MHS Chemistry
Chemical Reactions

In order to make sense of the huge array of chemical reactions observed, chemists classify them into a few basic categories. The biggest categories are “organic” and “inorganic” reactions. Organic reactions involve the millions of compounds of carbon, and there are thousands of “named” reactions, as well as every biochemical reaction. The scope of this course only includes the most basic organic chemistry, and only one kind of classified reaction.
             

Inorganic reactions include all the other compounds, including some of carbon. Typical compounds include mineral acids, bases, salts, and other compounds with metals or ionic bonds.
             
All chemical reactions involve chemical changes. They start with some reactant chemicals, which change to become some product chemicals. We show them by listing the reactants on the left side and the products on the right, and we use an arrow (“ ® “) to indicate that a change is taking place. We never ever use an equals sign, because the point of a chemical change is that you end up with new compounds with new properties. Here is the general form for writing a chemical reaction as a chemical equation.
             
Reactant A + Reactant B ® Product C + Product D
 

We will classify reactions into six types for now.

Synthesis reactions are reactions in which two or more reactants combine to form a single new product.
Reactant A + Reactant B ® Product C    
N2 + 3H2 ® 2NH3    
 

In the second reaction, with the actual chemicals, the numbers in front of the hydrogen and ammonia are called coefficients. They are necessary to account for all the mass present at the start and finish of a chemical reaction. According to the Law of Conservation of Mass, “matter cannot be created nor destroyed in ordinary chemical reactions” (this is not entirely true in nuclear reactions, but they are not “ordinary” chemical reactions either).

Balancing chemical equations means finding the coefficients so that the total number of each kind of atom on each side is the same.  The most common way to do this at first is by the "guess and check" method:  See if the first atom you see is the same on both sides, and adjust coefficients as needed.  Then check the second atom you see, and so on.  This may take several repetitions to complete.

Time can be saved if you recognize that in many cases, polyatomic ions do not change during the reaction.  So right after you balance Na+, you can balance NO3- all at once.  Coefficients distribute to everything in the molecule.  Subscripts only apply to whatever they immediately follow.
 
Decomposition reactions are the opposite of synthesis reactions: A single reactant breaks down into one or more simpler products.  These reactions often fit a few patterns, which are described briefly here.
Reactant A ® Product B + Product C + …
2NH3 ® 3H2 + N2    
2H2O2 ® 2H2O + O2    
N2O4 ® 2NO2        
             
Replacement reactions are one in which an element replaces another in a compound. These are also sometimes called "Single Displacement" or plain "Replacement" or "Displacement" reactions.
Element A + Reactant ® Element B + Product
Mg + HCl ® H2 + MgCl2
MgCl2 + Br2 ® MgBr2 + Cl2
             
Double Replacement reactions are one in which parts of two compounds trade places. These are also known as “Double Displacement” or “Metathesis” reactions.
Reactant A + Reactant B ® Product C + Product D
WX + YZ ® WZ + YX
AgNO3 + ZnCl2 ® AgCl + Zn(NO3)2
NaOH + HCl ® NaCl + HOH
 
The last reaction above is a special category of double replacement reaction, because it involves and acid (HCl) and a base (NaOH). This can also be called a "neutralization" reaction, because the products are neither acid nor base. We will concentrate on neutralization reactions when we study acids and bases.
             
The fifth type of reaction we will be concerned with is called a Combustion reaction. All combustion reactions are burning reactions, but not all burning is a combustion reaction. Combustion is specifically a hydrocarbon combining with oxygen to form water and carbon dioxide. A hydrocarbon is a compound made of just hydrogen and carbon (surprise, surprise). Combustion reactions can also start with a carbohydrate (C, H, & O) instead of just a hydrocarbon.
Hydrocarbon + Oxygen ® Water + Carbon dioxide
CH4 + O2 ® H2O + CO2
C6H12O6 + O2 ® H2O + CO2
             
NOT COMBUSTION:
Mg + O2 ® MgO    
CH4 + O2 ® H2O + CO
H2CO3     ® H2O + CO2
             
The sixth reaction type, for reactions that do not fit any of the patterns above, is Other.
             
For many reactions to practice balancing, and categorizing, click here.
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