Topic 6.1 Notes – Endothermic and Exothermic Processes

Energy in Chemical and Physical Changes

Energy is the capacity to do work or transfer heat. In AP Chem, we mostly track how energy moves during reactions or physical changes.

Two types matter most:

• Kinetic energy (KE) = energy of motion
  • Temperature is proportional to the average KE of particles.
  • Higher temperature → particles moving faster.
• Potential energy (PE) = stored energy due to position/interactions
  • In chemistry, this is mainly energy in chemical bonds and electrostatic attractions.
  • Lower PE → more stable arrangement of particles.

So when something “releases energy,” what’s usually happening is that particles move to a lower potential energy state.

Law of Conservation of Energy

Energy cannot be created or destroyed. It only transfers or changes form.

In any process:

• Energy lost by the system = energy gained by the surroundings
• Total energy of the universe stays constant

This idea drives everything in this unit.

System and Surroundings

• System = what you are studying (usually the reacting chemicals)
• Surroundings = everything else (solution, container, air, etc.)

If you mix $\ce{HCl(aq)}$ and $\ce{NaOH(aq)}$, the reacting ions are the system. The water and beaker are surroundings.

On tests, mistakes almost always happen because students forget to answer from the system’s perspective.

Enthalpy and Energy Changes

To track heat flow at constant pressure (typical lab conditions), we use enthalpy (H).

$$\Delta H = H_{\text{final}} - H_{\text{initial}}$$

• $\Delta H$ depends only on initial and final states.
• That makes enthalpy a state function.
• Heat $q$ and work $w$ depend on how you get there, so they are not state functions.

At constant pressure:

• $\Delta H \approx q$ (heat transferred)

If $\Delta H$ is negative, the system’s energy decreased. If positive, it increased.

Endothermic and Exothermic Processes

Here’s the core comparison you need to know:

Exothermic

• Heat released by system
• Products have lower potential energy than reactants
• Surroundings warm up

Common examples:

• Combustion
• Many acid-base neutralizations
• Freezing or condensation

Endothermic

• Heat absorbed by system
• Products have higher potential energy than reactants
• Surroundings cool down

Common examples:

• Melting, vaporization
• Some dissolutions
• Thermal decomposition

Now connect that to a reaction coordinate diagram. The left panel shows an exothermic reaction and the right panel shows an endothermic reaction.

Exothermic and endothermic energy diagrams

Focus on the vertical difference between reactants and products in each case. That vertical gap is ΔH. Downward means ΔH is negative. Upward means ΔH is positive.

What Temperature Changes Tell You

In lab problems, you’re often given a temperature change and asked what type of process occurred.

• If solution temperature increases
  • surroundings gained heat
  • system released heat
  • exothermic
• If solution temperature decreases
  • surroundings lost heat
  • system absorbed heat
  • endothermic

If a metal reacts in solution and the beaker gets warm, that reaction is exothermic. The AP loves giving short experimental descriptions and asking you to classify the energy change.

Types of Processes That Can Be Endo or Exo

All energy changes fall into these categories:

Heating or Cooling a Substance

• Temperature change only
• KE changes
• No change in identity

Phase Changes

Energy changes involve intermolecular forces, not covalent bonds.

• Melting, vaporization, sublimation → endothermic
• Freezing, condensation, deposition → exothermic

Energy is required to separate particles and released when attractions form.

Chemical Reactions

• Breaking bonds absorbs energy
• Forming bonds releases energy
• Net ΔH depends on which effect is larger

Dissolution

When something dissolves, three energy steps occur:

1. Separate solute particles
2. Separate solvent particles
3. Form solute-solvent attractions

If new attractions are stronger overall → exothermic.

If weaker → endothermic.

Cold packs use endothermic dissolution. Some hot packs rely on exothermic dissolution or crystallization.