In our introduction to statistical mechanics, we defined the state function called entropy. It does not explain why heat spontaneously flows from a hot to a cold object, or why a gas fills all available space in a container. However, it does not predict the spontaneous direction of change. The First Law is a statement of energy conservation. That state function, as we shall see, is entropy and unlocking it will require the Second Law of Thermodynamics. There must be another parameter, another state function that remains to be discovered that will unlock the key to describing spontaneity. Once a body or solution is brought into thermal equilibrium at a uniform temperature, spontaneous temperature polarization into hot and cold regions does not occur even, though it conserves energy. Once mixed, ideal gases do not spontaneously unmix even though such a process conserves energy. We always observe mixing of gases upon opening a valve. But even in systems with minimal or no chemical interactions, there are spontaneous changes that occur. Some compounds dissolve completely, others establish reagent-favored equilibria. Some systems react completely, others establish equilibria. We need a new state function that describes the evolution of chemical and physical parameters. They are also insufficient to determine whether an equilibrium exists and whether that equilibrium favors the reactants or products. They do not determine when a reaction will occur. The Zeroth and First Laws are insufficient for determining the direction of spontaneous change. We need to discover one or more new state functions along the way to finding what truly is the chemical potential. Therefore, neither Δ U nor Δ H plays the role of the chemical potential. Sometimes, an equilibrium that favors the reactants is established when a process exhibits a positive Δ U, but not always. Sometimes, spontaneous reactivity corresponds to minimization of Δ H, but not always.
‘Sometimes’ is not good enough when establishing a law of Nature. Again, the process is spontaneous even though it is endothermic. But if we add heat to the system so that the temperature reaches the boiling point, the water spontaneously evaporates.
SECOND LAW OF THERMODYNAMICS STATEMENT FULL
Take a beaker full of water, it will remain in the liquid phase at atmospheric pressure and room temperature once it has established its equilibrium vapor pressure above the liquid. The reaction is endothermic, Δ H is positive. Nonetheless, the temperature drops upon the dissolution of KCl. Over 35 g of KCl can be dissolved in 100 g of water at room temperature. Similarly, when KCl is added to water, it dissolves spontaneously. The system moves toward a state of lower energy. The Δ U and Δ H values for dissolving H 2SO 4 in water are negative. It dissolves with great evolution of heat – remember always add acid to water not water to acid when diluting from a concentrated solution. Many reactions that proceed spontaneously are exothermic. Willard Gibbs much like the gravitational potential – that indicates which direction is ‘downhill’ for chemical reactions.ĭoes U or H act as this chemical potential? This was one of the first arguments used by chemists to explain reactivity. There must be a chemical potential – a concept introduced by J. Then a fraction of this energy is converted into vibrations and sound waves, while another portion is used to overcome the cohesive energy of the ceramic as the mug strikes the floor. Gravitational potential energy is converted into translational energy as the system accelerates toward the floor.
SECOND LAW OF THERMODYNAMICS STATEMENT PLUS
The system (ceramic plus finely brewed dark roasted aqueous extract of Arabica beans) spontaneously falls to the ground to lower its energy. Raise a coffee mug one meter off the ground. We have an instinctive feeling that systems tend to lower their energy. We quantify the changes in U and H by measuring temperature changes at either constant volume or constant pressure. The temperature increases when an exothermic reaction occurs in a thermos flask and decreases during the course of an endothermic reaction. Certainly, temperature changes upon mixing and subsequent reaction are indicative of chemical changes. From the Zeroth and First Laws of Thermodynamics we have defined a state variable (temperature T) and two state functions (internal energy U and enthalpy H) that we know exist and are rather fundamental. Not much chemistry is performed without making products.
When two chemicals are mixed together do they react? This is an existential question for a synthetic chemist.
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