Chemistry Reference

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 (ℓ) denotes a liquid, and (aq) indicates the substance is aqueous, which means dissolved in water.

1.  Combustion – 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.

• 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:

4.  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.

• Always consult a solubility table to make sure the reaction works. 
• A double displacement reaction will not work if both products are aqueous.

• 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.

1. The following are gases at room temperature:
   • Elements − hydrogen, nitrogen, oxygen, fluorine, chlorine
   • ammonia
   • carbon monoxide and carbon dioxide
   • nitrogen monoxide and nitrogen dioxide
   • sulfur dioxide and sulfur trioxide
   • hydrogen-compounds (e.g. hydrogen chloride and hydrogen cyanide)
2. Acids are aqueous.  Any phrases that refer to being dissolved or in solution means the compound is aqueous.
3. Liquids − bromine, mercury, and water
4. All other elements are solids.  When in doubt, so are most other compounds, particularly ionic compounds.