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