Introduction: Why Do Reactions Feel Hot or Cold?
Imagine holding a hand warmer on a frosty morning, or pressing a cold pack against a sprain. Both of these rely on chemical reactions that transfer energy , one releasing heat, the other absorbing it. To understand why this happens, we need to explore the concept of enthalpy change and how chemists measure the energy exchanged during chemical reactions.
This subtopic covers the core ideas you need:
- How energy flows between a reaction and its surroundings
- The difference between exothermic and endothermic reactions
- How to use energy profile diagrams
- How to calculate enthalpy changes from experimental data
- The practical considerations of calorimetry experiments
These concepts underpin much of the rest of IB Chemistry's Reactivity strand , a solid grasp here will pay off throughout the course.
Energy Transfer: System, Surroundings, and Conservation of Energy
Every chemical reaction involves energy being exchanged between two parts of the universe:
System: The system is the specific part of the universe under study , typically the reactants and products involved in the reaction.
Surroundings: The surroundings are everything outside the system that can interact with it , the reaction flask, the water bath, the air, and so on.
The law of conservation of energy tells us that the total energy of the universe (system + surroundings) remains constant. Energy is never created or destroyed , it simply moves from one place to another.
Types of Systems
Systems are classified by what can cross their boundaries:
| System Type | Matter exchange | Energy exchange | Example |
|---|---|---|---|
| Open | ✓ | ✓ | Boiling water in an open pot |
| Closed | ✗ | ✓ | Sealed reaction flask |
| Isolated | ✗ | ✗ | Ideal thermos flask |
In most school calorimetry experiments, we aim for a closed system , we don't want matter to escape, but we accept that some heat may be lost to the surroundings (hence the need for insulation).
