Representing Chemical Changes (College Board AP Chemistry)

• Both, physical and chemical changes can be represented with balanced equations
• A balanced equation shows the chemical symbols of all the species and their state of matter
  • The state symbols are written as subscripts on the right side of each formula
  • Additional to the three states of matter, there is aqueous state which means dissolved in water
  • The table below show the four state symbols used in equations

The four state symbols that show the physical state of substances

• An equation is divided in reactants (starting substances) and products (substances produced)
  • In physical changes, the reactants are the same as the products but in different states of matter
  • In chemical changes, the reactants and the products have totally different identities

Balanced equations representing physical and chemical changes

A physical and a chemical change represented by balanced equations. All the reactants and products show their state symbols

• Chemical changes are represented by balanced chemical equations
• During a chemical reactions, atoms cannot be created or destroyed
  • This shows that the mass is conserved
• An equation is balanced when there is the same number of atoms of each element on both sides of the equation
• The numbers in front of chemical formulas are called coefficients
  • The coefficients show the amount of molecules involved during a chemical reaction

Balanced chemical equation

Balanced chemical equation showing the reaction between hydrogen gas and oxygen gas to produce water. The number of atoms of hydrogen and oxygen is the same in reactants and products

• When reactions occur in aqueous media, there are three different ways of representing a balanced equation: molecular, complete ionic and net ionic
• A molecular equation shows the complete chemical formula of reactants and products
• In aqueous solution, the ionic compounds dissociate into their ions. Therefore, they can be written in a separate way
• A complete ionic equation shows all the soluble compounds separated into their ions
• If an ion appears on both sides of the complete ionic equation, it is called an spectator ion
  • Spectator ions remain unchanged on each side of the equation
• A net ionic equation can be obtained when spectator ions are canceled out front he complete ionic equation
  • A net ionic equation just includes ions and molecules involved in the reaction
• The charge is also conserved in complete ionic equations and net ionic equations. Therefore, the charge must be the same on both sides when the equation is balanced

Three ways to represent a reaction in aqueous solution

Molecular equation, complete ionic equation and net ionic equation for the reaction of lead(II) nitrate and potassium iodide to produce lead(II) iodide and the potassium nitrate