Chapter 16 Chemical Equilibrium Different States a System

Chapter 16 Chemical Equilibrium

Different States a System Can Be In n n A state of change No change (there are several no change states

No Change States static system – no movement, no change (example: a rock in a jar) steady state system – is a flow through system with no net change, is an open system (example: the earth and the flow of energy from the sun to the earth and then into outer space) equilibrium – the forward process is equal to the reverse process so there is no net change, is a closed system

n n Equilibrium examples a reversible chemical reaction 2 NO 2(g) ↔ N 2 O 4(g) The forward and reverse reactions are taking place at the same time. When the rate of the forward reaction is equal to the rate of the reverse reaction, the system is at equilibrium.

The law of Chemical Equilibrium Every reversible reaction proceeds to an equilibrium state. n Regardless of how much of each chemical you start with the system will shift so that concentrations of the products divided by the concentrations of the reactants will always be a constant. n Do sample problem page 543 n Assign practice problems 1 & 2 on page 543 n

Equilibrium constant is Keq If Keq 1 the products are favored. Keq 1 the reactants are favored. The concentrations of the pure solids or liquids can be left out of the formula – Their concentration does not change and becomes part of the Keq. Do sample problem 2 on page 545 Assign practice problems 3 & 4 on page 545 (write Keq and give the equation)

n Reaction Quotient The reaction quotient is the same mathematical expression as Keq only it is taken when the system is not at equilibrium.

Le Chatelier’s Principle (leh-shaht-lee-ay) When pressure (stress) is placed on a system the system will shift to relieve the pressure (stress). Increasing the pressure will favor the side that requires less volume. 2 NO 2(g) ↔ N 2 O 4(g) Changing the pressure will change the equilibrium constant.

Adding a compound to an equilibrium This will “push” the reaction to favor the other side of the equation. 2 NO 2(g) ↔ N 2 O 4(g) This will remove some of the compound that has been added. This does not change the equilibrium constant.

n n n Removing a compound 2 NO 2(g) ↔ N 2 O 4(g) This will cause the equilibrium to shift to favor (make more of) the material that is being removed. This does not change the equilibrium constant.

Effects of changing Temperature H 2 + I 2 ↔ 2 HI + heat Adding heat will drive the reaction to the left. Removing heat (cooling) pulls the reaction to the right. This will change the equilibrium constant Chapter questions page 558 -559 (1 - 14, 17, 18, 20, 21, 23, 24, 26, 27, 28, 29)