Energy Profile: In an exothermic reaction, the products have a lower enthalpy than the reactants. This means the system has lost energy to the surroundings, usually in the form of heat.
Sign of : Because the final energy state is lower than the initial state, the enthalpy change is always negative ().
Stability: Products in an exothermic reaction are considered more energetically stable than the reactants because they exist at a lower potential energy level.
Observation: A thermometer placed in the surroundings will show a temperature increase as the chemical system 'sheds' its internal energy.
Energy Profile: In an endothermic reaction, the products possess more enthalpy than the reactants. The system must absorb energy from the surroundings to reach this higher energy state.
Sign of : Since the system gains energy, the enthalpy change is positive ().
Stability: The products are less energetically stable than the reactants. This often means the reaction requires a continuous input of energy to proceed or maintain the product state.
Observation: The temperature of the surroundings decreases as heat is drawn into the chemical bonds of the system.
It is critical to distinguish between an Enthalpy Level Diagram and a Reaction Profile Diagram. While they look similar, they serve different purposes in chemical analysis.
| Feature | Enthalpy Level Diagram | Reaction Profile Diagram |
|---|---|---|
| Focus | Thermodynamics (Start vs. End) | Kinetics (The path taken) |
| Activation Energy | Not shown | Shown as a 'hump' or peak |
| Transition State | Not represented | Represented at the peak |
| Purpose | Shows net energy change | Shows energy barriers to reaction |
An enthalpy level diagram only cares about the 'before' and 'after' states. It does not provide information on the speed of the reaction or the energy required to break initial bonds.
Arrow Direction: Always ensure the arrow for points from the reactant level to the product level. In exothermic diagrams, the arrow points down; in endothermic diagrams, it points up.
Labeling Axes: The vertical axis should be labeled 'Enthalpy' or 'Energy', and the horizontal axis should be labeled 'Progress of Reaction' or 'Extent of Reaction'.
State Symbols: Always include state symbols (s, l, g, aq) in the chemical equations on the diagram. A change in state (e.g., gas to liquid) involves its own enthalpy change, which can significantly alter the diagram's levels.
Sign Consistency: Double-check that the sign of your calculated matches the visual representation. A negative value must correspond to a downward step.
Confusing Temperature with Enthalpy: Students often think a temperature rise means is positive. In reality, a temperature rise in the surroundings means the system lost energy, so is negative.
Adding Activation Energy: Do not draw a curve or peak between the reactant and product levels unless specifically asked for a reaction profile. Enthalpy level diagrams use simple horizontal lines and vertical arrows.
Thermodynamic vs. Kinetic Feasibility: Just because a reaction is exothermic (thermodynamically favorable) doesn't mean it will happen quickly. If the activation energy is too high, the reaction is 'kinetically controlled' and may not occur at room temperature.
