Rates of Chemical Reactions Chapter 21 Section 4 Rates of Chemical Reactions

In a chemical reaction… Atoms are not created or destroyed, but are rearranged (Thank you, John Dalton!!) Energy is required to make and to break chemical bonds Some chemical reactions require more energy to take place than others

Activation Energy In order for a chemical reaction to take place, the reactants need to gain a certain amount of energy or become “activated” The energy needed for a reaction to move forward is called activation energy. Activation Energy Animation

Exothermic Reactions An exothermic reaction releases energy in the form of heat or light (feels warm). The products end up having less energy than the reactants because some of the original energy was lost

Endothermic Reactions An endothermic reaction absorbs energy (feels cold). The products end up having more energy than the reactants because energy was absorbed

Exothermic vs. Endothermic Energy Curves Based on what you know about exothermic and endothermic reactions, which graph represents an exothermic reaction, and which represents an endothermic reaction? Why?

Exothermic vs. Endothermic Energy Curves

How can we speed up a reaction? In order for a chemical reaction to take place, the reactants need to have the right amount of energy and orientation Collision Theory says the rate of a reaction is affected by four factors: Concentration Surface area Temperature Catalysts

How to Increase Reaction Rates (and get a date)

Concentration How much substance do you have in a given area? The higher the concentration, the more collisions that will take place, the faster the rate of the reaction You can increase concentration by adding more particles or decreasing the volume of the container Example: Walking in the school hallways during class vs. during passing time

Surface Area The area surrounding a reactant that is available for contact with another reactant The higher the surface area, the more contact the reactants have with each other, the faster the rate of reaction Example: Walking in the school hallways during passing time while holding hands with your significant other (you bump into more people than if you were by yourself). Sawdust Creamer Cannon

Temperature An increase in temperature will increase the rate of a reaction. Temperature is a measure of the average kinetic energy of the particles – the higher the temperature, the faster the particles are moving & the more collisions which result in a reaction Example: To prevent food from spoiling quickly, we refrigerate or freeze it to slow down the chemical reactions that make food go bad Temperature and Reaction Rate Animation

Catalysts Catalysts lower the activation energy needed to make a reaction occur. They lower the energy “hill” by helping the reactants line up in the correct orientation Catalysts do not participate in the actual chemical reaction, they are just bystanders (do not get used up) Examples: Your body has special catalysts called enzymes that assist in almost all chemical reactions in your body Going to a packed concert and having the band play their best song (is like a catalyst to get people moving & colliding)