23.4 Notes – Reaction Rates and Energy

Objectives Identify the source of energy changes in chemical reactions. Compare and contrast endothermic and exothermic reactions. Describe the factors that affect reaction rate. Discuss the effects of catalysts and inhibitors on the speed of chemical reactions

Chemical Kinetics The area of chemistry concerned with the speed at which reactions occur

Collision Theory In order for reactants to form products the reactants must collide with enough energy to start the reaction (activation energy)

Energy Changes Chemical reaction involve energy exchange –Breaking chemical bonds requires energy –Forming chemical bonds releases energy

More Energy Out In exergonic reactions, the energy required to break bonds is less than the energy released from new bonds –The energy given off is usually light

Exothermic Reaction Give off energy in the form of heat

More Energy In In endergonic reaction, more energy is required to break bonds than to form new ones, energy is needed for the reaction to occur

Endothermic Reaction If the energy needed is heat

Reaction rate How quickly a reaction occurs The time it takes reactants to turn into products The change in concentration of reactants and products over time

Factors affecting reaction (rxn) rates Temperature Concentration Surface area/size Catalyst

Temperature The higher the temperature, the faster the molecules are moving The faster the molecules are, the more collisions there are

Concentration The more particles there are within a given space, the more likely a collision is to occur This increases the frequency of collisions VideoinfoVideoinfo

Surface Area How much of a material is exposed –The larger the surface area of a reactant, the greater the number of particles that are exposed for reaction –This increases the frequency at which particles collide

Catalyst Speeds up a chemical reaction without itself being permanently changed Lowers the activation energy An inhibitor prevents or slows a chemical reaction

So…how can you speed up a reaction? Increase the temperature Stir the mixture Break the reactants into smaller pieces Higher concentration Add a catalyst

Let’s Practice Identify which factor determines the rate of each reaction. –Temp, Concentration, Surface Area, or Catalyst

Answers 1.Using kindling to start a fire 2.Why forest fires are particularly dangerous when weather conditions include high winds. 3.Using Goody’s headache powder, rather than an aspirin tablet 4.Accidentally leaving a gallon of milk on the counter overnight. Surface Area Concentration Temperature

Answers 5.Baking a cake 6.Digesting a cookie 7.Catalytic converter in your car 8.Termites destroying the foundation of a house. Temperature Catalyst

In-Class Assignment/Homework 23.3 Directed Reading 23.4 WKT