CHAPTER 19 CHEMICAL THERMODYNAMICS SECTION 1

Enthalpy change (ΔH) is the heat transferred between the system (reaction) and its surroundings in a constant pressure process. Reactions involve not only changes in enthalpy but also changes in entropy – another thermodynamic quantity. Entropy involves the Second Law of Thermodynamics, which provides insight into why chemical & physical changes tend to favor one direction over another.

Thermodynamics helps us understand the significance of this directional character of processes, whether they are exothermic or endothermic. The First Law of Thermodynamics states that energy is conserved: ΔE = q + w ΔE is the change in energy of a system q is the heat absorbed by the system from its surroundings w is the work done on the system by its surroundings

Directionality: 2Na(s) + Cl2  2NaCl (table salt) This is a common reaction where the reverse process does not take place. A process that occurs on its own without any outside influence is said to be spontaneous… - a spontaneous process is one that proceeds on its own without any outside assistance, and in a definite direction.

EXAMPLES OF SPONTANEOUS PROCESSES Gas: always moves from an area of high pressure to an area of low pressure. A shiny new nail exposed to the weather will turn rusty (it is hard to imagine a rusty nail somehow becoming shiny and new on its own.

Processes that are spontaneous in one direction are non-spontaneous in the reverse direction. Experimental conditions, such as temperature and pressure, can be important in determining whether a process is spontaneous. A spontaneous reaction does not necessarily occur at an observable rate (fast). It may proceed very slowly (iron rusting).

Thermodynamics can determine the direction of a reaction, but not the speed of the reaction. For that, we need reaction rate data from experimentation. Spontaneous processes or reactions are only “usually” exothermic and may be endothermic (absorb heat) like the melting of ice. If work (w) must be done to reverse a process and the original conditions of the system and the surroundings are not re-established, the process is irreversible. (See Fig. 19.5)

In reality… The reverse of any spontaneous process is a non-spontaneous process…or any real spontaneous process is irreversible. Even if the system is returned to the original condition, the surroundings will have changed.