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Chemistry 122

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.

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Chemistry 122

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  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

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