Writing Chemical Equations
- A chemical equation is a symbolic representation of what happens during a chemical reaction.
- To describe the reaction you did between baking soda and hydrochloric acid in words you would write: Sodium Bicarbonate reacts with Hydrochloric Acid to produce Carbon Dioxide, Water, and Sodium Chloride.
- In symbolic form we would write: NaHCO3 + HCl --> CO2 + H2O + NaCl
- Everything to the left of the arrow is called the reactants, everything to the right, the products.
- One can even add the state of each substance in the reaction.
(s) = solid (l) = liquid
(g) = gas (aq) = aqueous (dissolved in water)
Using the above, the reaction becomes:NaHCO3(s) + HCl(aq) --> CO2(g) + H2O(l) + NaCl(aq)
- Reactions with only ionic compounds as reactants.
- Typically ionic compounds won't react with each other unless they are dissolved in water. Therefore, most of our reactions with ionic compounds will be in the aqueous phase.
- A reaction only occurs if one of the products formed would be insoluble in water. When an insoluble compound is formed from a reaction between two aqueous solutions, we call this compound a precipitate. See the Precipitation Rules Sheet to learn if an insoluble compound would form.
- When combining two aqueous ionic compounds you basically have four different ions floating around in solution. The positive and negative ions from each compound have the opportunity to come in contact and react. If the new compound formed is insoluble then a precipitate forms.
- We can write the reaction between Sodium Chloride and Lead(II)Nitrate
in several ways.
- In words it would be:
Sodium Chloride + Lead(II) Nitrate --> Sodium Nitrate + Lead(II) Chloride
- In formulas it would be
NaCl(aq) + Pb(NO3)2(aq) --> NaNO3(aq) + PbCl2(s) (see an illustration of this below)
- Notice that the NaNO3 is still dissolved. Basically, the sodium and nitrate ions did not really do anything. They were floating around dissolved in solution before and after the reaction.
- In words it would be:
- Reactions can occur with all different kinds of substances. The one described
above is typical of how ionic substances react with each other. However, elements
and compounds (ionic, molecular, and acid) also react together, although in
more or less predictable ways. For example:
- CuSO4(aq) + Zn(s) --> ZnSO4(aq) + Cu(s)
- Ca(s) + Cl2(g) --> CaCl2(s)
- H2SO4(aq) + Mg(s) --> MgSO4(aq) + H2(g)
- NaHCO3(s) + HCl(aq) --> NaCl(aq) + CO2(g) + H2O(l)