Presentation on theme: "Kinetics and Equilibrium review (Items 114-132 of 200 ways..) 18.1 Kinetics deals with the rates of chemical reactions. In chemistry, the rate of chemical."— Presentation transcript:
Kinetics and Equilibrium review (Items 114-132 of 200 ways..) 18.1 Kinetics deals with the rates of chemical reactions. In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time. Equilibrium refers to the condition where forward and reverse reactions are occurring at the same rates.
The minimum energy that colliding particles must have in order to react is called the activation energy. 18.1 An activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. The activated complex is sometimes called the transition state.
Factors affecting Reaction Rates What four factors influence the rate of a chemical reaction? 18.1 The rate of a chemical reaction depends upon temperature, concentration, particle size, the use of a catalyst.
A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance called chemical equilibrium. The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction. At chemical equilibrium, no net change occurs in the actual concentration of the reactants and products – i.e. concentrations are constant. (Note this does not mean that the concentration of reactants and products are equal – avoid this common mistake).
Reversible Reactions How do the amounts of reactants and products change in a chemical system at equilibrium? At chemical equilibrium, no net change occurs in the actual amounts or concentration of the reactants and products. If the rate of the shoppers going up the escalator is equal to the rate of the shoppers going down, then the number of shoppers on each floor remains constant, and there is an equilibrium.
Three stresses can cause a change in the equilibrium position of a chemical system? changes in the concentration of reactants or products, changes in temperature, changes in pressure ( for equilibria involving gases ). 18.2 The French chemist Le Châtelier proposed what has come to be called Le Châtelier’s principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.
Physical Equilibria – Saturated Solutions Saturated solutions are another example involving physical equilibrium. The term “saturated solutions” refers to a solution containing the maximum amount of solute that will dissolve at a given temperature. The rate at which a substance crystallizes out of solution is equal to the rate at which it dissolves. pHET simulation for saturated solutions http://phet.colorado.edu/en/simulation/soluble-salts
Saturated Solution: = contains the maximum amount of solute for a given quantity of solvent at a constant temperature & pressure if additional solute is added, it will not dissolve; rather it will precipitate out Unsaturated Solution: = a solution that contains less solute than a saturated solution at a given temperature & pressure if additional solute is added, it will dissolve Supersaturated Solution: = contains more solute than it can theoretically hold at a given temperature crystallization will be initiate if a very small “seed crystal” of solute is added **very unstable**