Chemical Equations

Writing Chemical Equations

A chemical equation consists of two sides: the reactant and the product. It simply describes the inputs and the outputs, much like a function.


Specifying State of Matter

There are 4 states of matter, which tell the physical state of the species and can
affect the reaction rate + its energy change.

  • (g) Gas - e.g. O2
  • (l) Liquid - e.g. water
  • (s) Solid - e.g. solid NaOH
  • (aq) Aqueous Solution - used when x is dissolved in water

Balancing Chemical Equations


Stoichiometry deals with the relative qualities of reactants and products. Because atoms can neither be created nor destroyed, we need to ensure the number of each element on each side is equal. We manipulate these with co-efficients.

Rules for Chemical Equations


When we write these there are a few basic rules

  • Reactants on left, products on right
  • States of matter are included next to the element
  • We balance the equation by changing co-efficients
    • Numbers within a species are not modified.
  • Nett ionic equations are used - only essential changes are conveyed, anything that remains the same on each side is taken out.
  • Water reactions: H2O is added to balance oxygen, or H+ to balance hydrogen.
  • Ions in a solution are treated as separated - as they would be, being dissolved


Q: Aluminium Sulfate solution and Calcium Nitrate solution are reacted to give a precipitate of Calcium Sulfate and a solution of Aluminium Nitrate. Write the equation:

  • Al2(SO4)3 - solution = Al3+(aq) + SO42-(aq)
  • Al3+(aq) + SO42-(aq) + Ca2-(aq) + NO3-(aq) -> CaSO4(s) + Al3+(aq) + NO3-(aq)
  • SO42-(aq) + Ca2-(aq) -> CaSO4(s)