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The Meaning of a Chemical Equation

-Chemical equations give information in two major areas.

-First, they tell us what substances are reacting (those being used up) and what substances are products (those being made).

-Second, the coefficients of a balanced equation tell us in what ratio the substances react or are produced.

-The reactants are on the left side of a chemical equation and the products are on the right side.

-However, you might ask, "On the left and right side of what?"

-Answer - the arrow.

2 H2 + O2 ---> 2 H2O

-Coefficients are the numbers in front of the formulas.

-Here is the example equation again:

2 H2 + O2 ---> 2 H2O

-The coefficients of an equation tell us how many moles of each reactant are involved as well as how many moles of each product get produced.

Balancing chemical equation 


Balancing equations is another process of chemistry that requires some thought and problem solving skills. To balance an equation, you have to equalize everything on the reactant side with that on the product side. Every element has to be equally represented on either side of the equation.

Sometimes you will see a chemical equation that must be balanced. For example, suppose you were going to burn some propane gas (C3H8). Combining propane with oxygen results in carbon dioxide and water.

C3H8 + O2 --> CO2 + H2O ??

You can see that the number of carbon (C), oxygen (O) and hydrogen (H) atoms on the left of the equation does not equal the number on the right side. There are 3 C, 8 H, and 2 O on the left and 1 C, 3 O, and 2 H on the right.

Use trial-and-error

So, to balance the equation, you must do some clever trial-and-error guesses. Sometimes the unbalanced equation is written with unknowns, similar to what you would do in Algebra:

wC3H8 + xO2 --> yCO2 + zH2O

where w, x, y and z are the unknown numbers from of each molecule in the equation.

Logical approach

One logical, trial-and-error approach to balancing this chemical equation is as follows:

1.    Since there are 8 H on the left, perhaps there are 4 H2O on the right.

2.    Since there are 3 C on the left, perhaps there are 3 CO2 on the right. 

3.    The resulting equation is then: C3H8 + O2 --> 3CO2 + 4H2O ??

4.    The C's and H's balance, but there are 10 O on the right and only 2 on the left. So, let's try 5 O2 on the left.

Now the equation balances out.

C3H8 + 5O2 --> 3CO2 + 4H2O

Count the number of carbon atoms, hydrogen atoms, and oxygen atoms on the left and compare with the number on the right side of the equation.


Chemical equations are similar to algebraic equations, in that the total number of atoms of each element on the left side must equal the number for that element on the right side. You can have complex equations and series of equations for some chemical reactions. You usually can use a logical trial-and-error method to balance a chemical equation.


Types of Chemical Reaction


-Single Replacement:


During single replacement, one element replaces another element in a compound. There are two different possibilities:

1. One cation replaces another. Written using generic symbols, it is:

AX + Y ---> YX + A

Element Y has replaced A (in the compound AX) to form a new compound YX and the free element A. Remember that A and Y are both cations (postively-charged ions) in this example.

Some examples are:

Cu + AgNO3 ---> Ag + Cu(NO3)2
Fe + Cu(NO3)2 ---> Fe(NO3)2 + Cu
Ca + H2O ---> Ca(OH)2 + H2

2. One anion replaces another. Written using generic symbols, it is:

A + XY ---> XA + Y

Element A has replaced Y (in the compound XY) to form a new compound XA and the free element Y. Remember that A and Y are both anions (negatively-charged ions) in this example.

Cl2 + NaBr ---> NaCl + Br2
Br2 + KI ---> KBr + I2

In single replacement, one reactant is always an element. It does not matter if the element is written first or second on the reactant side. The other reactant will be a compound.

-Double Replacement:

During double replacement, the cations and anions of two different compounds switch places.

Written using generic symbols, it is:

AB + XY ---> AY + XB

A and X are the cations (postively-charged ions) in this example, with B and Y being the anions (negatively-charged ions).

Some examples are:

KOH + H2SO4 ---> K2SO4 + H2O
FeS + HCl ---> FeCl2 + H2S
NaCl + H2SO4 ---> Na2SO4 + HCl
AgNO3 + NaCl ---> AgCl + NaNO3

In double replacement, both reactants are compounds, each with a cation part and an anion part. Diatomic elements do not count; they are included in the single replacent category.


During decomposition, one compound splits apart into two (or more pieces). These pieces can be elements or simpler compounds

Written using generic symbols, it is usually shown as:

AB ---> A + B

However, that really only works for splitting apart into the elements, like these examples.

HgO ---> Hg + O2
H2O ---> H2 + O2
MgCl2 ---> Mg + Cl2
FeS ---> Fe + S

Decomposition can also split one compound into two simpler compounds (or compound and an element) as in these examples:

CaCO3 ---> CaO + CO2
Na2CO3 ---> Na2O + CO2

1) All binary compounds will break down into their elements.
2) All carbonates break down to the oxide and carbon dioxide.
3. Chlorates (like KClO3 and Ba(ClO3)2 ) will break down to the binary salt and oxygen.


Synthesis are, at this introductory level, almost always the reverse of a decomposition reaction. That means that two pieces join together to produce one, more complex compound. These pieces can be elements or simpler compounds. Complex simply means that the product compound has more atoms than the reactant molecules.

Written using generic symbols, it is usually shown as:

A + B ---> AB

These are some examples:

Mg + O2 ---> MgO
H2 + O2 ---> H2O
K + Cl2 ---> KCl
Fe + O2 ---> Fe2O3

Since synthesis reactions are the reverse of decomposition

1) Direct union of two elements will produce a binary compound.
2) Metallic oxides and carbon dioxide react to produce carbonates.
3. Binary salts and oxygen react to produce a chlorate.


Combustion, at its most general, can mean the reaction of oxygen gas (O2) with anything.

However, we will understand combustion to mean the reaction of oxygen with an compound containing carbon and hydrogen. A common synonym for combustion is burn.

Written using generic symbols, it is usually shown as:

CxHy + O2 ---> CO2 + H2O

These are some examples:

CH4 + O2 ---> CO2 + H2O
C2H6 + O2 ---> CO2 + H2O
C6H12O6 + O2 ---> CO2 + H2O

Activity series of the elements


Activity series is a list of elements organized according to the ease with which the elements undergo certain chemical equation.


The Activity of Metals

(Li) Lithium

(Rb) Rubidium

(K) Potassium Reacts with liquid water and

(Ba) Barium acids, and replaces Hydrogen

(Sr) Strontium

(Ca) Calcium

(Na) Sodium

(Mg) Magnesium

(Al) Aluminum

(Mn) Manganese Reacts with steam

(Zn) Zinc and acids, and

(Cr) Chromium replaces Hydrogen

(Fe) Iron

(Cd) Cadmium

(Co) Cobalt

(Ni) Nickel Reacts with acids

(Sn) Tin replaces Hydrogen

(Pb) Lead

(H2) Hydrogen

(Sb) Antimony Reacts with oxygen

(Bi) Bismuth to make oxides

(Cu) Copper

(Hg) Mercury

(Ag) Silver

(Pt) Platinum Unreactive. Forms

(Au) Gold oxides undirectly


The Activity of Nonmetal Halogens

F2 Fluorine

Cl2 Chlorine

Br2 Bromine

I2 Iodine

An element at the top of a list will generally be more likely to replace an element below it in a single replacement reaction. Elements at the top, therefore, are unlikely to be found free in nature, but elements at the bottom are unlikely to be found free in nature.



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This site was last updated 09/26/03