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(2Al(ss) + 3Br2() + 3Br2(ll) → 2 AlBr3) → 2 AlBr3

• The physical state of each substance can The physical state of each substance can be indicated by using (be indicated by using (ss) for solid, () for solid, (ll) for ) for liquid, (liquid, (gg) for gas, and () for gas, and (aqaq) for an aqueous ) for an aqueous solution.solution.

Introduction (continued)

• 2Al(s) + 3Br2(l) → 2AlBr3(s)

• The coefficients can be interpreted in The coefficients can be interpreted in terms of: terms of: – Individual species Individual species

• 2 Al atoms react with 3 Br2 molecules 2 Al atoms react with 3 Br2 molecules – Amounts in moles of species Amounts in moles of species

– 2 moles of Al react with2 moles of Al react with

Introduction (continued)

• Chemical change does not result in Chemical change does not result in destruction or creation of atoms. Thus all destruction or creation of atoms. Thus all atoms in the reactants must also be atoms in the reactants must also be present in the products.present in the products.

• 2Al(s) + 3Br2(l) → 2AlBr3(s)

• In the above equation there are 2 Al atoms In the above equation there are 2 Al atoms and 6 Br atoms, thus and 6 Br atoms, thus matter is conserved.matter is conserved.

Step 1

• Identify all substances in the given Identify all substances in the given problem problem

• Write their chemical symbols and/or Write their chemical symbols and/or formulas formulas – Pay attention to your charges!! Pay attention to your charges!! – This means use subscripts but do not use This means use subscripts but do not use

coefficients at this pointcoefficients at this point

Step 1 - Example

• The reaction of zinc with aqueous The reaction of zinc with aqueous hydrochloric acid produces a solution of hydrochloric acid produces a solution of zinc chloride and hydrogen gas.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)