EQUILIBRIUM

How Chemical Reactions Occur Collision model – molecules must collide in order for a reaction to occur Rate depends on concentrations of reactants and temperature.

Conditions That Affect Reaction Rates Concentration – increases rate because more molecules lead to more collisions Temperature – increases rate Why?

Conditions That Affect Reaction Rates Activation energy – minimum energy required for a reaction to occur

Conditions That Affect Reaction Rates Catalyst – a substance that speeds up a reaction without being consumed Enzyme – catalyst in a biological system

Reactions Homogeneous reaction – all reactants and products are in one phase Gas Solution Heterogeneous reaction – reactants in two phases

Reactions

The Equilibrium Condition Equilibrium – the exact balancing of two processes, one of which is the opposite of the other

The Equilibrium Condition Chemical equilibrium – a dynamic state where the concentrations of all reactants and products remain constant

Chemical Equilibrium: A Dynamic Condition Equal numbers of moles of H2O and CO are mixed in a closed container. The reaction begins to occur, and some products (H2 and CO2) are formed. The reaction continues as time passes and more reactants are changed to products. Although time continues to pass, the numbers of reactant and product molecules are the same as in (c). No further changes are seen as time continues to pass. The system has reached equilibrium.

Chemical Equilibrium: A Dynamic Condition Why does equilibrium occur?

The Equilibrium Constant: An Introduction Law of chemical equilibrium For a reaction of the type aA + bB  cC + dD Equilibrium expression Each set of equilibrium concentrations is called an equilibrium position.

Applications Involving the Equilibrium Constant The Meaning of K K > 1  the equilibrium position is far to the right *Favors products K < 1  the equilibrium position is far to the left *Favors reactants

Heterogeneous equilibria – an equilibrium system where the products and reactants are not all in the same state *Their value is 1

Le Chatelier’s Principle Le Chatelier’s Principle – when a change is imposed on a system at equilibrium the position of the equilibrium shifts in a direction that tends to reduce the effect of that change

Effect of a Change in Concentration

Effect of a Change in Concentration When a reactant or product is added the system shifts away from that added component. If a reactant or product is removed, the system shifts toward the removed component.

Effect of a Change in Volume The system is initially at equilibrium. The piston is pushed in, decreasing the volume and increasing the pressure. The system shifts in the direction that consumes CO2 molecules, lowering the pressure again.

Effect of a Change in Volume Decreasing the volume The system shifts in the direction that gives the fewest number of gas molecules.

Effect of a Change in Volume Increasing the volume The system shifts in the direction that increases its pressure.

Effect of a Change in Temperature The value of K changes with temperature. We can use this to predict the direction of this change. Exothermic reaction – (heat is a product) Adding energy shifts the equilibrium to the left (away from the heat term). Endothermic reaction – (heat is a reactant) Adding energy shifts the equilibrium to the right (away from the heat term).

The value of K for a system can be calculated from a known set of equilibrium concentrations. Unknown equilibrium concentrations can be calculated if the value of K and the remaining equilibrium concentrations are known.