I. CHEMICAL EQUATIONS—Chemical equations are a shorthand method used to illustrate and summarize what happens during a chemical reaction.
A. THE BASIC ANATOMY OF A CHEMICAL EQUATION
1. The formulae of reactants are written to the left of the arrow.
2. The formulae of the products are written to the right of the arrow.
3. Stoichiometric numbers in front of each reactant and product show the molar ratio of reactants and products.
B. WRITING AN EQUATION—There are a number of steps to writing a chemical equation.
1. Write the equation in words (this may or may not be required).
2. Replace the words with correct formulae for all the substances.
3. Add state symbols as subscripts (this may or may not be required).
(s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous
(meaning in solution with water)
II. BALANCING CHEMICAL EQUATIONS
A. CONSERVATION OF MASS
1. Once the equation has been written, it must be balanced because it is not possible to create or destroy matter.
2. All atoms that exist at the start of a reaction must also exist (and be accounted for) at the end of the reaction.
B. METHOD OF BALANCING
1. Balance each element in turn, remembering to multiply any brackets out carefully. This process is essentially one of trial and error, but the following tips can help.
i. If any element appears in only one compound on each side of the equation, try balancing that first.
ii. Any elements that appear in more than one compound should be balanced last.
iii. If one of the reactants or products appears as the free element, try balancing that last.
2. When balancing, only place numbers in front of whole formulae. Do not change the (correct) formulae of any of the reactants or products, or add any extra formulae.
If you are having difficulty in balancing an equation, go back and carefully check all of the formulae. If mistakes are made in constructing formulae, this can sometimes make an equation impossible to balance.
3. For example, balance the equation that summarizes the reaction between sulfuric acid and potassium hydrogen carbonate, to produce potassium sulfate, water, and carbon dioxide.
i. Write the equation and assess atoms initially present.
|
Left-hand side |
Right-hand side | |
|
3 H |
2 H | |
|
1 S |
1 S | |
|
7 O |
7 O | |
|
1 K |
2 K | |
|
1 C |
1 C |
ii. Work from left to right, as shown in the following section.
a) H appears in more than one compound, so leave until later.
c) O is balanced.
d) There is insufficient K on the left, so change the 1 in front of KHCO3 to a 2, to give the following equation.
|
Left-hand side |
Right-hand side | |
|
4 H |
2 H | |
|
1 S |
1 S | |
|
10 O |
7 O | |
|
2 K |
2 K | |
|
2 C |
1 C |
iii. Now the C and O are unbalanced. Change the 1 in front of CO2 to a 2, as shown in the following equation.
|
Left-hand side |
Right-hand side | |
|
4 H |
2 H | |
|
1 S |
1 S | |
|
10 O |
9 O | |
|
2 K |
2 K | |
|
2 C |
2 C |
iv. Finally, address the H and O. By simply changing the 1 in front of the H2O to a 2, both are balanced.
|
Left-hand side |
Right-hand side | |
|
4 H |
4 H | |
|
1 S |
1 S | |
|
10 O |
10 O | |
|
2 K |
2 K | |
|
2 C |
2 C |
When counting the number of atoms present, take care to multiply out brackets carefully, and to apply stoichiometric numbers carefully. For example, in the following reaction, the subscript 3 that follows the nitrate ion (NO3−) in Al(NO3)3 applies only to the N atoms and the O atoms to give a total of 1 Al, but 3 Ns and 9 Os in Al(NO3 )3. However, the stoichiometric number 3 in front of HNO3 means all atoms within HNO3 must be multiplied by 3 to give a total of 3 Hs, 3 Ns, and 9 Os in 3HNO3.
