1. How does one complete equations when given reactants only?
Completing Balanced Chemical Equations that Represent Chemical Reactions
How does one complete equations when given reactants only?

2. Decomposition If given only one reactant (a binary compound)
The products are the elements that make up the compound
Remember which elements are diatomic (H O F Br I N Cl)
For phases of matter of elements:
Liquids: Hg and Br2
Gases: H2, N2, O2, F2, Cl2, and the Noble Gases
All other elements are solids
Then balance the equation!

3. Decomposition Example
The decomposition of mercury (II) oxide
Unbalanced: HgO(s)  Hg(l) + O2(g)
Balanced: 2 HgO(s)  2 Hg(l) + O2(g)

4. Synthesis (Combination)
If given only two reactants (both are elements)
The only product is the binary compound formed from those elements
Look up the charges of ions from those elements. Determine the formula of the neutral compound based on the charges.
For phases of matter of compounds:
Ionic Compounds (M to NM): solid if there is no indication of water (aq) on the reactant side
Covalent (Molecular) Compounds: Many should be Common Knowledge: CO2(g) and H2O(l) for example
Then balance the equation!

5. Synthesis (Combination) Example
The reaction between sodium and chlorine
Reactants: Na(s) and Cl2(g)
Charges of ions (in product): Na+ and Cl-
Formula of Product: NaCl(s)
Unbalanced: Na(s) + Cl2(g)  NaCl(s)
Balanced: 2 Na(s) + Cl2(g)  2 NaCl(s)

6. Single Replacement
If given two reactants (one element and one compound)
Focus on the element from the reactants
If the element is a metal, it would replace the compound’s first element (another metal or H)
If the element is a non-metal, it would replace the compound’s second element (another non-metal)
For the products, write the formula of the element that is “kicked out” of the original compound
Then determine the formula of the new compound based on the charges of the elements/ions involved
** Check Table J to determine if reaction will occur spontaneously

7. Determining Whether a Single Replacement Reaction Will Occur
A reaction will occur spontaneously (actually happen) only if the element is more reactive than what it would replace
Example: Mg(s) + HCl(aq) 
Mg (a metal) replaces H because Mg is more reactive than H2
The new compound involves Mg2+ and Cl-
Mg(s) + 2 HCl(aq)  H2 (g) + MgCl2(aq)
Example: Cl2(g) + 2 NaI(aq) 
Cl2 (a non-metal) replaces I because Cl2 is more reactive than I2
The new compound involves Na+ and Cl-
Cl2(g) + 2 NaI(aq)  I2(s) + 2 NaCl(aq)

8. Determining Whether a Single Replacement Reaction Will Occur
A reaction will not occur spontaneously (there is no reaction) if the element is less reactive than what it would replace
Example: Cu(s) + HCl(aq) 
Cu (a metal) would replace H, but there is no reaction because Cu is less reactive than H2
Example: I2(s) + NaCl(aq) 
I2 (a non-metal) would replace Cl, but there is no reaction because I2 is less reactive than Cl2

9. Double Replacement Reactions
If given two reactants that are both compounds
Determine the charge of each ion in the reactants (ignore any subscripts that are not part of the formula of a polyatomic ion)
Use ion charges to determine the formula of the products (positive ion first, negative ion second)
Ex. CuSO4(aq) + Na2CO3(aq) 
Ion Charges: Cu2+ , SO42- , Na+ and CO32-
Recombine ions: CuCO3 and Na2SO4
Without phases: CuSO4 + Na2CO3  CuCO3 + Na2SO4

10. Double Replacement Reactions
Check Reference Table F: Solubility Guidelines to determine the solubility of the products
*A reaction will occur spontaneously (actually happen) when a product is an insoluble (solid) compound, a gas (CO2 or H2S), or a molecular substance (H2O)
There is a visible reaction when CuSO4(aq) and Na2CO3(aq) are mixed because CuCO3 is as insoluble solid
CuSO4(aq) + Na2CO3(aq)  CuCO3(s) + Na2SO4 (aq)
*A reaction will not occur (no reaction) if both potential products are soluble (aqueous)
Ex. KCl(aq) + Na2CO3(aq) 
K2CO3 is soluble (aq) and NaCl is soluble (aq)
Therefore: KCl(aq) + Na2CO3(aq)  no reaction as all ions remain dissolved

11. Completing Organic Combustion Reactions
If given 2 reactants (compound with C,H and the element O2)
*Do not mistake this for Single Replacement!
Products will be CO2(g) and H2O(l)
Then balance…
Ex. CH4(g) + O2(g) 