Collision Model • Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy). • Reactants must have proper orientation to allow the formation of new bonds.
Reactions can occur: • Very fast – such as a firecracker • Very slow – such as the time it took for dead plants to make coal • Moderately – such as food spoilage • A “rate” is a measure of the speed of any change that occurs within an interval of time • In chemistry, reaction rate is expressed as the amount ofreactant changing per unit time. Example: 3 moles/year, or 5 grams/second
Activation Energy The minimum energy required to transform reactants into the activated complex (The minimum energy required to produce an effective collision) Flame, spark, high temperature, radiation are all sources of activation energy
Collision Model • Collisions must have enough energy to produce the reaction: must equal or exceed the “activation energy”, which is the minimum energy needed to react. • Will aAA battery start a car? • Think of clay clumps thrown together gently – they don’t stick, but if thrown together forcefully, they stick tightly to each other.
Processes in which energy is released as it proceeds, and surroundings become warmer Exothermic Processes Reactants Products + energy
Processes in which energy is absorbed as it proceeds, and surroundings become colder Endothermic Processes Reactants + energy Products
Reaction Rates: 2NO2(g) 2NO(g) + O2(g) 1. Can measure disappearance of reactants 2. Can measure appearance of products 3. Are proportional stoichiometrically
The Reaction Mechanism • Thereaction mechanism is the series of steps by which a chemical reaction occurs. • A chemical equation does not tell us how reactants become products; it is a summary of the overall process. Reactants Products The sign has represents the reaction mechanism, but gives no indication of the steps in the mechanism
The Rate-Determining Step In a multi-step reaction, the slowest step is the rate-determining step. It therefore determines the rate of reaction.
Factors Affecting Rate • Temperature Increasing temperature always increases the rate of a reaction.
Things that Affect Rate • More and harder collisions. • Faster Reactions. • .
2. Surface Area Increasing surface area increases the rate of a reaction
3. Concentration Increasing concentration USUALLY increases the rate of a reaction
Things that Affect Rate • More concentrated molecules closer together • Collide more often. • Faster reaction.
Things that Affect Rate 4. Catalysts-substances that speed up a reaction without being used up.(enzyme). • Speeds up reaction by giving the reaction a new path. • The new path has a lower activation energy. • More molecules have this energy. • The reaction goes faster. • Inhibitor- a substance that blocks a catalyst.
Catalysis • Catalyst: A substance that speeds up a reaction by lowering activation energy • Enzyme: A large molecule (usually a protein) that catalyzes biological reactions. • Homogeneous catalyst: Present in the same phase as the reacting molecules. • Heterogeneous catalyst: Present in a different phase than the reacting molecules.