GENERAL CHEMISTRY GENERAL CHEMISTRY Chapter 6 Chapter 6 Chemical Equilibrium Chemical Equilibrium Dr. Huynh Ky Ph ng H ươ a Office: 112 B2 Building Phone: 38650484 Email: [email protected]
• Consider colorless frozen N 2 O 4 . At room temperature, it decomposes to brown NO 2 : N 2 O 4 ( g ) 2NO 2 ( g ). • After some time, the color stops changing and we have a mixture of N 2 O 4 and NO 2 . • Chemical equilibrium is the point at which the concentrations of all species are constant. • Chemical equilibrium occurs when the reaction in the reverse direction proceed at equal rates. The Concept of Equilibrium The Concept of Equilibrium
• Using the collision model: • At the beginning of the reaction, there is no NO 2 so the reverse reaction (2NO 2 ( g ) N 2 O 4 ( g )) does not occur. • Only the production of NO 2 will occur. • As the amount of NO 2 builds up, there is a chance that two NO 2 molecules will collide to form N 2 O 4 . • Thus, the forward chemical reaction has an opposing reaction that will increase with the increase in product formation. Collision Model: Understanding Collision Model: Understanding Equilibrium Equilibrium
• The point at which the rate of decomposition: N 2 O 4 ( g ) 2NO 2 ( g ) equals the rate of dimerization: 2NO 2 ( g ) N 2 O 4 ( g ). is dynamic equilibrium. • The equilibrium is dynamic because the reaction has not stopped: the opposing rates are equal. Dynamic Equilibrium Dynamic Equilibrium
• At equilibrium, as much N 2 O 4 reacts to form NO 2 as NO 2 reacts to re-form N 2 O 4 : • The double arrow implies the process is dynamic. • Consider Forward reaction: A B Rate = k f [A] Reverse reaction: B A Rate = k r [B] • At equilibrium k f [A] = k r [B], which implies • The mixture at equilibrium is called an equilibrium mixture. Expressing Equilibrium Reactions Expressing Equilibrium Reactions N 2 O 4 ( g ) 2NO 2 ( g ) A B
The Equilibrium Constant The Equilibrium Constant • No matter the starting composition of reactants and products, the same ratio of concentrations is achieved at equilibrium.
Defining the Equilibrium Constant Defining the Equilibrium Constant • For a general reaction in the gas phase the equilibrium constant expression is where K eq is the equilibrium constant. a A + b B c C + d D b a d c eq P P P P K B A D C
The Equilibrium Constant The Equilibrium Constant • For a general reaction the equilibrium constant expression for everything in solution is where K eq is the equilibrium constant, A, B, C, and D are the reactants and products, and a, b, c, and d are the stoichiometric coefficients. a A + b B c C + d D b a d c eq K B A D C
Meaning of Equilibrium Meaning of Equilibrium Constant Constant • K>>1: reaction is product-favored; equilibrium concentrations of products are greater than equilibrium concentrations of reactants.
- Spring '19