Measuring enthalpy changes: R1.1.2 Endothermic and exothermic reactions IB DP Chemistry Study Notes - New Syllabus 2025

Reactivity 1.1.2 – Exothermic and Endothermic Reactions

Energy Transfer in Chemical Reactions

Chemical reactions are always accompanied by a transfer of energy between the system (the chemicals reacting) and the surroundings (everything outside the reaction vessel). This energy is most often transferred in the form of heat.

Types of Energy Change

Based on the direction of energy flow, reactions are classified as either:

• Exothermic Reactions — energy is released to the surroundings.
• Endothermic Reactions — energy is absorbed from the surroundings.

Exothermic Reactions

• The system loses energy, which is transferred to the surroundings as heat.
• The enthalpy change \( \Delta H \) is negative: \( \Delta H < 0 \).
• Surroundings gain energy.
• Examples: combustion, neutralization, respiration.

Endothermic Reactions

• The system absorbs energy from the surroundings.
• The enthalpy change \( \Delta H \) is positive: \( \Delta H > 0 \).
• Surroundings lose energy.
• Examples: thermal decomposition, photosynthesis, evaporation.

Enthalpy (\( H \)) and Enthalpy Change (\( \Delta H \))

Enthalpy is a measure of the total energy of a system at constant pressure. The change in enthalpy during a reaction is:

\( \Delta H = H_{\text{products}} – H_{\text{reactants}} \)

• Negative \( \Delta H \): exothermic → heat released.
• Positive \( \Delta H \): endothermic → heat absorbed.

Enthalpy Profile Diagrams (Conceptual)

• Exothermic: reactants start higher than products; diagram slopes down.
• Endothermic: products are higher in energy than reactants; diagram slopes up.
  

Temperature Changes in Endothermic and Exothermic Reactions

Effect on Temperature

During a chemical reaction, the energy transferred between the system and surroundings leads to measurable changes in temperature:

Linking to Kinetic Theory

Temperature is a measure of the average kinetic energy of the particles in a substance. In the kinetic theory of matter, all particles (atoms, ions, or molecules) are in constant motion. The faster these particles move, the higher their kinetic energy — and therefore, the higher the temperature.

In an Exothermic Reaction:

• • The chemical system releases energy (often in the form of heat) into the surroundings.
  • This added energy increases the motion of the particles in the surroundings (e.g. solvent molecules).
  • As particle motion increases, so does their average kinetic energy.
  • This leads to a rise in temperature of the surroundings.
  • In kinetic theory terms:
    More energetic vibrations, rotations, or translations of particles = higher temperature.

In an Endothermic Reaction:

• • The system absorbs energy from the surroundings.
  • The surroundings lose energy, which slows down particle motion.
  • As the particles move more slowly, their average kinetic energy decreases.
  • This results in a drop in temperature of the surroundings.
  • In kinetic theory terms:
    Reduced particle motion = lower temperature.

Units and Measurement

• Temperature: Measured in Kelvin (K) or degrees Celsius (°C).
• Heat (q): Measured in joules (J) or kilojoules (kJ).
• Calculated using \( q = mc\Delta T \), where:
  • \( m \): mass (g)
  • \( c \): specific heat capacity $\text{J·kg}^{-1}\text{·K}^{-1}$ or  $\text{J·kg}^{-1}\text{·}^{\circ}\text{C}^{-1}$
  • \( \Delta T \): temperature change (K or °C)