
Conservaton of mass justifies balancing method
Balancing a chemical equation is a fundamental first step to solving any chemical problem. Conservation of mass means that each atomic species on the reactant and product side of the equations must be balanced. One can applya set of coefficients a, b, c etc. for the reactants and x, y, z etc. for the products in order to set up the problem. Then one can set up "atom equations", which are composed of the coefficient times the number of atoms of the given type that appear in the molecule.
Independent net reactions
The algebraic method works in all cases. However, there are instances where the solution is not unique. The reason for this is that there can be more than one chemical reaction reaction contained in the given chemical equation. If the chemical equation consists of two or more independent net reactions, then there will be more than one possible solution. You can recognize this if you encounter a situation where the variables in are not completely specificed while carrying out the algebraic method. One of your sample problems is an example of this type. I will let you determine which one. The recommended procedure for separation of independent net reactions is to use the matrix method. We have demonstrated the matrix method in the lecture. However, we will not work problems using this method. It is not frequently needed although it is formally correct and it is the best way to establish that one has more than one independnet net reaction.
Algebraic method for balancing chemical equations
 Write the equation with coefficients a M1 + b M2 > x M3 + y M4.
 Set up atomic mass balance equations by counting the atoms of each type multiplying each coefficient.
 Make an initial guess using the equation that has the largest coefficient.
 Solve for the remaining coefficients and test for consistency where possible.
 


