CHEMISTRY 122 Reversible Reactions and Equilibrium

Reversible Reactions  Some reactions are capable of being reversible.  When this happens, the conversion of reactants to products as well as the conversion of products to reactants is occurring simultaneously.  Ex.2SO 2(g) + O 2(g) → 2SO 3(g) - forward reaction 2SO 2(g) → 2SO 2(g) + O 2(g) - reverse reaction  They can be combined using a double arrow (reversible reaction is taking place).

Chemical Equilibrium  Figure 18.10, p  The formation of sulfur trioxide occurs when sulfur dioxide and oxygen begin to react.  At first, there is no presence of sulfur trioxide.  When it starts to form, it also begins to decompose back to sulfur dioxide and oxygen.  This continues to happen until the forward and reverse reactions are equal (occurring at the same rate).  This is called chemical equilibrium or a state of balance.

Equilibrium and Concentration  The concentrations of all three substances changes until equilibrium is met.  Remember, concentration is measured by how much there is of something, not its particular strength through dilution.  At chemical equilibrium, no net change occurs in the actual amounts of the components of the system.

Has the reaction stopped at equilibrium?  It is a dynamic state.  Both the forward and reverse reactions continue, but their rates are equal.  What variables can we manipulate to alter the concentrations of substances in reversible reactions?

Can you compare concentrations?  The concentrations of substances can vary from one to the next in a reaction  The relative concentrations of substances at equilibrium determine the equilibrium position of a reaction  This determines whether the reactants or products are favoured  Many reactions reach an equilibrium state in which the reaction mixture contains both reactant and product particles  Equilibrium refers to rate, not quantities

Comparing Reactants to Products  After the reaction has reached equilibrium, there may be more product than reactant  In this case, the products are favoured  Page 551 on how to write a favoured reaction  In other cases, when equilibrium is reached, there may be more reactant present  The reactants are favoured  Many times, one side is favoured so much, the other is hardly detectible

Comparing Reaction Types  When a reaction has changed the reactants to the point where none are detected, the reaction is said to be irreverisible  When two substances are mixed and nothing happens, there is no reaction  Reversible reactions are somewhere in the middle

Catalysts  When a catalyst is added to a reversible reaction, it influences the rate of change on both counts.  They do not influence the amounts of reactant and product, only the time it takes for the reaction to take place.

LeChatelier's Principle  When a disturbance is introduced to a reaction at equilibrium, adjustments are made to restore order to the system  The equilibrium position will be different from its original placement  The amount of product or reactant will be changed  This is called a shift in the equilibrium position " Stresses that upset the equilibrium of a chemical system include changes in the concentration of reactants or products, changes in temperature, and changes in pressure"

LeChatelier - Concentration  Changing the amount of reactant or product disturbs equilibrium  Ex. the decomposition of carbonic acid  Without disturbance, the products are favoured. Carbon dioxide and water constitute 99% of the amount present at equilibrium  If more carbon dioxide is introduced into the reaction, more carbonic acid will form  The new equilibrium will be set showing a favour towards the reactant  Adding a product to a reaction at equilibrium pushes a reversible reaction in the direction of the reactants

On the other hand…  If product is removed, its concentration decreases  The rate of the reverse reaction will decrease  If the amount of reactant is lessened, the amount of product will also lessen  Equilibrium will be restored, but with a new equilibrium position  Removing a product always pushes a reversible reaction towards the direction of the products

Practical Application  There is a balance of carbonic acid and carbon dioxide in the body. During times of exercise, carbon dioxide builds up. In order to restore order and not have a buildup of carbonic acid, our bodies respond. What do we do?  Where do you think carbonic acid is stored in the body?

One more time!  When a reactant is added to a system at equilibrium, the reaction shifts in the direction of the products.  When a product is added to a system at equilibrium, the reaction shifts in the direction of formation of the reactants.

LeChatelier - Temperature  When temp