A thermochemical equation is a balanced stoichiometric chemical equation which shows both mass relationships and enthalpy change (delta H) between products and reactants. In variable form, a thermochemical equation would look like this: A + B → C
ΔH = (±) #
Where {A, B, C} are the usual agents of a chemical equation with coefficients and “(±) #” is a positive or negative numerical value, usually with units of kJ.
An endothermic reaction refers to a chemical reaction in which a system receives heat from its environment. They must absorb energy in order to proceed. They cannot occur spontaneously. They are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH). The endothermic chemical reaction creates a product that has a higher energy level than the original materials, causing the reactant's stored energy to decrease. The resulting product of the reaction is less stable because, the higher the energy bond, the less strength its molecules possess.
Exothermic reactions refers to chemical reactions which release energy in the form of heat, light, or sound. They may occur spontaneously and result in higher randomness or entropy of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy. The reactants contain more stored energy than the product because energy from external sources is not required, but given off. This gives the product more stability due to the lower amount of energy needed.
At a more advanced level, heat change is called the enthalpy change. It is denoted by delta H, ΔH.
Enthalpy is considered as the heat content of the system.
ΔH represents the difference between the enthalpy of the system before and after the process and is represented as :
- ΔH is negative (-ve) for exothermic reactions i.e. heat energy is given out and lost from the system to the surroundings which warm up.
- ΔH is positive (+ve) for endothermic reactions i.e. heat energy is gained by the system and taken in from the surroundings which cool down OR, as is more likely, the system is heated to provide the energy needed to effect the change.
The first law of thermodynamics, often called as the law of conservation of energy, states that energy can be transformed, but cannot be created nor destroyed. This law suggests that energy can be transferred from one system to another in many forms. The amount of energy in the universe is constant – merely changing from one form to another. However, this energy cannot be created from nothing or reduced to nothing. Every natural process transforms energy and moves energy, but cannot create or eliminate it.
It is usually formulated by stating that the total energy lost by the system is equal to the total energy absorbed by its surroundings. Likewise, the total energy absorbed by the system is equal to the energy lost bythe surroundings.
