Using Chemical Demonstrations to Demonstrate Concepts in Physical Science II

Holt Science & Technology: Physical Science

Chapter 14 Chemical Reactions Section 4 Energy and Rates of Reactions Section Outline Reactions and Energy - Exothermic reactions Figure 1. Types of Chemical Reactions -Law of Conservation of Energy Figure 2. No title Rates of Reaction - Activation energy Figure 3. No title -Sources of Activation Energy Figure 4. Energy Diagrams

Chapter 14 Chemical Reactions Section 4 Energy and Rates of Reactions Section Outline (cont.) Factors Affecting Rate of Reaction - Temperature Figure 5. No title - Concentration Figure 6. Concentration of solutions - Catalyst Figure 7. No title

Reactions and Energy Chemical energy is part of all chemical reactions. Energy is needed to break chemical bonds in reactions. Energy is released when new chemical bonds form in chemical reactions.

Reactions and Energy Exothermic reaction – a chemical reaction that releases energy Exo means “go out” or “exit” Endothermic reaction – a chemical reaction that takes energy in Endo means “go in”

Demonstration 1 – Energy Release Types of Energy release Light Electrical energy Light and thermal energy

Demonstration 1 – Light Snapping a glowstick breaks a small glass container in the glowstick, allowing chemicals in the glass container to mix with chemicals outside the glass (but inside the chamber of the glowstick). The resulting reaction gives off light.

Demonstration 2 – Electrical Energy Electrical energy is released in an exothermic reaction that takes place in the battery See if the batteries get warm. 2 MnO2(s) + 2 H2O(l) + Zn(s)  2 MnO(OH)(s) + Zn(OH)2(s)

Demonstration 3 – Heat and Light Release Energy is released during some chemical reactions. The fire of a Bunsen burner gives off light energy and chemical energy. (An alcohol burner, candle, lighter, etc. could also be used, although reaction would change.) CH4 + 2 O2  CO2 + 2 H2O

Signs of Chemical Reactions Gas formation Solid (precipitate) formation Color change Energy change Light Thermal energy (heat) Electrical energy

Law of Conservation of Energy Law of conservation of energy – energy cannot be created or destroyed Energy can change forms Energy can be transferred from one object to another in the same manner that a baton is transferred from one runner to another in a relay race.

Rates of Reaction Activation energy – smallest amount of energy that molecules need to react. Chemical reactions need energy to get started In the same way that a bowling ball needs a push to get rolling.

Activation Energy Activation energy Reactants Energy Activation energy given off Energy absorbed Products Reaction progress Reaction progress

Factors Affecting Rates of Reaction Demonstration 4 – Glow Sticks Temperature A higher temperature causes a faster reaction rate. At higher T, particles of reactants move rapidly. Rapid movement causes particles to collide often and with a lot of energy. Many particles have the activation energy to react and change into products in a short time. The light stick on the right glows brighter than the one on the left because the one on the right is warmer.

Factors Affecting Rates of Reaction Concentration In general, a high concentration of reactants causes a fast rate of reaction. Concentration is a measure of the amount of one substance dissolved in another substance. When the concentration is high, there are many reactant particles in a given volume and a small distance between them. Thus, the particles run into each other often, leading particles to react faster.

Factors Affecting Rates of Reaction Surface Area Surface area is the amount of exposed surface of a substance Increasing surface area increases rate of reaction. Greater surface area exposes more particles of the reactant to other reactant particles. These leads to more collisions and faster rates.

Demonstration 5 – Mentos and Diet Coke http://www.eepybird.com/featured-video/the-extreme-diet-coke-mentos-experiments-ii-the-domino-effect//

Factors Affecting Rates of Reaction Inhibitors An inhibitor is a substance that slows down or stops a reaction Food preservatives – slow down growth of bacteria and fungi

Factors Affecting Rates of Reaction Catalysts Catalyst – speeds up a chemical reaction without being consumed/ A catalyst is not consumed because it is not a reactant. A catalyst lowers the activation barrier, which allows a reaction to happen more quickly. This catalytic converter contains platinum and palladium. These two catalysts increase the rate of reactions that make the car’s exhaust less harmful.

Demonstration 6 – Catalyst Hydrogen peroxide, H2O2, is unstable but decomposes very slowly. The decomposition reaction gives off heat. If Cu2+ ions are added (as a catalyst) to add aqueous solution of H2O2, the reaction proceeds rapidly and can become hot enough to boil the water. If dishwashing detergent is added to the solution before the catalyst, then a version of the elephant toothpaste demonstration is produced. 2 H2O2  O2 + 2 H2O

Demonstration 7 – Clock Reaction Concentration, temperature, inhibitor

Demonstration 8 – Alka Seltzer Concentration, temperature, surface area