Quimica nivelacion Tema 09

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium.

Most Chemical Reactions are Reversible (can go backwards). A + B

k1

C + D

k2

At the start of a Reversible Process, the reaction proceeds toward the formation of products. As soon as some product molecules are formed, the Reverse Process begins to take place and reactant molecules are formed from product molecules. Chemical Equilibrium is achieved when the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant.

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium.

Most Chemical Reactions are Reversible (can go backwards). A + B

k1

C + D

k2

Chemical Equilibrium is achieved when the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant. Chemical equilibrium is a dynamic process because reactant molecules continue to form product molecules while product molecules react to yield reactant molecules.

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. An Example is the the reversible reaction involving nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4). N2O4 (g) 2 NO2 (g) (coloreless) (brown)

At equilibrium, the conversions of N2O4 to NO2 and of NO2 to N2O4 are still going on, but no color change is seen because the two rates (forward and reverse reactions) are equal.

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant The concentration of all species in the system involved in the reaction (reactants and products) at the moment when equilibrium is reached are related by the Equilibrium Constant. N2O4 (g)

2 NO2 (g)

For this reaction, the equilibrium constant can be written as: KC =

[NO2]2

= 4.63 x 10-3 M (25°C)

[N2O4]

Its numerical value only depends on the temperature.

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant For a generic reaction at equilibrium at a particular temperature, the following expression can be written. aA + bB

cC + dD

(a,b,c,d are stoichiometric coefficients)

No matter what the individual equilibrium concentrations may be in a particular experiment, the Equilibrium Constant for a reaction at a particular temperature always has the same value.

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant The magnitude of the equilibrium constant tells us whether an equilibrium reaction favors (or not) the generation of products.

Any number greater than 10 is considered to be much greater than 1 (product formation favored, equilibrium lies to the right) and any number less than 0.1 (equilibrium lies to the left) is much less than 1.

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant. Homogeneous Equilibria The term Homogeneous Equilibrium applies to reactions in which all reacting species are in the same phase. An example would be the ionization of acetic acid (CH3COOH) in water (acid-base reaction)

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant. Heterogeneous Equilibria A Heterogeneous Equilibrium results from a reversible reaction involving reactants and products that are in different phases. An example is when calcium carbonate is heated in a closed vessel, the following equilibrium is attained

The two solids and one gas constitute three separate phases. As the concentration of pure solid and liquid phases (e.g. solids CaCO3 and CaO) are considered constant, the equilibrium constant is written as:

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant. Multiple Equilibria If a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions.

(overall)

KC = K’CK”C

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant. Multiple Equilibria An example is the ionization of a Diprotic Acid (such as H2CO3) in water. M

(1st ionization)

M

(2nd ionization)

The overall reaction is the sum of these two reactions: M2

Theme 9. CHEMICAL EQUILIBRIUM Concept of Chemical Equilibrium. Equilibrium Constant. Criteria to write Equilibrium Constants 1. The Concentrations of the Reacting Species are expressed in mol/L (molarity, M). 2. The Concentrations of Pure Solids and Pure Liquids in heterogeneous equilibria, and that of Solvents in homogeneous equilibria, are considered invariable (constant) and are not included in the expressions of the Equilibrium Constant. 3. In quoting a value for the equilibrium constant, the balanced equation (stoichiometry) and the temperature must be specified. 4. If a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual ones.

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Constant. Relationship with Chemical Kinetics Chemical Equilibrium is defined as the situation in which Forward and Reverse Reaction Rates are equal (no net change occurs).

The rates for both reactions are: at equilibrium thus and

and

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Constant. Relationship with Chemical Kinetics The rates for both reactions are:

and

at equilibrium thus and In terms of chemical kinetics, the equilibrium constant of a reaction can be expressed as a ratio of the rate constants of the forward and reverse reactions, which are themselves constant at a given temperature.

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Alterations. Le Chatêlier´s Principle Le Chatêlier´s Principle dictates that when those factors that influence the equilibrium are altered, the equilibrium is shifted to a new position in such a way so that those alterations are minimized. Those factors are: - Concentration of reactants and/or products - Pressure - Volume - Temperature

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Alterations. Le Chatêlier´s Principle Changes in Pressure do not affect the concentration of reactants and/or products in solution, as liquids are virtually incompressible. Yet changes in pressure have a dramatic effect in the concentration of gases.

P and V are related to each other inversely: The greater the pressure, the smaller the volume, and viceversa. The term (n/V) is the concentration of the gas in mol/L, and it varies directly with pressure.

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Alterations. Le Chatêlier´s Principle Changes in Temperature have a dramatic effect in the concentration of gases. endothermic reaction

- A temperature increase favors the endothermic direction of the reaction (from left to right), which decreases [N2O4] and increases [NO2]. - A temperature decrease favors the exothermic direction of the reaction (from right to left), which decreases [NO2] and increases [N2O4].

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Alterations. Le Chatêlier´s Principle Changes in Temperature have a dramatic effect in the concentration of gases. endothermic reaction

Thus, the Equilibrium Constant, given by

is increased by increasing the temperature, and decreased when the system is cooled down.

Theme 9. CHEMICAL EQUILIBRIUM Chemical Equilibrium. Equilibrium Alterations. Le Chatêlier´s Principle Changes in Concentration of Products can shift the Equilibrium. For instance:

FeSCN2+ (aq)

Fe3+ (aq) + SCN- (aq)

If NaSCN is added, then [SCN-] increases. As the numerical value of the Equilibrium Constant is the same, if [SCN-] goes up, then [Fe3+] has to go down, which means that less FeSCN2+ gets dissociated (equilibrium shifted to the left, to reactants). But if the amount (concentration) of the products is decreased (e.g. by adding an agent that removes Fe3+ from the system), then [SCN-] goes up, which means that more FeSCN2+ gets dissociated (equilibrium shifted to the right, to products).