1. Writing Chemical Equations
From Words to Numbers

2. Introduction
Chemical change is represented by an equation in which the formulas for reacting substances (reactants) are written to the left. An arrow separates these from the formulas for products.
2Al(s) + 3Br2(l) → 2 AlBr3(s)

3. Introduction (continued)
2Al(s) + 3Br2(l) → 2 AlBr3
The physical state of each substance can be indicated by using (s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution.

4. Introduction (continued)
2Al(s) + 3Br2(l) → 2AlBr3(s)
The coefficients can be interpreted in terms of:
Individual species
2 Al atoms react with 3 Br2 molecules
Amounts in moles of species
2 moles of Al react with

5. Introduction (continued)
Chemical change does not result in destruction or creation of atoms. Thus all atoms in the reactants must also be present in the products.
2Al(s) + 3Br2(l) → 2AlBr3(s)
In the above equation there are 2 Al atoms and 6 Br atoms, thus matter is conserved.

6. Step 1 Identify all substances in the given problem
Write their chemical symbols and/or formulas
Pay attention to your charges!!
This means use subscripts but do not use coefficients at this point

7. Step 1 - Example
The reaction of zinc with aqueous hydrochloric acid produces a solution of zinc chloride and hydrogen gas.
Zn(s)
HCl(aq)
ZnCl2(aq)
H2(g)

8. Step 2 Determine the sequence of events in the reaction
Identify the reactants and products
Set them up with an arrow going from reactants to products

9. Step 2 - Example
Zn(s) + HCl(aq)
ZnCl2(aq) + H2(g)