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Balancing Chemical Equations
 

A balanced equation has equal numbers of each type of atom on each side of the equation.

The Law of Conservation of Matter is the rationale for balancing a chemical equation. It is defined as: the total mass of materials present after a chemical reaction is the same as the total mass before the reaction. Last chapter we learned how to measure atoms and molecules in terms of moles. Rather than compare the mass of the reactants to the mass of the products, we will compare the number of moles of each atom before and after a chemical reaction.

An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. It is important to note that ONLY the coefficients can be changed, NEVER a subscript .

Lets look at the following example: H2 + O2 --> H2O
The correct way to balance the above equation is as follows: 2 H2 + O2 --> 2 H2O
Note that in order to balance this equation coefficients were placed in front of H2 and H2O. The number one is never used as a coefficient. If no coefficient is written, it is understood to be one.

After balancing an equation you should always check to see that there are the same number of each atom on either side of the arrow. If we check our equation above we see that there are 4 hydrogen atoms on the left (2x2=4) and 4 hydrogen atoms on the right (2x2=4); there are 2 oxygen atoms on the left and 2 oxygen atoms on the right (2x1=2).

Two things you CANNOT do when balancing an equation.
1. You cannot change a subscript. For example, in the following equation you cannot change oxygen's subscript in water from one to two, as in: H2 + O2 --> H2O2
True, this balances the equation, but you have changed the substances in it. H2O2 is a completely different substance from H2O.
2. You cannot place a coefficient in the middle of a formula. The subscript goes at the beginning of a formula, not in the
middle, as in: H2 + O2 --> H22O

Three rules to follow when balancing chemical equations. (They do not work every time, but they do work most times and most people find them helpful.)
1. If there is an element that is not bonded to any other atom or is diatomic (H2, N2, O2, F2, Cl2, Br2, I2) balance it last. For example, in the equation: CO2 + H2O --> C6H12O6 + O2, oxygen is diatomic(O2). When balancing this equation, balance oxygen last.
2. If you are balancing an equation with both hydrogen and oxygen and neither is a diatomic molecule, balance hydrogen last and balance oxygen second to last. For example, in the equation: CO2 + NH3 --> OC(NH2)2 + H2O, both hydrogen and oxygen are used and neither exist as a diatomic molecule. When balancing this equation, balance hydrogen last and oxygen second to last.
3. If you come to the point where you have an odd number of a certain element on one side of the equation and an even number of the same element on the opposite side of the equation, try doubling the coefficient. If the coefficient is one, change it to two. For example, in the equation: Fe + O2 --> Fe2O3 , there are 2 oxygen atoms on the left side of the equation and three oxygen atoms on the right side of the equation. Putting a 2 in front of Fe2O3 gives you an even number of oxygen atoms on each side of the equation and makes it easier to balance.

Additional Hints.

  • Make sure that your final set of coefficients is reduced to the lowest whole number ratio.
  • The only way to become skilled at balancing chemical equations is to practice. Practice as many as you can. Before long balancing will be like second nature.
  • Also, you may want to use pencil instead of pen. You will be doing a good deal of erasing at first.

    Practice Problems:
    Balance each of the following.
    K + Br2 --> KBr Al + O2 --> Al2O3
    P4 + O2 --> P2O5 H2O2 --> H2O + O2
    K2O + H2O -->KOH C2H2 + O2 --> CO2 + H2O

    Click here for the answers.

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