Section 4: Chemical Reactions and Energy

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Presentation transcript:

Section 4: Chemical Reactions and Energy Chapter 24: Chemical Reactions Section 4: Chemical Reactions and Energy

Section 4: Chemical Changes Chemical reactions involve energy exchange Breaking chemical bonds requires energy Forming chemical bonds releases energy More energy out (ex-) Exergonic reactions: energy required to break bonds is less than the energy released from new bonds that are formed; energy given off is usually light. http://www.youtube.com/watch?v=qVfAoRYmOpc

More energy out (ex-) More energy in (end-) Exothermic reactions: energy given off in the form of heat. http://www.youtube.com/watch?v=zePrQamwLw0 More energy in (end-) Endergonic reactions: more energy is required to break bonds than to form new ones; need energy for the reaction to occur. If the energy needed is heat, the reaction is endothermic. http://www.youtube.com/watch?v=4_ful2tcJcY

Changing the Speed of Reactions Classify the following as endergonic, exergonic, endothermic or exothermic: Sunlight decomposes hydrogen peroxide into water and oxygen Endergonic Sodium hydroxide is added to hydrochloric acid and gets warm Exothermic A campfire Exergonic and exothermic

Changing the Speed of Reactions A catalyst speeds up a chemical reaction without being permanently changed itself. http://www.youtube.com/watch?v=GSCJ2EYkRqQ An inhibitor prevents or slows a chemical reaction or interferes with a catalyst’s reaction. (lemon juice on apples to prevent browning; food preservatives such as BHT) Ever wonder why the shelf life of a Twinkie is years long?

Review Questions, p.750-754 Answer these questions (p.754) for tomorrow: Discuss the differences between exergonic and endergonic reactions. What happens to a catalyst in a reaction? Crackers containing BHT stay fresh longer than those without it. Explain why. Classify the reaction that makes a firefly glow in terms of energy input or output. To develop a product that warms people’s hands, would you choose an exothermic or endothermic reaction to use? Why?