1. Using Chemical Demonstrations to Demonstrate Concepts in Physical Science II

2. Holt Science & Technology: Physical Science

3. 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

4. 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

5. 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.

6. 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”

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

8. 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.

9. 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)

10. 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

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

12. 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.

13. 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.

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

15. 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.

16. 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.

17. 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.

18. Demonstration 5 – Mentos and Diet Coke

19. 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

20. 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.

21. 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

22. Demonstration 7 – Clock Reaction
Concentration, temperature, inhibitor

23. Demonstration 8 – Alka Seltzer
Concentration, temperature, surface area