4.3 Representations of Reactions

Molecular vs. particulate views

Chemical reactions can be represented by chemical equations (symbols and formulas) or particulate diagrams (drawings showing atoms/molecules). Particulate representations show relative numbers, types, and arrangements of particles, helping visualize conservation of atoms and identify reactants, products, and physical states (solid, liquid, gas, aqueous) during a reaction.

Balancing and conservation of mass

A correctly written equation must be balanced — equal numbers of each atom type on both sides — reflecting the law of conservation of mass. Coefficients (not subscripts) are adjusted to balance atoms. Particulate models must also show the same total number of each atom before and after the reaction.

Interpreting particulate diagrams

In particulate diagrams, different colors/shapes represent different elements or molecules. You can determine the limiting reactant, identify excess reactant, and check stoichiometric ratios by counting particles. These diagrams reveal information not obvious from formulas alone, such as molecular arrangement or reaction mechanism steps.

Key terms

Particulate diagram

A drawing representing atoms/molecules with shapes or colors to show relative numbers and arrangements before and after a reaction.

Law of conservation of mass

Matter is neither created nor destroyed in a chemical reaction; atoms present in reactants must equal atoms in products.

Limiting reactant

The reactant that is completely consumed first, limiting the amount of product formed.

Excess reactant

The reactant that remains unreacted after the limiting reactant is used up.

Balancing coefficients

Numbers placed in front of chemical formulas to equalize the number of each atom type on both sides of an equation.

Try an AP-style question

What is present after the reaction goes to completion?

1. 4 H₂O and 1 O₂
2. 4 H₂O and 1 H₂
3. 6 H₂O only
4. 3 H₂O and 1 H₂