Enthalpy Profile Diagrams Overview

Overview

Topic: Enthalpy profile diagrams for chemical reactions.
Objectives: Define system and surroundings, distinguish exothermic/endothermic reactions, and represent enthalpy changes on profile diagrams.
Context: B Tech applied science, unit on chemistry (enthalpy basics and diagrams).

Enthalpy (H): Thermal energy stored within a chemical system; conceptually H = internal energy (U) + pressure × volume.
System: The chemical species and their bonds where the reaction occurs.
Surroundings: External environment that exchanges energy with the system (practically water in lab measurements).
Enthalpy Change (ΔH): Measurable change in enthalpy during a reaction; absolute enthalpy cannot be measured directly.
Activation Energy (Ea): The energy barrier that must be overcome for a reaction to proceed; measured from reactants up to the peak of the energy profile.

Energy flow: From chemical system to surroundings.
System enthalpy: Decreases → ΔH is negative.
Surroundings: Gain energy; temperature increases (positive ΔT).
Energy conservation: Energy lost by system equals energy gained by surroundings.
Diagram features:
- Reactants start at higher enthalpy than products.
- Products are lower on the y-axis (enthalpy).
- Activation energy shown as vertical distance from reactants to peak.

Energy flow: From surroundings to chemical system.
System enthalpy: Increases → ΔH is positive.
Surroundings: Lose energy; temperature decreases (negative ΔT).
Diagram features:
- Products are higher on the y-axis than reactants.
- Activation energy measured from reactants to the peak (not from products to peak).

Axes:
- x-axis: Reaction pathway (progress of reaction).
- y-axis: Enthalpy (H).
Required labels to practice:
- Reactants and products positions (relative enthalpy).
- ΔH sign and direction (negative for exothermic, positive for endothermic).
- Activation energy arrow from reactants to the highest point of the curve.

Term	Definition
Enthalpy (H)	Thermal energy in a chemical system; H = U + pV (conceptual).
System	The reacting chemical species and their internal energy.
Surroundings	The external medium that exchanges energy with the system (e.g., water).
ΔH (Enthalpy Change)	Change in enthalpy during reaction; measurable, positive or negative.
Activation Energy (Ea)	Energy barrier from reactants to reaction peak; must be overcome to start reaction.
Exothermic	Reaction that releases energy to surroundings; ΔH < 0.
Endothermic	Reaction that absorbs energy from surroundings; ΔH > 0.

When sketching, always show activation energy from reactants to peak.
Use surrounding temperature change (measured in experiments) to infer sign of ΔH.
Practice drawing both diagram types and labeling ΔH and Ea clearly.
Remember: temperature measurements record changes in the surroundings, not the chemical system directly.

Practice sketching and labeling exothermic and endothermic enthalpy profile diagrams.
Be able to explain energy transfer direction using system and surroundings terminology.
Prepare for calculations in later lessons where water will be used as the surroundings for temperature measurements.