Topic 4.3 Notes – Representations of Reactions

1. Chemical Equations as Particle Accounting

A balanced chemical equation is a bookkeeping system for atoms. It follows the law of conservation of mass, which says atoms are rearranged in a reaction, not created or destroyed.

Take this reaction:

$$\ce{2H2(g) + O2(g) -> 2H2O(g)}$$

That equation tells you:

• Ratios of particles (moles): 2 molecules of $\ce{H2}$ react with 1 molecule of $\ce{O2}$.
• Atom conservation: 4 H atoms and 2 O atoms on both sides.
• Bond changes: H-H and O=O bonds break; O-H bonds form.
• Physical states: (g), (l), (s), (aq) tell you how particles exist.

Important reminders:

• Coefficients change amounts. Subscripts change identity.
  $\ce{2NO2}$ is two nitrogen dioxide molecules.
  $\ce{N2O4}$ is a different compound.
• Coefficients represent the simplest whole-number ratio.
• If the equation is unbalanced, any diagram based on it will also violate conservation.

That’s why balancing comes first.

2. Types of Reaction Representations

These all describe the same reaction, just at different levels.

Molecular Equation

Shows all compounds intact, including states.

Example:

$$\ce{BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)}$$

This gives the full picture of reactants and products.

Complete Ionic Equation

Break strong electrolytes (aq) into ions. Leave solids, liquids, and gases intact.

$$\ce{Ba^{2+}(aq) + 2Cl^{-}(aq) + 2Na^{+}(aq) + SO4^{2-}(aq) -> BaSO4(s) + 2Na^{+}(aq) + 2Cl^{-}(aq)}$$

Now you see every particle in solution.

Net Ionic Equation

Remove spectator ions (unchanged on both sides).

$$\ce{Ba^{2+}(aq) + SO4^{2-}(aq) -> BaSO4(s)}$$

This shows what actually changed.

If every ion stays dissolved and unchanged, you write no reaction. That shows up often in multiple choice.

Particulate Model

A diagram that shows:

• Atoms as spheres
• Molecules as bonded spheres
• Ions separated (with charges if shown)
• Correct particle counts

Here’s what the precipitation reaction above would look like before and after at the particle level:

Particulate view of a precipitation reaction forming BaSO₄(s)

Notice that $\ce{Ba^{2+}}$ and $\ce{SO4^{2-}}$ come together to form the solid at the bottom, while $\ce{Na+}$ and $\ce{Cl-}$ remain dispersed in solution. Those are the spectator ions.

3. How to Balance Chemical Equations

Balancing is systematic, not guessing.

1. Check if already balanced.
2. Balance elements in only one compound per side.
3. Balance elements appearing in multiple compounds.
4. Leave H and O for last (usually).
5. Recount everything.
6. Reduce to lowest whole-number ratio.

Example:

$$\ce{C3H8 + O2 -> CO2 + H2O}$$

Balance C → 3 $\ce{CO2}$
Balance H → 4 $\ce{H2O}$
Now count O: $3 \times 2 + 4 \times 1 = 10$ O atoms → 5 $\ce{O2}$

Final:

$$\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}$$

Common patterns:

• Diatomic elements: $\ce{H2, N2, O2, F2, Cl2, Br2, I2}$
• Combustion: hydrocarbon + $\ce{O2}$ → $\ce{CO2}$ + $\ce{H2O}$

For AP free response, always double-check atom counts before moving on.

4. Translating Between Equations and Particles

This is the heart of 4.3.

From Equation to Diagram

If you have:

$$\ce{2NO(g) + O2(g) -> 2NO2(g)}$$

Your drawing must show:

• 2 separate NO molecules
• 1 O₂ molecule
• After reaction: 2 NO₂ molecules
• Same total N and O atoms before and after

That is exactly what the particulate diagram below represents: two NO molecules and one O₂ molecule before collision, rearranging into two NO₂ molecules with atom counts conserved.

Particulate representation of 2NO(g) + O₂(g) → 2NO₂(g)

For aqueous ionic reactions, separate strong electrolytes into ions in the drawing.

From Diagram to Equation

1. Count each type of particle.
2. Identify what bonds changed.
3. Write correct formulas.
4. Add states.
5. Reduce coefficients.

AP trick: diagrams often show multiple “sets.” If you see 4 identical reaction events, divide to lowest whole-number ratio before writing the equation.

5. Physical vs Chemical Changes

Not every diagram shows a reaction.

Chemical Change

• Bonds break/form
• New substances appear
• Atom rearrangement

Example: gas bubbles forming or a solid precipitating.

Physical Process

Particle identity stays the same.

Dissolving $\ce{KBr(s)}$:

• Crystal lattice breaks apart
• $\ce{K+}$ and $\ce{Br-}$ separate
• Charges stay the same
• Ions disperse in water

No new substance formed. Just separated particles.

On exams, students often call dissolving a “reaction.” It isn’t unless a new substance forms.

6. Common AP Mistakes

• Changing subscripts instead of coefficients
• Forgetting diatomic elements
• Drawing intact ionic compounds in aqueous solution
• Not conserving atoms in diagrams
• Forgetting to reduce coefficients

If the particle count in your diagram doesn’t match the equation exactly, something is wrong.