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)