Kinetics (Reaction Rate) How Fast Does the Reaction Go

Rates of reaction KINETICS Objectives To understand that a chemical reaction involves collisions between particles To be able to describe the four factors which will affect the rate of a chemical reaction. To describe the factor that can not be changed and affect the rate of a chemical reaction.

Collision Theory In order to react molecules and atoms must COLLIDE with each other. They must hit each other with enough energy and with the correct orientation. The more “frequently” the particles hit, the faster the reaction will go. A COLLISION THAT RESULTS IN PRODUCT IS CALLED EFFECTIVE COLLISION Anything that increase these effective collisions will make the reaction faster.

How do we make the reaction go faster? There are four things that we can change to make the reaction go faster. Temperature Surface area Concentration Using a catalyst

Temperature When we increase the temperature we give the particles energy, this makes them move faster, which means they collide with other particles more often. So the reaction goes faster.

Surface area If we make the pieces of the reactants smaller we increase the number of particles on the surface which can react. This makes the reaction faster. The particles on the surface can react When cut into smaller pieces the particles on the inside can react

Concentration There are more particles in the same volume to react If we make one reactant more concentrated (like making a drink of orange squash more concentrated) There are more particles in the same volume to react So the reaction goes faster. There are less red particles in the same volume so there is less chance of a collision There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Factors that Affect Rate 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.

Factors that Affect Rate Summary Temperature Higher temperature faster particles. More and harder collisions. Faster Reactions. Concentration More concentrated means molecules closed together. Collide more often. Faster reaction.

Particle size Molecules can only collide at the surface. Smaller particles bigger surface area. Smaller particles faster reaction. Smallest possible are molecules or ions. Dissolving speeds up reactions. Getting two solids to react with each other is slow.

Using a catalyst A catalyst is a chemical which is added to a reaction. It makes the reaction go faster. The catalyst does not get used up in the reaction.

Potential Energy Diagrams Represent the changes in potential energy that occur during a chemical reaction. The reaction coordinate represents the progress of the reaction.

Remember in terms of energy content HIGH ENERGY =UNSTABLE LOW ENERGY = STABLE

Classification of reactions Exothermic Rx: releases energy. Heat part of the products. Endothermic Rx: absorbs energy. Heat part of the reactants.

Definitions Activation Energy: the minimum amount of energy needed to start up a chemical reaction. Activated Complex: a temporary intermediate product. Highest point of the diagram. Is UNSTABLE.

Catalyzed Reactions Need less energy to start up the reaction. The presence of a catalyst LOWERS THE ACTIVATION ENERGY!!!

Energy Reactants Products Reaction coordinate

Activation Energy - Minimum energy to make the reaction happen Reactants Products Reaction coordinate

Activated Complex or Transition State Energy Reactants Products Reaction coordinate

Energy Reactants Overall energy change (DH) Products Reaction coordinate

Thermochemistry The study of the changes in heat energy that accompany chemical reactions and physical changes.

What is DH? DH is the enthalpy, or “heat of reaction.” The amount of energy released or absorbed during a chemical reaction. By convention DHrxn = Hproducts - Hreactants

What is an exothermic reaction? A reaction that gives off or releases heat. Since... DHrxn = Hproducts - Hreactants then DHrxn will be a negative number

Energy Reactants Overall energy change (DH) Products Reaction coordinate

What is an endothermic reaction? A reaction that absorbs heat. Since… DHrxn = Hproducts – Hreactants then DH will be a positive number.

Products Energy Overall energy change (DH) Reactants Reaction coordinate

Heats of Reaction Table I The new reference table (Table I) shows different heats of reaction for some common reactions.

EXOTHERMIC RX Release heat. DH for reaction is negative The value for DH is the amount of heat released during the rx We write the heat it as PRODUCT in the chemical equation The greater the magnitude of DH the more stable the product will be!!!!

Exothermic vs Endothermic

ENDOTHERMIC RX Absorb heat DH for reaction is positive The value for DH is the amount of heat absorbed during the rx We write the heat it as REACTANT in the chemical equation The greater the magnitude of DH the more UNstable the product will be!!!!

MC Q-FACTORS 2 1 4 3 4 2 1

INTERPRETING REACTION COORDINATES handout B A G F B E I D J C H A

HEATS OF REACTION MC 1 4 2 4 1

PE DIAGRAMS FOR RX EXO -30 20 10 60 ENDO 25 30 SAME H E B G I F A C D