Exothermic & Endothermic Reactions (College Board AP Chemistry)

Essential Definitions in Thermodynamics

• Thermodynamics is a branch of chemistry that studies the relationship between heat energy and other forms of energy
• A sub-branch of thermodynamics is thermochemistry which looks at the quantity of heat absorbed or released by chemical reactions
• Some important keywords associated with thermodynamics and thermochemistry include:
  • System
  • Surrounding
  • Heat,
  • Work
  • Temperature

System & Surroundings

• A system is what is being studied or undergoing some physical or chemical change
• Everything in the vicinity of the system is known as the surroundings
  • For example, the reactants and the products of a chemical reaction are the system, while the container and everything beyond it are the surroundings
• Based on the nature of interaction between a system and its surroundings, systems may be classified into three types:
  • Open systems
    • In these systems, both energy and matter can be exchanged between the system and surroundings
  • Closed systems
    • These systems only allow the exchange of energy between the systems and surroundings
  • Isolated Systems
    • Neither energy nor matter is allowed to be exchanged in such systems

Types of Systems

Diagrammatic representations of the types of systems. Open systems are the most commonly encountered systems in chemical reactions

Heat, Work and Temperature

• As defined by the kinetic molecular theory, the temperature of a substance is a measure of the average kinetic energy of the particles that make up such a substance
• On the other hand, work (w) and heat (q) are forms in which energy can be transferred
  • Work is said to be done when a force applied to an object causes the object to move in the direction of the applied force
  • Heat is the energy that flows into or out of a system because of a difference in temperature
• Heat will always flow from a body at higher temperature (hotter) to a body at lower temperature (colder)
  • Essentially, a change in temperature is an indication of heat change or energy change

Endothermic and Exothermic Processes

• During a chemical or physical process, energy changes either in the form of heat or work done occur in the system
• Based on this, we may classify chemical or physical processes into two categories:
  • Endothermic
  • Exothermic
• When a process occurs in which the system absorbs heat it is endothermic
  • During an endothermic process, such as the melting of ice, heat flows into the system from its surroundings
  • There is also a consequent decrease in the temperature of the system
    • This is why the container (surroundings) in which ice is melting feels cold to us
• Examples of physical and chemical endothermic processes include
  • Melting of ice
    • H₂O (s) → H₂O (l)
  • Evaporation
    • H₂O (l) → H₂O (g)
  • Dissolution of ammonium nitrate, NH₄NO₃
    • NH₄NO₃ (s) → N₂O (g) + 2H₂O (l)
  • Thermal decomposition of CaCO₃
    • CaCO₃ (s) → CaO (s) + CO₂ (g)
• On the other hand, a process in which the system releases heat is exothermic
  • During an exothermic process, such as the combustion of gasoline, heat exits or flows out of the system into the surroundings
  • The temperature of the system increases
    • This explains why the container (surroundings) feels hotter to touch after a combustion reaction
• Examples of chemical and physical exothermic processes include:
  • Combustion reactions
    • CH₄ + 2O₂ → CO₂ (g) + 2H₂O (l)
  • Neutralization reactions
    • NaOH + HCl → NaCl (aq) + H₂O (l)
  • Freezing of liquid
    • H₂O (l) → H₂O (s)
  • Condensation of vapor
    • H₂O (g) → H₂O (l)

• Physical processes such as the melting of ice or condensation of a vapor also involve absorbing or releasing heat energy
• In the same manner, when a solute is dissolved in a solvent to make a solution, energy changes also occur
• At constant pressure, the energy change associated with the dissolution of a solute is known as the enthalpy of solution (ΔH_solution) or the heat of solution
•  ΔH_solution refers to the heat absorbed or released when a given amount of a solute dissolves in a given amount of solution
• When heat is absorbed for a dissolution process, ΔH_solution is positive and the process is said to be endothermic
• When heat is released, then ΔH_solution is negative and the process is said to be exothermic

Dissolution of Ionic Compounds

• Dissolving an ionic compound such as NaCl in water involves complex interactions among the solute and solvent species
• However, for the sake of analysis, we can simply the process into two separate steps:
• In the first step, Na⁺ and Cl^- ions present in the NaCl crystal are separated
• In the second step, the separated ions are stabilized in the solution by their interaction with water molecules
• The energy involved in the first step is known as the lattice energy (ΔH_lattice)
• Lattice energy is the energy required to completely separate one mole of a solid ionic compound into gaseous ions

NaCl (s) → Na⁺(g) + Cl^- (g)

• Lattice energy is used to measure the strength of ionic bonds in an ionic compound
• It is an endothermic process
• So, ΔH_lattice is always positive

• For example, 788 kJ of energy are required to separate one mole of NaCl into its gaseous ions

NaCl (s) → Na⁺(g) + Cl^- (g)           ΔH_lattice = 788 kJ/mol

• The second step which involves the interaction of the ions with water is referred to as hydration
  • The enthalpy change associated with the hydration process is called the heat of hydration or enthalpy of hydration, ΔH_hydration
  • Hydration of ions is an exothermic process
  • So, ΔH_hydration is always negative
• For example, 784kJ of energy are released when gaseous Na⁺ and Cl^- ions are converted into aqueous Na⁺ and Cl^- ions

Na⁺(g) + Cl^- (g) → Na⁺ (aq) + Cl^- (aq)     ΔH_hydration = -784 kJ/mol

Dissolution of Solid NaCl

The solution process for NaCl. The process can be considered to occur in two separate steps: (1) separation of ions from the crystal state to the gaseous state and (2) hydration of the gaseous ions

• The overall energy change associated with these steps determines whether the dissolution of the ionic solid is exothermic or endothermic
  • ΔH_solution for the formation of an ionic solution is a sum of the lattice and hydration energies involved in the dissolution process of the ionic solid
  • Mathematically, we can express this as:

ΔH_solution = ΔH_lattice + ΔH_hydration

• Therefore, ΔH_solution for the formation of sodium chloride solution will be calculated as:

NaCl (s) → Na⁺(g) + Cl^- (g)                       ΔH_lattice = 788 kJ/mol

Na⁺(g) + Cl^- (g) → Na⁺ (aq) + Cl^- (aq)     ΔH_hydration = -784 kJ/mol

NaCl (s) → Na⁺ (aq) + Cl^- (aq)                  ΔH_solution = 4kJ/mol

• In general,
  • If lattice energy is greater than the hydration energy, the dissolution process will be endothermic
  • If lattice energy is less than the hydration energy, the dissolution process will be exothermic