4.3 Representations of Reactions

TLDR

In AP Chemistry, representing reactions means translating a balanced chemical equation into a particulate model that shows individual atoms, molecules, and ions before and after a reaction. The key rule is consistency: the same atoms have to appear on both sides, the coefficients have to match the number of particles you draw, and charges have to be conserved.

Why This Matters for the AP Chemistry Exam

This topic connects the symbolic level (balanced equations) to the particle level (what is actually happening to atoms and molecules). That connection is exactly the kind of reasoning the exam rewards.

You will see particulate diagrams in multiple-choice questions where you match a balanced equation to the correct "after" picture, or pick the equation that matches a drawing. On free-response questions, you may need to draw or interpret particle-level representations and explain how they show conservation of mass and charge. Getting comfortable moving between equations and particle pictures also sets you up for later units like kinetics and equilibrium, where particle diagrams show up again.

Key Takeaways

• A balanced equation can be translated into a particulate model, and the model must match the equation's coefficients exactly.
• Atoms are conserved: the number of each type of atom is the same before and after, even though atoms rearrange into new substances.
• Charge is conserved too, so ions in your drawing must add up to the same total charge on both sides.
• Coefficients tell you how many particles to draw, while subscripts tell you how atoms are bonded within one molecule or formula unit.
• Particulate models can show physical processes (dissolving, phase changes, mixing) where identity stays the same, as well as chemical reactions where new substances form.
• Use consistent colors and sizes for each element and include a legend so your model is clear.

Why Balance Reactions First

You can't build an accurate particle picture without a correctly balanced equation, so balancing comes first.

The law of conservation of mass states that matter is not created or destroyed in a closed system. A chemical reaction is a closed system, so the number of atoms going in must equal the number coming out. Atoms rearrange into new combinations, but the total count of each element stays the same. That means the number of atoms of each element on the reactant side must equal the number on the product side.

Example 1

Here's the equation for forming carbon dioxide: CO (g) + O₂ (g) → CO₂ (g)

Step 1. Check whether it is already balanced. Count each element on both sides. There is 1 carbon on each side, so carbon is balanced. There are 3 oxygen atoms on the reactant side and 2 on the product side, so oxygen is not.

Step 2. Look at elements that appear in only one compound on each side and are already balanced. Here that is carbon, so CO and CO₂ will need the same coefficient. Leave both at 1 for now.

Step 3. Balance elements that appear in only one compound on each side but have different counts. None apply here (the next example has one).

Step 4. Balance elements that appear in multiple compounds on one side. That's oxygen. There is less oxygen on the product side, so raise the coefficient of CO₂ to 2:

CO (g) + O₂ (g) → 2CO₂ (g)

Because carbon must stay balanced, raise CO to 2 as well:

2CO (g) + O₂ (g) → 2CO₂ (g)

Double-check:

Both sides match, so conservation of mass is satisfied.

Example 2

Here's the equation: Li (s) + N₂ (g) → Li₃N (s)

Step 1. Check whether it is balanced. There is 1 lithium on the reactant side and 3 on the product side. There are 2 nitrogen atoms on the reactant side and 1 on the product side. Neither is balanced.

Step 2. Look for elements in only one compound on each side with equal counts. None exist here.

Step 3. Balance elements in only one compound on each side with different counts. Both lithium and nitrogen fit. This is partly guess-and-check, but you can work it strategically.

There are 3 lithium atoms on the right, so raise Li to 3:

3Li (s) + N₂ (g) → Li₃N (s)

Lithium is balanced. Now nitrogen: 2 on the left, 1 on the right. Raise Li₃N to 2:

3Li (s) + N₂ (g) → 2Li₃N (s)

Nitrogen is balanced, but lithium is off again. There are now 6 lithium atoms on the right, so raise Li to 6:

6Li (s) + N₂ (g) → 2Li₃N (s)

Step 4. Balance elements in more than one compound. None apply here.

Double-check:

Steps to Balancing Equations

The more you practice, the faster this gets. Keep these steps in mind:

1. Check whether the equation is already balanced.
2. Find elements that appear in only one compound on each side with equal counts. These need equal coefficients.
3. Balance elements that appear in only one compound on each side with different counts.
4. Balance elements that appear in more than one compound.
5. Double-check by confirming each element has the same count on both sides, satisfying conservation of mass.

Particulate Representations of Reactions

A balanced equation can be translated into a particulate model, a drawing that shows individual atoms, molecules, or ions before and after a reaction. These models let you see what is happening at the particle level.

Key Principles for Particulate Models

1. Conservation of atoms: the same number of each type of atom appears on both sides.

2. Conservation of charge: if you show ions, the total charge must be equal before and after.

3. Proper representation:

   • Atoms are shown as spheres, often color-coded by element.
   • Molecules show atoms bonded together.
   • Ions are shown with + or - charges.

4. Consistent scale: use the same relative sizes and colors for each element throughout.

Example: CO + O₂ to CO₂

For the balanced equation 2CO (g) + O₂ (g) → 2CO₂ (g):

• Before: 2 CO molecules (each 1 carbon bonded to 1 oxygen) and 1 O₂ molecule (2 oxygens bonded together).
• After: 2 CO₂ molecules (each 1 carbon bonded to 2 oxygens).
• Total each side: 2 carbon atoms and 4 oxygen atoms.

Example: Formation of Lithium Nitride

For 6Li (s) + N₂ (g) → 2Li₃N (s):

• Before: 6 separate Li atoms and 1 N₂ molecule.
• After: 2 formula units of Li₃N (each with 3 Li⁺ ions and 1 N³⁻ ion in an ionic structure).
• The atoms rearrange, but the total number stays constant.

Physical Processes vs. Chemical Reactions

Particulate models can also show physical processes:

• Dissolving: a solid ionic compound separating into individual ions surrounded by water molecules.
• Phase changes: particles moving closer (condensation) or farther apart (evaporation).
• Mixing: different types of molecules spread throughout the space.

In a physical process, the chemical identity stays the same. Only the arrangement or state of the particles changes. (Note that dissolving an ionic solid can be argued either way, since it involves breaking ionic bonds and forming ion-dipole interactions.)

How to Use This on the AP Chemistry Exam

MCQ

• Match a balanced equation to the correct "after" particle diagram, or do the reverse.
• Count atoms in a diagram and check that they match the coefficients. A picture that doesn't conserve atoms is wrong.
• For ionic reactions, also confirm the total charge is the same on both sides.

Free Response

• If asked to draw, start from the balanced equation so your particle counts are right.
• Label or use a legend so it is clear what each sphere represents.
• Show molecules with atoms bonded together and show free ions as separate particles with their charges.
• Be ready to explain in words how your drawing demonstrates conservation of mass (and charge, for ionic systems).

Drawing Particulate Models

1. Start with the balanced equation so you know exactly how many of each particle to draw.
2. Use a legend showing what each colored sphere represents.
3. Keep it simple and show only the essential features.
4. Check conservation by counting atoms before and after.
5. Show proper bonding: atoms within a molecule are connected, while separate ions are drawn apart.

Common Trap

A diagram can have the right total atom count but still be wrong if the atoms are bonded incorrectly. For example, leftover separate O atoms instead of O₂ molecules, or the wrong number of molecules, both break the match with the equation.

Common Misconceptions

• Changing subscripts to balance. You can only change coefficients. Editing a subscript changes the actual substance into something different.
• Thinking coefficients and subscripts mean the same thing. Coefficients count whole particles; subscripts count atoms within one particle. In a drawing, the coefficient tells you how many molecules to draw, and the subscript tells you how to build each one.
• Forgetting charge conservation. It is easy to balance atoms and ignore ion charges. For ionic reactions, the total charge must match on both sides of your model.
• Drawing the wrong number of particles. If the equation says 2CO₂, you draw 2 separate CO₂ molecules, not 1 molecule with a "2" floating next to it.
• Assuming dissolving is purely physical or purely chemical. Dissolving an ionic solid breaks ionic bonds and forms ion-dipole interactions, so it can reasonably be argued either way.
• Showing atoms appearing or disappearing. A particulate model must conserve atoms. If your "after" picture has more or fewer atoms than the "before," it is incorrect.

Review Activity

Part A: Balance the following equations and include the states of matter for reactants and products.

1. Na₃PO₄ + AgNO₃ → Ag₃PO₄ + NaNO₃
2. A reaction between iron (III) oxide and carbon monoxide
3. The combustion of ethane (C₂H₆)
4. The synthesis of sulfur trioxide
5. The decomposition of potassium chlorate

Part B: Practice with Particulate Models

1. Draw a particulate representation of the synthesis of water: 2H₂ (g) + O₂ (g) → 2H₂O (g)
2. Given a particulate diagram showing 4 NH₃ molecules and 3 O₂ molecules as reactants, draw the products after the reaction (assuming complete reaction)
3. Sketch a particulate model showing NaCl dissolving in water (physical process)

Answers to Balancing Practice Problems

Question 1) Na₃PO₄ (aq) + 3AgNO₃ (aq) → Ag₃PO₄ (s) + 3NaNO₃ (aq)

Question 2) Fe₂O₃ (s) + 3CO (g) → 2Fe (s) + 3CO₂ (g)

Question 3) 2C₂H₆ (g) + 7O₂ (g) → 4CO₂ (g) + 6H₂O (l)

Question 4) 2SO₂ (g) + O₂ (g) → 2SO₃ (g)

Question 5) 2KClO₃ (s) → 2KCl (aq) + 3O₂ (g)

Related AP Chemistry Guides

• Unit 4 Overview: Chemical Reactions
• 4.1 Introduction to Reactions
• 4.2 Net Ionic Equations
• 4.6 Introduction to Titration
• 4.5 Stoichiometry
• 4.7 Types of Chemical Reactions