Thermochemistry Essay

3881 WordsFeb 18, 201416 Pages
Thermochemistry April 18, 2013 Introduction Thermochemistry is a branch of chemistry that studies relationships between chemical actions and the heat that is generated or absorbed by the action and includes calculations as the heat capacity, heat of combustion, heat of formation, etc. In thermochemistry the system is a specific part of the universe that is studied and often is a chemical reaction. The surroundings are the other parts of the universe that are not in the system, typically outside of the system. The systems can be: open – when both energy and matter can be exchanged with the surroundings (a full glass of water), closed – when only the energy can be transferred (a sealed wine bottle) and isolated – when neither the energy nor the mater can be exchanged (an sealed thermos bottle). An important aspect of thermochemistry is the First Law of Thermodynamics that states the energy can’t be created or destroyed. In other words when a system releases or absorbs energy the total energy of the universe will be constant and it will not change, and therefore the energy will be transferred. This concept can be mathematically expressed as: ∆E=q+w where ∆E is the difference in energy, q is heat and w is work. From this equation there are few conventions that apply: when q is positive the heat is absorbed by the system, when q is negative the heat is lost by the system, when w is positive work is done on the system and when w is negative the work is done by the system. Enthalpy, notated with H, is a state function that describes the heat change that occur in a reaction. If a reaction is termed exothermic, it has a negative value of H and if it is endothermic, it has a positive value of H. This flow of heat is the enthalpy exchange notated with ∆H. The enthalpy change is measured in Kj/mol. ΔH is dependent upon three quantities: • The mass, m, of the