Measuring enthalpy changes: R1.1.1 Energy transfer in chemical reactions IB DP Chemistry Study Notes - New Syllabus 2025

Reactivity 1.1.1 – Energy Transfer in Chemical Reactions

Energy Changes During Reactions

All chemical reactions involve energy changes. These changes occur as chemical bonds are broken and formed. Breaking bonds requires energy input, while forming bonds releases energy. The total energy change depends on the balance between these two processes.
 

The System and Its Environment
In thermodynamics, we focus on a defined portion of the universe known as the system, which typically includes the reacting chemicals. Everything outside this defined boundary is called the surroundings. Energy is exchanged between the system and its surroundings during a reaction.

Work

Types of Systems

• Open System: Both energy and matter can move in and out (e.g., uncovered beaker).
• Closed System: Only energy can be exchanged; matter stays contained (e.g., sealed tube).
• Isolated System: No energy or matter can enter or leave (ideal scenario; e.g., perfect calorimeter).
  

Common Forms of Energy in Chemistry

Reactions can involve various forms of energy—thermal (heat), light, electrical—but thermal energy is most common in laboratory reactions. The energy observed is usually the result of bond-making and bond-breaking processes.

Exothermic vs. Endothermic Reactions

• Exothermic Reaction: The system releases energy to the surroundings. These reactions typically raise the temperature of the surrounding environment.
  • Example: Combustion of methane:
    \( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{energy} \)
• Endothermic Reaction: The system absorbs energy from the surroundings. These reactions often result in a temperature drop.
  • Example: Thermal decomposition of calcium carbonate:
    \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)
    

Law of Conservation of Energy

One of the core principles of chemistry is that energy cannot be created or destroyed. It can only be transferred between different parts of the system or converted between forms. If a reaction appears to gain or lose energy, that energy must be accounted for in the surroundings.

Why Particles Have Energy

Particles in a substance are always in motion, and this motion gives them kinetic energy. The faster they move, the more energy they carry. When substances react, the energy of these particles influences how bonds are broken and formed. Temperature reflects how much kinetic energy the average particle has, while heat measures the total energy of all particles combined.

Understanding Heat vs. Temperature

Temperature: A Measure of Particle Motion

Temperature reflects the average kinetic energy of the particles in a substance. It tells us how fast the particles are vibrating, rotating, or translating. Higher temperatures mean that particles are moving more energetically.

• Temperature is an intensive property—it does not depend on the amount of substance present.
• Units: Kelvin (K) is the SI unit, but Celsius (°C) is commonly used in the lab.
• A substance with fewer particles can still have a high temperature if the average motion of each particle is high.
  

Heat: The Transfer of Energy

Heat is the total energy transferred between substances or systems due to a temperature difference. It involves the movement of thermal energy from a hotter object to a cooler one until thermal equilibrium is reached.

• Heat is an extensive property—it depends on the amount of matter involved.
• Units: Joules (J) or kilojoules (kJ).
• Heat always flows spontaneously from high temperature to low temperature.
  

Key Differences Between Heat and Temperature

Types of Heat Transfer

Heat transfer is the movement of thermal energy from a hotter object or region to a cooler one. There are three main methods by which heat can be transferred:

Scientific Implication in Reactions

In chemical reactions:

• Heat is the quantity transferred to or from the surroundings when bonds are formed or broken.
• Temperature tells us how energetic the reacting particles are. Faster particles often lead to faster reaction rates.

Measuring Heat in Practice

In calorimetry, heat transfer is measured using the formula:

\( q = mc\Delta T \)

• \( q \): heat (J)
• \( m \): mass of substance (g)
• \( c \): specific heat capacity $\text{J·kg}^{-1}\text{·K}^{-1}$ or $ \text{J·kg}^{-1}\text{·}^{\circ}\text{C}^{-1}$
• \( \Delta T \): change in temperature (K or °C)