Chemical Reactions

Reaction Equations

Consider the reaction equation below:

This equation is like a recipe for reactions.  It says that one methane (CH4) reacts with two oxygen molecules (O2) to produce one carbon dioxide molecule (CO2) and two water molecules (H2O).  The arrow, referred to specifically as a completion arrow, separates the reactants from the products.  The reactants are the compounds to the left of the arrow.  They are the compounds used to initiate a reaction.  The compounds to the right of the arrow are the products, the compounds produced from the reaction.  Sometimes a formula, or symbol like the triangle above will appear above or below the completion arrow.  These symbols or notation refer to a catalyst necessary for the reaction.  The usage of the triangle as seen above specifically indicates that heat is a catalyst.  The (g) indicates the state of matter for a particular reactant or product, in this case all compounds are gases.  A (s) denotes a solid, a () denotes a liquid, and (aq) indicates the substance is aqueous, which means dissolved in water.

A full list of reactions and their specific properties can be found here.


The reaction of a hydrocarbon (or more broadly, an organic compound) with oxygen in the presence of heat to produce carbon dioxide and water vapor. 
• If the supply of oxygen is limited (incomplete combustion), carbon monoxide is produced instead of carbon dioxide.
• If the supply of oxygen is sufficient, or in excess (complete combustion), carbon dioxide is produced as stated above.
• When nothing else is indicated, go with carbon dioxide.
• If elements such as nitrogen and sulfur are part of the reactant, nitrogen dioxide and sulfur dioxide will also be produced.

Below are some examples of combustion reactions:

Frequently, when water is shown as a product in a combustion reaction, its given state of matter is a liquid. This is not wrong since the combustion products, if left to cool to room temperature, will allow the water vapor to condense. The carbon dioxide will still be far above its sublimation point and remain a gas. Trapping a lit candle under a glass jar and then allowing it to cool after the flame has extinguished will prove the presence of water. Small droplets of it should be noticed condensing on the side of the glass.

Combustion reactions are strongly exothermic, meaning ΔHrxn is very negative. Due to the large number of moles of gaseous products, there is a large positive entropy change also associated with combustion reactions. For these reasons, combustion reaction are strongly spontaneous, meaning that the values of ΔGrxn are very negative. Additionally, the values of K are unreasonable to calculate as they often exceed 10100, if not prove too large an order of magnitude for most computers and calculators to handle.

Synthesis (Direct Combination)

The combination of two or more elements/compounds to produce one compound. Below are some examples of synthesis reactions:

Often synthesis reactions require a catalyst, as seen in the third example above. This is especially true when two solids are reacting with each other. Solid particles do not provide enough contact or have enough activation energy to react on their own. Catalysts such as water (helps increase surface contact between particles) and heat (helps overcome activation energy) help mitigate these factors. Since synthesis reactions by definition have fewer moles of products than reactants (a net loss of particles) they are usually associated with a negative change in entropy. Consider that many synthesis reactions also produce solids and liquids (low entropy states of matter) and occasionally consume gasesous reactants (high entropy state of matter) to do so, and the matter is exacerbated. In order for these reactions to be spontaneous at room temperature they must also be exothermic, and in fact many of them have very negative ΔHrxn values.


The breaking down of a compound (frequently with heat or electricity) into its elements or other smaller compounds.
Chlorates - decompose into metal chlorides and oxygen.  The metallic chloride produced will initially be a liquid sue to heat, but will eventually cool back to a solid.  Heat is required.  Example:

Carbonates - decompose into metal oxides and carbon dioxide:

The decomposition of a compound will always increase the number of particles in a reaction. That is, there will always be more moles of products than reactants. As in the special cases noted above, carbonates and chlorates (both are solids) yield a gaseous product upon decomposition. These are the reasons for decomposition reactions causing a very positive entropy change. However, these reactions are not often spontaneous at 25°C. This is because these reactions are usually endothermic, meaning that substantial energy must be added to reactant in order for the decomposition to proceed. Realize that the heat required for a decomposition is different than the heat needed for combustion to take place. In combustion, heat is merely a catalyst, and once the reaction starts, the source of heat does not need to be continuously applied. For decomposition reactions, the reactant will only decompose for as long as the source of heat is continuously applied.

Single Displacement/Replacement

The more active metal or halogen takes the place of another metal or halogen in a compound.  The replaced metal or halogen is left in its free state.
• Always consult an activity series to make sure the reaction works.
• The more active metal will always replace a metal of lower reactivity.
• The more active nonmetal will always replace a nonmetal of lower reactivity.

This reaction works because magnesium is more reactive (listed higher on the activity series) than the metal it is trying to replace, copper.

Similarly, this reaction also works because bromine is a more reactive (listed higher on the activity series) than the nonmetal it is trying to replace, iodine.

Double Displacement/Replacement or Metathesis

The ions of two aqueous compounds switch partners to produce at least one non-aqueous product (gas, water, or precipitate).
• Always consult a solubility table to make sure the reaction works.
• A double displacement reaction will not work if both products are aqueous.

Acid-Base - Acid/base reactions are a special type of double displacement reaction. Recall that at least one product cannot be aqueous. In the case of an acid/base reaction, that product is liquid water. Liquid water will always be produced because it is formed from the union of the H+ ion from the acid and the hydroxide (OH-) ion from the base. More information about the reactions of acids and bases can be found on the acid/base page.

Acid-Carbonate - The first of three hybrid double displacement/decomposition reactions. In this reaction, carbonic acid or H2CO3, is a product when the hydrogen ion from the acid combines with the carbonate ion. However, due the lack of stability of carbonic acid at room temperature, it immediately decomposes into carbon dioxide gas and water.

Acid-Sulfite - The second of three hybrid double displacement/decomposition reactions. In this reaction, sulfurous acid or H2SO3, is a product when the hydrogen ion from the acid combines with the sulfite ion. However, due the lack of stability of sulfurous acid at room temperature, it immediately decomposes into sulfur dioxide gas and water.

Base-Ammonium - The last of three hybrid double displacement/decomposition reactions. In this reaction, ammonium hydroxide or NH4OH, is a product when the hydroxide ion from the base combines with the ammonium ion. However, due the lack of stability of ammonium hydroxide at room temperature, it immediately decomposes into ammonia gas and water.


A reaction where one element loses electrons (oxidized) from reactants to products, and another element gains electrons (reduced) from reactants to products.
• Single displacement and combustion reactions are ALWAYS redox.
• Reactions involving free elements as either reactants or products are redox. This will include many synthesis and decomposition reactions.
• Double displacement reactions are NEVER redox.  The charges of the ions do not change when they switch.
• Look for polyatomic ions that have been broken down because they usually contain an element that has been oxidized or reduced.

More information on redox reactions can be found here.