Understanding Reversible Reactions and Equilibrium in Chemistry

1. Chemistry 122 Reversible Reactions and Equilibrium

2. 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. 2SO2(g) + O2(g) → 2SO3(g)- forward reaction 2SO2(g) → 2SO2(g) + O2(g) - reverse reaction • They can be combined using a double arrow (reversible reaction is taking place).

3. Chemical Equilibrium • Figure 18.10, p. 550. • 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.

4. 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.

5. 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?

6. 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

7. 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

8. 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

9. 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.

10. 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"

11. 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

12. 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

13. 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?

14. 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.

15. LeChatelier - Temperature • When temp