Preview Version - To order please call 1.866.422.7310 or email info@edvantageinteractive.com
The Shift Mechanism: Effects of Stress on Forward and Reverse Reaction Rates
Le Châtelier’s principle describes how an equilibrium system responds to a stress without offering any explanation of the response. The explanation is related to the effect of the stress on the equilibrium’s forward and reverse reaction rates. To an equilibrium system, a stress is any action that has a different effect on the forward reaction rate than it does on the reverse reaction rate, thus disrupting the equilibrium. In other words, a disrupted or stressed equilibrium system is no longer at equilibrium because its forward and reverse reaction rates are not equal.
Sample Problem — Describing the Shift Mechanism Explain in terms of forward and reverse reaction rates how the following equilibrium system would respond to adding some iron(III) chloride. (FeCl3 dissociates into independent Fe3+ and Cl− ions in solution.) Fe3+(aq) + SCN−(aq) FeSCN2+(aq)
What to Think About
How to Do It
1. Determine the immediate effect of the stress on the forward and/or reverse reaction rates
Adding some Fe3+ increases the forward reaction rate (rf).
2. Decide if this results in a net forward or net reverse reaction.
This results in a net forward reaction, also known as a shift right.
The system in Sample Problem 2.2.2(a) would re-equilibrate in the same manner that it established equilibrium in the first place. Figure 2.2.1(a) shows the rates when the system is initially at equilibrium (Ei), when the system is stressed (S), and when the system restores equilibrium (Ef). The net forward reaction would cause the reactant concentrations and the forward rate (rf) to decrease, while the product concentrations and the reverse rate (rr) increase, until rf once again equals rr. The graph in Figure 2.2.1(b) is the more traditional way of depicting the same information shown in the arrow diagram in (a). In (b), the solid line represents the forward rate and the dotted line represents the reverse rate.
Ei S
Rate
Reaction Rates
Ef
Time
(a) (b) Figure 2.2.1 Two different ways of representing a reaction in a diagram
© Edvantage Interactive 2019
Chapter 2 Chemical Equilibrium 77