Chapter 7.4

* Reaction Rates tell you:  the rate that reactants change into products * Fast or slow, controlled or uncontrolled - total amount of energy released is the same in all cases Reaction Rates

* Changes over time are expressed as rates. Examples : * changes in location are often expressed as speed: * mph (miles per hour)kph (kilometers per hour) * changes in volume are often expressed as flow: * gps (gallons per second)lps (liters per second) * Reaction rates are often expressed as: * How fast reactants are consumed * How fast products are formed * How fast energy is absorbed or released Reaction Rates

Some Factors Affecting Chemical Reaction Rates: * Temperature * Surface area * Concentration * Stirring * Catalysts

* Reaction rate depends on how often reactant particles collide. If collision are more frequent  reaction rate increases. If collisions are less frequent  reaction rate decreases. * Reaction rates can be changed Vary conditions so collisions increase or decrease Factors Affecting Reaction Rates: General

* Increasing temperature increases reaction rate * Generally (but not always) * Decreasing temperature decreases reaction rate * Example: Milk stored in a refrigerator slows down the reactions that cause the milk to spoil. WHY? * Increased temperature  particles moving faster * Collide more often  Collide with more energy  faster rate of reaction Factors Affecting Reaction Rates: Temperature

* For a given mass of material: smaller particle sizes  greater surface area * Increased surface  increased collisions  More particles reacting  Faster reaction rate * Examples: * Crushed ice melts much more quickly than large block of ice * Small twigs and branches burn much more quickly than big logs Factors Affecting Reaction Rates: Surface Area

* Stirring reactants: * Increases particles exposure to each other * More exposure  faster reaction rate * Example: Clothes in washing machine * Particles of detergent react with particles of the stains on clothes * A washing machine speeds up the reaction by stirring the contents back and forth. Factors Affecting Reaction Rates: Stirring

* Concentration = # of particles in a given volume. * More particles in a given volume  more opportunities for collisions  reaction rate is faster * Greater the concentration  faster the reaction rate Factors Affecting Reaction Rates: Concentration

* Dye solution in left beaker is more concentrated than solution in right. * Increasing concentration of dye increases rate of color change in the material. Factors Affecting Reaction Rates: Concentration

* For gases, concentration changes with pressure. * Greater the pressure of a gaseous reactant  Greater is its concentration  Faster reaction rate. Factors Affecting Reaction Rates: Gas Pressure

* Catalyst: substance that affects the reaction rate without being used up in the reaction. * Catalysts can: * Speed up a reaction * Enable a reaction to occur at lower temperature. Factors Affecting Reaction Rates: Catalysts

Ex: One of the ways to make sulfuric acid is to react sulfur dioxide with oxygen to form sulfur trioxide. * This reaction happens very slowly without a catalyst * [ vanadium(V) oxide is a good catalyst for this reaction] * The catalyst is neither a reactant nor a product, so it is written over the arrow. Factors Affecting Reaction Rates: Catalysts

* Catalyst lowers energy required for effective collisions Factors Affecting Reaction Rates: Catalysts

Review - Factors Affecting Chemical Reaction Rates: Exit Slips Next Slide * Temperature * Surface area * Concentration * Stirring * Catalysts

Exit Slip Questions 1. A reaction rate is the rate at which reactants change into products over time. True False

2. What are two things that might happen to a reaction if a catalyst is added to the reaction? Exit Slip Questions

What impact will each of the following changes have on the rate of most chemical reactions (increase or decrease)? 3) decreasing the temperature 4) grinding a reactant into a fine powder 5) stirring the reaction mixture 6) decreasing the concentration of one of the reactants Exit Slip Questions