Equilibrium. Chapter How Reactions Occur How Reactions Occur

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1 Copyright 2004 by Houghton Mifflin Company. Equilibrium Chapter 16 ll rights reserved How Reactions Occur Kinetics: the study of the factors that effect speed of a rxn mechanism by which a rxn proceeds. Collision model: molecules must collide for a rxn to occur Effective collisions occur if collision has enough energy to break the necessary bonds they collide in the proper orientation for new bonds to form 2 BrNO Br NO How Reactions Occur Kinetics: the study of the factors that effect speed of a rxn mechanism by which a rxn proceeds. Collision model: molecules must collide for a rxn to occur Effective collisions occur if collision has enough energy to break the necessary bonds they collide in the proper orientation for new bonds to form The higher the frequency of effective collisions the faster the rxn rate. 3

2 How Reactions Occur ctivated complex: a temporary, high energy (unstable) chemical species formed when two molecules have an effective collision Not a true molecule Bonds to be broken in the rxn are weakened Bonds to be formed are not yet complete 16.2 Rate ffecting Conditions ctivation energy, E a : The difference in potential energy between the reactant molecules and the activated complex The larger the E a, the slower the rxn The E to overcome the E a comes from the collision or from the environment Different rxns have different activated complexes and therefore different E a Rate ffecting Conditions Catalysts: substances that affect the speed of a reaction without being consumed. Most catalysts are used to speed up a rxn. provides a pathway for the rxn with a lower E a Lower E a means more molecules have sufficient E to react Rxn goes faster 6

3 Rate ffecting Conditions Molecular conditions: Small molecules tend to react faster than large Better chance of proper orientation Gases tend to react faster than liquids which react faster than solids More movement, increased rate of effective collisions Powdered solids more reactive than chunks More surface area for contact with other reactants Certain types of chemicals are more reactive than others Ions react faster than molecules 16.2 Rate ffecting Conditions Concentration: The larger the concentration of reactant molecules, the faster the reaction will go. Increases the frequency of collision Concentration of gases depends on the partial pressure of the gas; Higher pressure = Higher concencentration Concentration of solutions depends on the solute to solution ratio (molarity). Temperature: Increasing T always increases rxn rate Higher T means faster movement & more collisions Equilibrium Conditions If the products of a rxn are removed from the system as they are made, then a rxn will proceed until the LRs are used up. If the products are allowed to accumulate, they will start reacting together to form the original reactants - called the reverse reaction. 9

4 Equilibrium Conditions Chemical equilibrium: the forward rxn is using reactants & making products as fast as the reverse rxn is using products & making reactants. rate forward = rate reverse 16.4 Chemical Equilibrium Equilibrium only occurs in a closed system!! When a system reaches equilibrium, the amounts of reactants & products stays constant the forward & reverse rxns still continue, but at the same rate There is a mathematical relationship between the amounts of reactants & products at equilibrium no matter how much reactants or products you start with. 11 [ a + bb cc + dd C ] c [ D] d Equilibrium constant, K: a mathematical relationship between the amounts of reactants & products at equilibrium Does not include l or s, only sol ns & g K depends on the rxn the temperature K is independent of the amounts of reactants & products you start with. 12

5 13 Write the equilibrium expression for each of the following reactions: 1) HC 2 H 3 O 2(aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 - (aq) 2) copper(ii) nitrate and sodium iodide 3) The autoionization of water Calculate the equilibrium constant for the reaction of nitrogen tribromide gas reacting to form nitrogen gas and bromine gas, given the equilibrium concentrations of 2.07x10-3 M, 4.11x10-2 M & 1.06x10 3- M, respectively. 14 Size of K >1 [products]>[reactants] Products favored Forward reaction favored HCl + H 2 O H 3 O + + Cl - K= 15

6 16 Size of K >1 Forward reaction favored <1 [reactants]>[products] Reactants favored Reverse reaction favored HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O - 2 K=1.78x10-5 Size of K >1 Forward reaction favored <1 Reverse reaction favored ~1 [products] [reactants] Neither reactants nor products favored Neither reaction direction favored N H 2 2 NH 3 K= Equilibrium position: The relative [reactants] & [products] when a reaction reaches equilibrium Different initial amounts of reactants will result in different equilibrium concentrations The same K will be reached each time N 2(g) + 3 H 2(g) 2 NH 3(g) at 500 C [ NH 3 ] 2 Exp [N 2 ] 0 (M) [H 2 ] 0 (M) [NH 3 ] 0 (M) [ N 2 ][ H 2 ] 3 K ( 0.157) ( 0.921) ( 0.763) ( ) ( 0.399) ( 1.197) ( 1.82) ( 2.59) ( 2.77)

7 19 Le Châtelier's Principle when a change is imposed on a system at equilibrium, the position of equilibrium will shift in the direction that will reduce the effect of that change Predicts effect changes have on equilibrium position while K remains the same Concentration Volume Temperature Le Châtelier's Principle when a change is imposed on a system at equilibrium, the position of equilibrium will shift in the direction that will reduce the effect of that change Concentration Increase [reactant] Larger denominator Rxn shifts forward Reduce reactants Increase products a + bb cc + dd [ C ] c D [ ] d 20 Le Châtelier's Principle when a change is imposed on a system at equilibrium, the position of equilibrium will shift in the direction that will reduce the effect of that change Concentration Increase [product] Larger numerator Rxn shifts back Reduce products Increase reactants a + bb cc + dd [ C ] c D [ ] d 21

8 22 Le Châtelier's Principle when a change is imposed on a system at equilibrium, the position of equilibrium will shift in the direction that will reduce the effect of that change Concentration Decrease [product] Smaller numerator Rxn shifts forward Reduce reactants Increase products a + bb cc + dd [ C ] c D [ ] d Le Châtelier's Principle when a change is imposed on a system at equilibrium, the position of equilibrium will shift in the direction that will reduce the effect of that change Concentration Decrease [reactants] Smaller denominator Rxn shifts back Reduce products Increase reactants a + bb cc + dd [ C ] c D [ ] d 23 a + bb cc + dd Volume Decreasing volume increases pressure increases [gas] Effect is greatest where there are the most C moles of gas N H 2 2 NH 3 Decreasing volume would affect reactants most increasing [reactants] shifting equilibrium forward [ ] c [ D] d 24

9 25 a + bb cc + dd + heat Temperature Think of energy as a reactant or a product Exothermic Heat given off C Heat is a product Increasing heat would shift equilibrium back Cooling would shift equilibrium forward [ ] c [ D] d a + bb + heat cc + dd Temperature Think of energy as a reactant or a product Exothermic Heat is a product C Endothermic Heat is taken in Heat is a reactant Increasing heat would shift equilibrium forward Cooling would shift equilibrium back [ ] c [ D] d 26 Indicate how each of the following equilibrium positions would be affect by the change listed 1)dd sulfur dioxide to the reaction of sulfur dioxide and oxygen to form sulfur trioxide 2)Increase the volume for the above reaction 3)dding hydrochloric acid to a solution of silver nitrate solid in equilibrium with it s ions. 4)dding heat to the exothermic neutralization of sodium hydroxide with sulfuric acid. 27

10 Solubility Equilibria Soluble salts can form a saturated sol n Soluble: at least 0.1 g will dissolve per 100 g H 2 O Insoluble salts dissolve somewhat in water insoluble = less than 0.1 g per 100 g H 2 O The solubility of salts is described by the equilibrium between undissolved solid & its ions (aq) Sol n must be saturated n X m(s) n + (aq) + m Y- (aq) Solubility product: the equilibrium constant associated with dissolution K sp = [ + ] n [Y - ] m 16.9 Solubility Equilibria Determine the solubility of zinc(ii) carbonate (the maximum molarity that can be achieved) given K sp = 5.1 x Solubility Equilibria Determine the solubility of lead(ii) iodide given K sp = x