In this case, increasing the pressure has no effect on the position of the equilibrium. Because there are equal numbers of molecules on both sides, the equilibrium cannot move in any way that will reduce the pressure again. Again, this is not a rigorous explanation of why the position of equilibrium moves in the ways described. A mathematical treatment of the explanation can be found on this page. Three ways to change the pressure of an equilibrium mixture are: 1.
Add or remove a gaseous reactant or product, 2. Add an inert gas to the constant-volume reaction mixture, or 3. Change the volume of the system. To understand how temperature changes affect equilibrium conditions, the sign of the reaction enthalpy must be known. Assume that the forward reaction is exothermic heat is evolved :. In this reaction, kJ is evolved indicated by the negative sign when 1 mole of A reacts completely with 2 moles of B. For reversible reactions, the enthalpy value is always given as if the reaction was one-way in the forward direction.
The back reaction the conversion of C and D into A and B would be endothermic, absorbing the same amount of heat. The main effect of temperature on equilibrium is in changing the value of the equilibrium constant. It is not uncommon that textbooks and instructors to consider heat as a independent "species" in a reaction. While this is rigorously incorrect because one cannot "add or remove heat" to a reaction as with species, it serves as a convenient mechanism to predict the shift of reactions with changing temperature.
A more accurate, and hence preferred, description is discussed below. If the temperature is increased, then the position of equilibrium will move so that the temperature is reduced again. To cool down, it needs to absorb the extra heat added.
In the case, the back reaction is that in which heat is absorbed. The position of equilibrium therefore moves to the left. The new equilibrium mixture contains more A and B, and less C and D. If the goal is to maximize the amounts of C and D formed, increasing the temperature on a reversible reaction in which the forward reaction is exothermic is a poor approach.
The conditions and properties of a system at equilibrium are summarized below. The description of equilibrium in this concept refers primarily to equilibrium between reactants and products in a chemical reaction.
Other types of equilibrium include phase equilibrium and solution equilibrium. A phase equilibrium occurs when a substance is in equilibrium between two states. For example, a stoppered flask of water attains equilibrium when the rate of evaporation is equal to the rate of condensation. A solution equilibrium occurs when a solid substance is in a saturated solution. At this point, the rate of dissolution is equal to the rate of recrystallization.
Although these are all different types of transformations, most of the rules regarding equilibrium apply to any situation in which a process occurs reversibly. Read the material at ChemGuide. Skip to main content. Search for:. Chemical Equilibrium Learning Objectives Define chemical equilibrium. List conditions for equilibrium. Define chemical equilibrium and explain how it is achieved.
Explain Le Chatelier's principle. Equilibrium is achieved when the forward rate of a reaction is equal to the reverse rate of a reaction. This very simple principle can be observed in a closed container of liquid. In the container the liquid has vapor pressure that is influenced by the pressure above the liquid.
Species in brackets represent the concentrations of products, which are always in the numerator, and reactants, which are always in the denominator. Each of the concentrations is raised to a power equal to the stoichiometric coefficient for each species.
Assuming this reaction is an elementary step, we can write the rate laws for both the forward and reverse reactions:. Rearranging this equation and separating the rate constants from the concentration terms, we get:.
Notice that the left side of the equation is the quotient of two constants, which is simply another constant. We simplify and write this constant as K eq :. Keep in mind that the only species that should be included in the K eq expression are reactants and products that exist as gases or are in aqueous solution.
Reactants and products in the solid and liquid phases, even if they are involved in the reaction, are not included in the K eq expression, as these species have activities of 1. The activity for solids and liquids is 1, so they essentially have a constant concentration of 1, and thereby have no effect on the K eq expression.
As such, they are omitted. Note that because it is a solid, the activity of C s is 1, and it is omitted from the final K expression. When looking at the K eq expression, we should notice that it is essentially a ratio relating the concentrations of products to the concentrations of reactants at equilibrium. If we know the value of K eq , we can draw some conclusions about the thermodynamics of the forward and reverse reactions.
These conclusions are summarized as follows:.
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