Thermochemistry and Kinetics

Kinetics The branch of chemistry that is concerned with reaction rates and reaction mechanisms Reaction rate:Reaction rate: change in concentration of reactants per unit of time as a reaction proceeds Reaction mechanisms:Reaction mechanisms: the step-by-step sequence of reactions by which the overall chemical change occurs

Collision Theory In order for reactions to occur between substances, their particles (molecules, atoms, or ions) must collide. These collisions result in interactions between particles. collision theory.The set of assumptions regarding collisions and reactions is known as collision theory.

Activation Energy sufficient energyWhen a collision is violent enough, old bonds can be broken and new bonds can form, but only collisions that occur with sufficient energy can be effective. Activation Energy is the minimum energy needed for a reaction to occur.

Collision Theory Activated complex

Endothermic Reaction +ΔH (heat of reaction) value; enthalpy Products less stable than reactantsProducts less stable than reactants Exothermic Reaction -ΔH (heat of reaction) value; enthalpy Products more stable than reactantsProducts more stable than reactants

Chemical Kinetics The activation energy often acts like a barrier preventing a reaction from happening easily. Are there ways of overcoming this barrier without the addition of the required energy? Activation Energy

E a Diagram with catalyst Catalyst = Decreases Activation Energy

Rate-Influencing Factors Nature of reactants (Increases Rxn.) Surface area (Increases Rxn.) Concentration(Increases Rxn.) Temperature (Increases Rxn.) Presence of catalysts (Decreases activation energy)Presence of catalysts (Decreases activation energy)

Factors Affecting Reaction Rate Temperature Two Things Happen: (1)The collisions are more forceful resulting in more collisions having the minimum energy (E a ) to be successful. This is shown on the graph at the left.

Factors Affecting Reaction Rate Temperature Two Things Happen: (2) Since the particles have more energy, they are moving faster and the collision frequency is increased. This results in more collisions per time unit thus increasing the odds of a successful collision. ©Mr. D. Scott; CHS

Factors Affecting Reaction Rate Concentration This factor is mainly a surface area issue. Let’s start with a simple idea: Building a camp fire.

Factors Affecting Reaction Rate Concentration This factor is mainly a surface area issue. First, These must be split into smaller pieces.

Factors Affecting Reaction Rate Concentration This factor is mainly a surface area issue. Next, Some of these need to be placed under the larger logs Before these are used to start the fire.

Factors Affecting Reaction Rate Concentration This factor is mainly a surface area issue. Smaller pieces of material react faster than larger pieces.

Factors Affecting Reaction Rate Concentration This factor is mainly a surface area issue. Higher concentration of material reacts faster than lower concentration

Factors Affecting Reaction Rate Concentration This factor is mainly a surface area issue. Gases react faster than liquids which react faster than solids.

EXOTHERMIC & ENDOTHERMIC REACTIONS Exothermic process: a change (e.g. a chemical reaction) that releases heat. A release of heat corresponds to a decrease in enthalpy Exothermic process:  H < 0 (at constant pressure) Burning fossil fuels is an exothermic reaction

Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat. An input of heat corresponds to an increase in enthalpy Endothermic process:  H > 0 (at constant pressure) Endothermic Photosynthesis is an Endothermic reaction (requires energy input from sun) Exothermic Cellular Respiration is an Exothermic reaction (releases energy)