Unit 10: Energy in Chemical Reactions Cypress Creek High School Chemistry 1L Chapter 10

Reaction Rate Reaction Rate is the number of atoms, ions or molecules that react in a given time to form products We measure either the rate of disappearance of the reactants or the rate of appearance of one of the products In simpler terms: The average rate is the change in a given quantity during a specific period of time. 2

Reaction Rate Graph For the reaction H2 + I2 → 2HI, What is happening to the concentration of the reactants? What is happening to the concentration of the products? 3

Measuring Reaction Rates Change in Electrical Conductivity Change in Color Change in Pressure Change in Volume 4

Factors that Effect Reaction Rate Physical State of the Reactants The more freedom of movement, the more collisions As state of matter changes from solid to gas, rate increases

Factors that Effect Reaction Rate Concentration of Reactants The more particles, the more collisions As concentration increases, rate increases 6

Factors that Effect Reaction Rate Temperature of Reactants As temperature increases, particles move faster; there are more high-energy collisions 7

Factors that Effect Reaction Rate Particle Size (surface area) For solids, breaking up big pieces increases surface area, increasing rate by having more places for the molecules to interact Ionic compounds have more surface area than covalent 8

Factors that Effect Reaction Rate Pressure – Gases only! As pressure increases the concentration increases, so you will have more collisions 9

Factors that Effect Reaction Rate Catalysts – compounds added to speed up reaction but not used in the reaction. Lowers activation energy but does not change ΔH Examples: enzymes, catalytic converters (Pt, plates that convert NO → N2 + O2 and CO → CO2) 10

Catalyst 11

Catalysts Without a catalyst Enthalpy Time (min)

Catalysts Lowers activation Without a energy catalyst Enthalpy With a Time (min)

Catalysts DOES THE CATALYST CHANGE THE DH? NO! Lowers activation energy Without a catalyst Enthalpy H reactants With a catalyst H products Time (min)

Factors that Effect Reaction Rate Inhibitors Compounds added to a reaction that slow it down Examples: Lead in diesel, preservatives in food. 15

Reaction Rates Discussion Why do you add lighter fluid to charcoal in a grill? How does opening and closing air vents on a charcoal grill effect how fast the coals burn? How can an explosion occur in a grain elevator when grain is not considered flammable? 16

Chemical Reactions Example Remember the classic science fair project - the homemade volcano? It’s done by combining vinegar and baking soda (and some red food coloring for effect). These substances undergo a chemical reaction, evident by the overflow and bubbles. This reaction produces three new substances: water, carbon dioxide, and sodium acetate (used in hot packs) Reaction of baking soda and vinegar: http://www.metacafe.com/watch/706897/vinegar_grenade/