The Effect of a Change in Concentration

When a system at equilibrium is disturbed by a change in concentration of one of the components, the system reacts in the direction that reduces the change  

Of course, the component must be one that appears in Q thus, pure liquids and solids, which do not appear in Q because their concentrations are constant, are not involved. 

At 523 K, the PCI3-CI2-PCI5 system reaches equilibrium when 

What happens if we now inject some CI2 gas, one of the reactants The system will always act to reduce the disturbance, so it will reduce the increase in reactant by proceeding toward the product side, thereby consuming some of the additional CI2. In terms of the reaction quotient, when we add CL, the [CL] term increases, so the value of immediately falls as the denominator 

In general, whenever the concentration of a component changes, the equilibrium system reacts to consume some of the added substance or produce some of the removed substance. In this way, the system reduces the effect of the disturbance. The effect is not completely eliminated, however, as we will see next from a quantitative comparison of original and new equilibrium positions. 

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As chemists, we are most often concerned with reactions proceeding under conditions in which the temperature and pressure are the variables we control. Therefore, it is useful to have a set of properties that describe the effect of a change in concentration on the various thermodynamic quantities under conditions of constant temperature and pressure. We refer to these properties as the partial molar quantities. 

To see how we can predict the effects of a change in concentration on a system at equilibrium, we will consider the ammonia synthesis reaction. Suppose there is an equilibrium position described by these concentrations ... 

The effect of a change in concentration on the equihbrium position is shown in the following example. 

SAMPLE PROBLEM 17.9 Predicting the Effect of a Change in Concentration... 

The effect of a Change in Concentration The effect of a Change in Pressure... 

What is the effect of a change in concentration of a reactant or product on a chemical reaction initially at equihbrium ... 

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