CMH 121 Luca Preziati Chapter 7: Reaction Rates and Chemical Equilibrium Chemical kinetics: Chemical kinetics: The study of the rates of chemical reactions. Consider the reaction that takes place when chloromethane and sodium iodide are dissolved in acetone; the net ionic equation for this reaction is: To determine the rate of this reaction, we measure the concentration of iodomethane at periodic time intervals, say every 10 minutes.

CMH 121 Luca Preziati Chapter 7: Reaction Rates and Chemical Equilibrium The rate of reaction is the increase in concentration of iodomethane divided by the time interval. For example, the concentration might increase from 0 to 0.12 mol/L over a 30 minute time period. The reaction rate over this period is: This unit is read mole per liter per minute.

CMH 121 Luca Preziati Chapter 7: Reaction Rates and Chemical Equilibrium The rates of chemical reactions are affected by the following factors: On a microscopic scale: 1.Molecular collisions 2.Activation energy On a macroscopic scale: 1.Nature of the reactants 2.Concentration of the reactants 3.Temperature 4.Presence of a catalyst

CMH 121 Luca Preziati Chapter 7: Molecular collisions Effective collision: Effective collision: A collision that results in a reaction An effective collision happens when two conditions are met: properly oriented 1.The colliding particles must be properly oriented for bond breaking and bond making energy 2.The colliding particles must have enough kinetic energy Activation energy: Activation energy: The minimum energy required for a reaction to take place.

CMH 121 Luca Preziati Chapter 7: Reaction Rates and Energy Energy diagram for an exothermic reaction. Energy diagram for an endothermic reaction. Transition state: Transition state: A maximum on an energy diagram.

CMH 121 Luca Preziati Chapter 7: Reaction Rates Nature of reactants In general, reactions between ions in aqueous solution are very fast (activation energies are very low). In general, reaction between covalent compounds, whether in water or another solvent, are slower (their activation energies are higher). Concentration In most cases, the reaction rate increases when the concentration of either or both reactants increases. For many reactions, there is a direct relationship between concentration and reaction rate. When concentration doubles the rate doubles.

CMH 121 Luca Preziati Chapter 7: Reaction Rates Temperature In virtually all reactions, rate increases as temperature increases. An approximate rule for many reactions is that for a 10°C increase in temperature, the reaction rate doubles. When temperature increases, molecules move faster (they have more kinetic energy), which means that they collide more frequently. More frequent collisions mean higher reaction rates. Not only do molecules move faster at higher temperatures, but the fraction of molecules with energy equal to or greater than the activation energy also increases.

CMH 121 Luca Preziati Chapter 7: Reaction Rates Catalyst: Catalyst: A substance that increases the rate of a chemical reaction without itself being used up.

CMH 121 Luca Preziati Chapter 7: Chemical Equilibrium Reversible reaction: Reversible reaction: A reaction that can be made to go in either direction. Equilibrium: Equilibrium: A dynamic state in which the rate of the forward reaction is equal to the rate of the reverse reaction. At equilibrium there is no change in concentration of either reactants or products. Reaction, however, is still taking place. Reactants are still being converted to products and products to reactants, but the rates of the two reactions are equal.

CMH 121 Luca Preziati Chapter 7: Chemical Equilibrium Equilibrium constant, K: Equilibrium constant, K: The product of the concentrations of products of a chemical equilibrium divided by the concentrations of reactants, each raised to the power equal to its coefficient in the balanced chemical equation. For the general reaction: The equilibrium constant expression is: There is no relationship between a reaction rate and the value of K.

CMH 121 Luca Preziati Chapter 7: Chemical Equilibrium Problem: Problem: When H 2 and I 2 react at 427°C, the following equilibrium is reached: The equilibrium concentrations are [I 2 ] = 0.42 mol/L, [H 2 ] = mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K.Solution: This K has no units because molarities cancel.

CMH 121 Luca Preziati Chapter 7: Le Chatelier’s Principle Le Chatelier’s Principle: Le Chatelier’s Principle: When a stress is applied to a chemical system at equilibrium, the position of the equilibrium shifts in the direction to relieve the applied stress. We are going to look at three types of stress that can be applied to a chemical equilibrium: addition of a reaction component removal of a reaction component change in temperature

CMH 121 Luca Preziati Chapter 7: Chemical Equilibrium Problem: Problem: When acid rain (H 2 SO 4 (aq))attacks marble (calcium carbonate), the following equilibrium can be written: How does the fact that CO 2 is a gas influence the equilibrium? Solution: Solution: CO 2 gas diffuses from the reaction site, and is removed from the equilibrium mixture. The equilibrium shifts to the right and the marble continues to erode.

CMH 121 Luca Preziati Chapter 7: Chemical Equilibrium