> AC+B. For example, in the reaction, Fe+CuSO₄ -> FeSO₄+Cu, iron which is more reactive than copper, replaces the copper in copper sulfate and forms iron sulfate. Metal plating is a process that uses a single-replacement reaction to coat one metal with another. An iron nail placed in a copper sulfate solution will be coated with copper.

In a double-replacement reaction (also known as double-displacement reaction) the ions of two compounds exchange places in an aqueous solution (dissolved in water) to form two new compounds. One of the resulting products from such a reaction is often a gas, a precipitate (solid), or water. A double-replacement reaction is represented by the general equation AB+CD -> AD+CB. The reaction needed to make a cake rise begins with a double replacement reaction. Baking soda and the acidic ingredients in cake batter exchange ions producing carbonic acid. This acid then decomposes into carbon dioxide gas and water. The carbon dioxide gas makes the cake rise.

In a combustion reaction, a compound and oxygen react to produce a new product and heat. Flame is the most recognizable form of a combustion reaction, while an explosion is a faster form of the reaction. Many of the reactants in a combustion reaction contain mostly carbon and hydrogen atoms which react with oxygen to produce heat, CO2 and H2O. Common examples of combustion reactions include the burning of wood or fuel, and the combustion of food nutrients that power our bodies. The following balanced equation represents the combustion of natural gas (methane) used to heat many homes: CH₄(g)+2O₂(g) -> CO₂(g)+2H₂O(g)+energy.