Energy, Thermodynamics, and Thermochemistry Concepts
Energy, Thermodynamics, and Thermochemistry
Energy: The ability of a body or system to produce work or heat transfer. Energy is neither created nor destroyed, only transformed.
Chemical Energy: The energy due to associations between atoms, ions, and molecules that make up each substance. It accumulates in the chemical bonds and joints caused by intermolecular forces.
Work: A mechanism of energy transfer between bodies based on the use of force.
Heat: Thermal energy is transferred naturally or spontaneously between two bodies found at different temperatures.
Variation in Temperature: q = CsmΔT.
Thermodynamics
Thermodynamics: The relationship between energy and two of the mechanisms to exchange it with its environment: heat and work.
State of Equilibrium: Classical thermodynamics deals only with transformations between steady-state systems, that is, those whose state variables do not change with time. It studies how variables change as the system transitions between two equilibrium states, one initial and one final, but does not address the process used to pass from one to another.
Thermodynamic Variables:
- Intensive: Whose value does not depend on size.
- Extensive: Whose value *does* depend on size.
State Function: A property whose value depends only on the initial and final states of the system and not the path that has been followed between the two. *Example:* Internal energy. Heat and work are *not* state functions.
Internal Energy: The sum of the kinetic and potential energy of the particles that compose it.
The First Law of Thermodynamics
Every system has a thermodynamic property called internal energy, which takes a characteristic value for each state and increases when the system absorbs heat or supports work. ΔU = q + W.
- Constant Volume Process (Isochoric Process): If the system volume does not change. ΔU = qv
- Constant Pressure Process: The constant pressure heat transfer coincides with the enthalpy change. This heat at constant pressure, qp, *is* a state function. H = U + pV.
- Isothermal Process: q = -W.
- Adiabatic Process: ΔU = W.
Thermochemistry
Thermochemistry: Deals with the study of heat changes that occur in chemical reactions.
- Endothermic Process: Absorbs heat.
- Exothermic Process: Releases heat.
Thermochemical Equation: A balanced chemical equation in which the thermodynamic parameters are expressed to the right of it.
Standard Conditions:
- Standard pressure is 1 bar = 105 Pa = 0.987 atm.
- Normal temperature is 25ºC.
- For dissolved substances, 1M.
Enthalpy Diagram: A schematic graphical representation of the enthalpy change of a chemical reaction.
Hess’s Law: The enthalpy of a chemical reaction depends only on the initial and final states, and its value is the same whether the reaction proceeds in one or more steps. *Definition:* When a chemical reaction can be expressed as an algebraic sum of others, the enthalpy of reaction is equal to the sum of the enthalpies of the partial reactions.
Binding Energy: The energy required to break one mole of a bond, assuming that the molecules are in the gas phase.
Entropy: Is interpreted as a measure of the degree of molecular disorder of the system. The higher the internal order, the lower its entropy.