Chapter 6 Section 3 Key Concepts: How is activation energy related to chemical reactions? What factors affect the rate of a chemical reaction? Key Terms: Activation energy, concentration, catalyst, enzyme, inhibitor

Energy and reactions Activation energy – the minimum mount of energy needed to start a chemical reaction ALL CHEMICAL REACTIONS NEED A CERTAIN AMOUNT OF ACCTIVATION ENERGY TO GET STARTED Waait. Ms. Fuentes, I thought you said some reactions are exothermic… that means energy would only be released, right?

Well yes, but… Consider this: You put hydrogen gas and oxygen gas into a container. They COULD make water, and energy would be released (exothermic). However, for the reaction to start, just a tiny little bit of energy is needed. OVERALL, energy is released. But a little bit was needed to break those original bonds and get everything going.

Endothermic and Exothermic Reactions EVERY CHEMICAL REACTION NEEDS ACTIVATION ENERGY. Whether or not a reaction needs still more energy from the environment to keep going depends on if it is exothermic or endothermic Look at figure 13 in your books. At the END of the exothermic reaction, the products have LESS energy than the reactants. Energy is released. At the END of the endothermic reaction, the products have MORE energy than the reactants. Energy was absorbed.

Memorize dat. No, really.

Rates of chemical reactions Chemists can control rates of reactions by changing factors such as surface area, temperature, concentration, and by using substances called catalysts and inhibitors.

Surface area If something is in a big chunk, only the surface particles will react when exposed to another substance. If it is broken up, more particles are exposed and the reaction will happen faster.

Temperature Will increase the rate of a reaction when increased Particles move faster when heated (we know this from when we studied particle movement) THEN, particles come in contact faster AND they have more energy, making activation energy easier to overcome So, lowering temperature will lower reaction rates

Concentration

No, not that kind. A different kind. Concentration: the amount of substance in a given volume. Increasing concentration of reactants supplies more particles to react. This increases the rate of reaction, because particles are more likely to come into contact with each other and reach activation level.

Catalysts A catalyst is a material that increases the rate of reaction by lowering the activation energy Catalysts affect a reaction’s rate but are not affected by the reaction, so they are NOT considered reactants

Enzymes Enzymes are biological catalysts They are needed because many chemical reactions happen at temperatures that would kill living things, but these reactions are needed for life.

Inhibitors Sometimes a reaction is more useful when slowed down rather than sped up. A material used to decrease the rate of a reaction is an inhibitor. Most inhibitors work by preventing reactants from coming together.