Section 1 Reversible Reactions and Equilibrium

2. Section 1 Reversible Reactions and Equilibrium Completion Reactions and Reversible Reactions Do all reactants change into products during a reaction? Sometimes only a trace of reactants remain after the reaction is over. Completion Reaction – the reactants are used to make products and the products are very stable and do not reverse and make any reactants. 2 C8H18 + 25O2 18 H2O + 16 CO2 Gasoline + oxygen  water + carbon dioxide This reaction is one directional. The gasoline will burn with oxygen and create water and carbon dioxide. None of the water and carbon dioxide will react and make gasoline and oxygen. (notice the one directional arrow between the reactants and products)

3. Reversible Reactions Reach Equilibrium A chemical reaction in which the products re-form the original reactants is called a reversible reaction In the equation below the reactants form the products and some of the product react to form reactants. Take note of the arrows between the products and reactants. CaCl2(aq) + Na2SO4(aq)  CaSO4(s) + 2NaCl(aq)

4. Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of products and reactants remain unchanged Opposing Reaction Rates Are Equal at Equilibrium When CO2 is dissolved in water to make carbonic acid (the fizz in soda), the reaction is written as seen below. CO2(g) + H2O(l)  H2CO3(aq) If the soda is left unopened some carbonic acid will go back to form carbon dioxide and water, as seen in the reaction below. H2CO3(aq) CO2(g) + H2O(l) This should be written as a reversible reaction. as written below. CO2(g) + H2O(l)  H2CO3(aq) When the equilibrium is reached the amount of product and reactants remains the same but both reactions continue as the same rate.

5. Section 2 - Skip Section 3 - Equilibrium Systems and Stress Le Châtelier’s Principle Le Châtelier’s principle the principle that states that a system in equilibrium will oppose a change in a way that helps eliminate the change. This principle is used to explain how a stress (change) move the point of equilibrium. Chemical equilibria respond to three kinds of stress: changes in the concentrations of reactants or products, changes in temperature, and changes in pressure

6. Before we get started we need to define some terms. Stress: any change in condition that will effect an equation that is at equilibrium. Shift: Shifts are said to be to the right or the left: Shift to the right favors the forward reaction causing more products to be formed and the amount of reactants to decrease. Shift to the left favors the reverse reaction causing more reactants to be formed and the amount of products to decrease. Push away / Pull toward If you push away the reaction shifts away from the stress. If you pull toward the reaction shift toward the stress When we study the effects of stress on equilibrium we will apply it to the equation below. This equation shows the formation of ammonia. N2(g) + 3H2(g)  2NH3(g) + 91.8 kilojoules

7. Change in Concentration If a chemical’s concentration is increased there are more particles of that chemical to react and push the reaction away from the chemical. If a chemical’s concentration is decreased there are fewer particles of that chemical to react and the reaction will pull toward the chemical. For example: N2(g) + 3H2(g)  2NH3(g) + 91.8 kilojoules What happens if I increase the concentration of Nitrogen gas? More nitrogen will be available to react with hydrogen gas to form more ammonia and heat. An increase in concentration causes the reaction to push away. The equilibrium will shift to the right. The concentration of hydrogen will go down, ammonia up, and an increase in heat production.

8. Another example: N2(g) + 3H2(g)  2NH3(g) + 91.8 kilojoules What happens if I decrease the amount of ammonia? Less ammonia will be available to combine with heat to form hydrogen gas and nitrogen gas. A decrease in concentration causes the reaction to pull toward. The equilibrium will shift to the right. The concentration of hydrogen will go down, nitrogen down, and an increase in heat production.

9. Answer the following question if I decrease the concentration of hydrogen gas. N2(g) + 3H2(g)  2NH3(g) + 91.8 kilojoules Will the stress favor the forward or reverse reaction? Will this result in a shift to the right or left? Reverse Left Will the amount of each substance increase or decrease? Nitrogen gas Ammonia Heat Increase Decrease Decrease

10. Change in Temperature Temperature will effect the equilibrium of a chemical reaction. Remember an endothermic reaction absorbs energy and has a positive H. An Exothermic reaction releases energy and has a negative H. Energy is located on the left (reactant) for endothermic reactions causing the reaction to cool, with exothermic reactions having energy on the right (products) causing the reaction to warm. The rules for applying Le Châtelier’s Principle to changes in temperature are the same for concentration. If the temperature in reaction is increased the reaction will push away from the energy. If the temperature in reaction is decreased the reaction will pull toward the energy.

11. For example: N2(g) + 3H2(g)  2NH3(g) + 91.8 kilojoules Is this reaction endothermic or exothermic? Exothermic Which direction will this reaction shift if I increase the temperature? Left What will happen to the concentration of ammonia if I decrease the temperature? Increase What will happen to the concentration of hydrogen if I increase the temperature? Increase What will happen to the concentration of nitrogen if I decrease the temperature? Decrease

12. Change in Pressure Pressure has almost no effect on equilibrium reactions that are in solution. Gases in equilibrium, however, may be affected by changes in pressure. If you increase pressure the side of the reaction with few moles of gas will be favored and will shift in that direction. If you decrease pressure the side of the reaction with more moles of gas will be favored and will shift in that direction. N2(g) + 3H2(g)  2NH3(g) + 91.8 kilojoules The above reaction has 4 moles of reactants and 2 moles of products that are gases. Increasing pressure will favor the forward reaction and decreasing pressure will favor the reverse reaction.

13. If I increase the pressure how will it affect each of the following reactions and the amount of each reactant and product? 2Al(s) + 3HCl(aq) → 2AlCl3(aq) + 3H2(g) Shift to the left. Increase in aluminum and hydrochloric acid, with a decrease in aluminum chloride and hydrogen gas. H2(g) + I2(g) → 2HI(g) No shift or change in concentration, due to equal moles of gas on both product and reactant side. N2(g) + 3H2(g)  2NH3(g) Shift to the right Increase in production of ammonia and decrease in nitrogen and hydrogen

14. The Common-Ion Effect The reduction of the solubility of a salt in the solution due to the addition of a common ion is called the common-ion effect. If in a beak we establish an equilibrium between copper(I) chloride and dissolved copper(I) ions and chloride ions in solution as seen in the equation Below. CuCl(s)  Cu+(aq) + Cl−(aq) If we add some NaCl to the beaker it will cause the production of Na+ ions and Cl- ions in solution as seen in the equation below. NaCl(s)  Na+(aq) + Cl−(aq) Both equations have a common ion, Cl-, This concentration will combine To create the effect of an increase in concentration of Cl-. The copper(I) chloride reaction will shift to the left causing a decrease in concentration of Cu+(aq) and an increase in concentration of CuCl(s).