The single replacement reaction between zinc metal and hydrochloric acid can be represented by the following unbalanced equation:Zn s + HCl aq ZnCl aq + H g To balance the equation, we need to make sure that the number of atoms of each element is the same on both sides of the equation. In this case, we need to balance the number of chlorine atoms. We can do this by adding a coefficient of 2 in front of HCl:Zn s + 2 HCl aq ZnCl aq + H g Now, the equation is balanced:1 Zn atom + 2 H atoms + 2 Cl atoms 1 Zn atom + 2 Cl atoms + 2 H atoms