Topic 7.1 – Dynamic Equilibrium

Equilibrium An equilibrium state is a situation in which two opposite processes take place at equal rates. For a system in a state of equilibrium, no net macroscopic changes can be detected. However, the system is dynamic, both forward and reverse processes are still occurring but in a manner such that they balance each other.

Equilibrium in Physical systems Evaporation and Equilibrium - The evaporative behavior of a liquid in a closed container is very different from that in an open container. - In an open container, as the molecules of the liquid evaporate, they escape. Consequently, the liquid eventually evaporates completely. 

Equilibrium in physical systems However, in a closed container, the vapor molecules cannot escape. The molecules that evaporate are unable to move completely away from the liquid as they did in the open container. The vapor molecules are trapped in the space immediately above the liquid. These vapor molecules undergo many random collisions with the container walls, other vapor molecules, and the liquid surface. Molecules colliding with the liquid surface may be recaptured by the liquid. Thus, two processes are taking place, evaporation and condensation, in a closed container.

Equilibrium in physical processes Initially, the rate of evaporation is greater than the rate of condensation and the liquid level drops. As more molecules evaporate, the rate of evaporation drops and the rate at which the molecules strike the liquid surface and become recaptured (condensation) increases. Eventually the rate of condensation becomes equal to the rate of evaporation. At this point there will be no net change in the amounts of liquid and gas present, and the liquid level will stop dropping. At this point, when the rate of evaporation equals the rate of condensation, the system has reached equilibrium.

Equilibrium in Chemical systems Reactions do not usually go to completion The reason for this is that product molecules begin to react with each other to form reactants (provided they are not allowed to escape). Eventually, the rate of formation of products is equal to the rate of formation of reactants. At this point, the concentrations of reactants and products remain constant, and the reaction has reached a state of equilibrium.

Equilibrium in Chemical systems Consider the reaction of dissociation between HI and its elements, H2 and I2. HI(g) H2(g)+ I2(g) If this reaction is carried out starting with the hydrogen iodide in a closed container, there will first be an increase in the amounts of hydrogen gas and iodine gas. After a while, the hydrogen gas and iodine gas will start reacting to form hydrogen iodide. Eventually, the rate at which hydrogen iodide is forming is equal to the rate at which hydrogen gas and iodine gas are forming; the reaction has reached equilibrium. At equilibrium, there is no net change observed, even though the reactions are still occurring.

Equilibrium in Chemical systems If the experiment is reversed, and we start with H2 and I2 instead of HI, we would find that eventually an equilibrium mixture would again be achieved in which the concentrations of H2, I2, and HI would remain constant. So, equilibrium can be achieved from either direction.

Chemical equilibrium SO3 decomposes to SO2 and O2 SO2 and O2 react to give SO3 SO3 decomposes to SO2 and O2

Equilibrium in Chemical systems N2O4 (g) 2 NO2 (g) Equilibrium is reached when the concentrations of reactants and products become constant.

Equilibrium in Chemical systems Note that the same equilibrium mixture is reached starting from N2O4 or from NO2.

The Concept of Equilibrium {Non-Equilibrium Reactions}: proceed in one direction, A  B 2 NO (g) + O2 (g) 2 NO2 (g) (clear gases) (red-brown gas) {Equilibrium Reactions}: proceed in both directions, A ↔ B  N2O4 (g) 2 NO2 (g) (clear gas) (red-brown gas) 2X Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. This does not mean chemicals are found in same concentration!

Equilibrium in Chemical systems Reactions that occur in both directions are reversible reactions. The reaction from left to right is described as the forward reaction and the reaction from right to left as the backward or reverse reaction. The symbol is used to denote the fact that the reaction is an equilibrium reaction. At equilibrium the rate of the forward reaction is equal to the rate of the backward reaction.

Characteristics of the equilibirium state Features of equilibrium state Feature of equilibrium state Explanation 1 Equilibrium is dynamic The reaction has not stopped. Both forward and backward reactions are still occurring. 2 Equilibrium is achieved in a closed system A closed system prevents exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other. 3 The concentrations of reactants and products remain constant at equilibrium The are being produced and destroyed at an equal rate 4 At equilibrium there is no change in macroscopic properties This refers to observable properties such as color and density. These do not change as they depend on the concentration of the components of the mixture. 5 Equilibrium can be reached from either direction The same equilibrium mixture will result under the same condition, no matter whether the reaction is started with all reactants, all products or a mixture of both

Equilibrium in chemical systems Even though the concentrations of reactants and product are constant at equilibrium, does not mean that they are equal. In fact, most commonly there will be a much higher concentration of either reactant or product in the equilibrium mixture, depending on the reaction and the conditions. The proportion of reactants and products in the equilibrium mixture is referred to as its equilibrium position. Reactions where the mixture contains predominantly products are said to “lie to the right”, and reactions with predominantly reactants are said to “lie to the left”.