Effect of Changes in Conditions on an Equilibrium System

Once a system has attained equilibrium, it is possible to change the ratio of products to reactants by changing the external conditions. We will consider three ways in which a chemical equilibrium can be disturbed  

You can deduce the direction in which an equilibrium will shift when one of these changes is made by applying the following principle  

If a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the system will, if possible, shift to partially counteract the change. 

This law was first stated, in a more complex form, in 1884 by Henri Le CMtelier (1850-1936), a French chemist who had studied a variety of industrial equilibria, including those involved in the production of iron in the blast furnace. The statement is commonly referred to as Le Ch telier s principle. 

Click Chemistry Interactive for the self-study module Le Chatelier s principle a water tank analogy. 

It is possible to use K to calculate the extent to which reaction occurs when an equilibrium is disturbed by adding or removing a product or reactant. To see how this is done, consider the effect of adding hydrogen iodide to the HI-H2-I2 system (Example 12.7). 

10 atm. Suppose enough HI is added to raise its pressure temporarily to 1.00 atm. When equilibrium is restored, what are Phi -Ph, and Pi  

Create a table. Note that the equilibrium partial pressures now become initial pressures for HI and I2. 

Since HI is added, the reaction goes in the direction of using up the HI, thus to the right. 

Substitute the value for x into the equilibrium pressures for all species. 

---

Although it is not an explanation, Le Chatelier s principle is used to predict the effect of changes in conditions on the position of equilibrium. One statement of Le Chatelier s principle is If a system in equilibrium is subjected to a change which disturbs the equilibrium, the system responds in such a way as to counteract the effect of the change . The factors that may change the position of an equilibrium are concentration, temperature and pressure. 

Table 7.2 summarizes the effect of a catalyst, and other effects of changing conditions, on a system at equilibrium. The Sample Problem that follows provides an opportunity for you to use Le Chatelier s principle to predict the equilibrium shift in response to various conditions. 

The coordinates of thermodynamics do not include time, ie, thermodynamics does not predict rates at which processes take place. It is concerned with equilibrium states and with the effects of temperature, pressure, and composition changes on such states. For example, the equilibrium yield of a chemical reaction can be calculated for given T and P, but not the time required to approach the equilibrium state. It is however true that the rate at which a system approaches equilibrium depends direcdy on its displacement from equilibrium. One can therefore imagine a limiting kind of process that occurs at an infinitesimal rate by virtue of never being displaced more than differentially from its equilibrium state. Such a process may be reversed in direction at any time by an infinitesimal change in external conditions, and is therefore said to be reversible. A system undeigoing a reversible process traverses equilibrium states characterized by the thermodynamic coordinates. 

The underlying idea is the restorative tendency of equilibrium, tending to counteract the effects of attempted changes on an original equilibrium system. This restorative tendency is associated with the stability of chemical equilibrium, and we therefore use the rigorous stability condition (8.13) to prove the above statement of Le Chatelier s principle in a general form. 

---