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Writing Chemical Equations
From Words to Numbers
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)
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.
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
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.
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
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)
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
Step 2 - Example
Zn(s) + HCl(aq)
ZnCl2(aq) + H2(g)