Chapter 4 Lesson 3

Chapter 4 Lesson 3

Controlling Chemical Reactions

​Activation Energy

​Activation energy is the minimum amount of energy needed to start a chemical reaction

• ​All chemical reactions need a certain amount of activation energy to get started

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​For a reaction to start, activation energy must be added. An electric spark or adding of heat can provide that energy

​Exothermic Reactions

​This type of reaction results in a release of energy

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​The burning of fuels, such as wood, natural gas, or oil, is an example of an exothermic reaction

​Endothermic Reaction

​This type of reaction must continually absorb energy to keep going

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​Most endothermic reactions require continuous energy

• ​For example, cooking a hamburger requires adding energy constantly until the meat is done

​What affects the rate of a chemical reaction?

​Chemical reactions don't all occur at the same rate.

• ​Explosions: fast

• ​Rusting of iron: slow

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​Factors

1. ​Surface area

2. ​Temperature

3. ​Concentration

4. The presence of catalysts and inhibitors

​Surface Area

• ​When a piece of solid substance reacts with a liquid or gas, only the particles on the surface of the solid come into contact with the other reactant

• ​If you break the solid into smaller pieces, more particles are exposed to the surface and the reaction happens faster

• ​Example:

  • ​Chewing your food breaks it into smaller pieces that your body can digest more easily and quickly

​Temperature

• ​When you heat a substance, its particles move faster. Faster moving particles have more energy, which helps reactants get over the activation energy barrier more quickly

• ​In contrast, reducing the temperature slows down reaction rates

• ​Example:

  • ​You store milk and other foods in the refrigerator because keeping foods cold slows down those reactions, so your foods stay fresh longer

​Concentration

• ​Concentration is the amount of a substance in a given volume

  • ​Ex: Sugar added to tea- the more sugar you add, the sweeter it tastes because the concentration of sugar is greater

• ​Increasing the concentration of reactants supplies more particles to react

  • ​Ex: "elephant toothpaste"

​Catalysts

​A catalyst increases the reaction rate by lowering the activation energy needed

• ​The cells in your body contain thousands of biological catalysts called enzymes that help these reactions occur at body temperature

• ​Chemical reactions using enzymes require less activation energy and can happen at lower temperatures

​Inhibitors

​A material used to decrease the rate of a chemical reaction is an in inhibitor

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• ​Inhibitors called preservatives are added to food to prevent spoiling

Multiple Choice

To slow down a reaction, you can ___ the concentration of the reactants.

• increase

• decrease

Multiple Choice

What would react more quickly in the air, a pile of grain or a cloud of grain dust?

• Pile of grain

• Grain dust