The boundaries of a system are based on the analysis being performed. In chemistry a system is often defined as a chemical reaction or a series of chemical reactions. In order to understand the properties of a system, the system must first be defined. Defining a system sets it apart as distinct from its surroundings. The boundaries of a system are chosen so that analysis of the interactions between the system and its environment or among multiple systems can be done. For example, a system could be a piston in a car engine, the entire engine, the engine and transmission and axles, or the entire car.
Thermodynamics is a branch of physical science that investigates the energy and work of systems. Work (w) is energy that is transferred when a force acts on an object over a distance. The heat, or thermal energy, of a reaction is related to the energy available to the reaction system to do work. Equilibrium is a vital consideration in thermodynamics. Equilibrium is a state in which the rates of the forward and reverse reactions are equal. A system, or chemical reaction that has reached equilibrium will come to rest, and no further changes to the system will occur. A system that has not yet reached equilibrium will continue to change until equilibrium is reached. The direction that the reaction equilibrium will shift is determined by the relative formation energies of reactants and products. Comparing the formation energies, the energies required to form bonds, of reactants and products gives a direction that the reaction will shift. If the formation energies are equal, then the reaction is at equilibrium; i.e., there is no energy available to shift the reaction toward reactants or products.
Spontaneous and Nonspontaneous Processes
When discussing thermodynamics, it is important to understand the concept of entropy. Entropy (S) is a measure of the disorder of a system. For example, a tower of blocks arranged one on top of the other has low entropy. There is little disorder: each block sits on top of another block, except for the bottom block, which sits on the ground. There is little uncertainty: below each block is another block, except for the ground beneath the bottom block. A toppling tower of blocks has high entropy. There is much disorder: each block's position relative to the other blocks and the ground is difficult to describe. There is uncertainty; the relationships between the positions of the blocks are random.
In real-world systems, macroscopic properties, such as temperature, pressure, volume, density, velocity, and mass, can be measured. The combination of these measurable macroscopic properties of a system is called the macrostate. Entropy describes how organized a system is as defined by microstates. A microstate is a possible energy and positional configuration of the particles of a system. Microstates represent probabilities; the exact positions and velocities of the molecules involved cannot be known, but probabilities for them can be known.
- Arrangement A: four particles in the left bulb and none in the right; there is one microstate with this arrangement
- Arrangement B: three particles in the left bulb and one in the right; there are four microstates with this arrangement
- Arrangement C: two particles in each bulb; there are six microstates with this arrangement
- Arrangement D: one particle in the left bulb and three in the right; there are four microstates with this arrangement
- Arrangement E: no particles in the left bulb and four in the right; there is one microstate with this arrangement
The probability for the system to be in arrangement A is 1 in 16, or 6.25%, and the probability for arrangement E is also 6.25%. The probability that the system is in arrangement B is 4 in 16, or 25%, and the probability for arrangement D is also 25%. The probability for arrangement C is 6 in 16, or 37.5%.
This simple system shows that the most probable state of the system has the particles distributed evenly between the two containers. However, real systems have many more than four particles, and they have many more than two ways the particles can be arranged and energy values of the particles. As the number of particles increases, the number of microstates becomes uncountably large. The probability of the system being in the state in which the particles are evenly distributed becomes so large that all other possibilities can be disregarded.
When considering chemical reactions, an analysis of the reactants and products allows scientists to predict whether the change in entropy for the reaction is positive or negative. If only gases are involved in the reaction, then the number of moles of reactants and products can be used to determine the sign of the entropy change. If the number of moles of products is less than the number of moles than the reactants, then entropy is negative. The reverse is also true. If the reaction involves multiple states of matter, then the side of the reaction with the gas typically has higher entropy than the side without.
For example, in the reaction , there are more moles on the reactants side than the products side, so the change in entropy is negative.
In the reaction , the gas is present on the product side, so the change in entropy is positive.