Exothermic & Endothermic Reactions
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
- Open 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)
- Melting of ice
- 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)
- Combustion reactions