Chemical Reaction – process that changes one set of chemicals into another 1. 2. 3.
Example: Fe + O2 Fe2O3 Reactants – elements or compounds that enter a chemical reaction Products – elements or compounds that are produced by a chemical reaction
Energy in Reactions
Energy can be released or absorbed whenever chemical bonds are formed or broken in chemical reactions
Sodium added to water releases a great amount of energy. Reaction of sulfuric acid and sugar creates a mound of black carbon and steam.
Energy Changes
EXOthermic-releases (expels) more energy than it absorbs. Chemical reactions that release energy often occur spontaneously
ex. Cellular respiration We release energy as heat
ENDOthermic-absorbs (takes in) more energy than it releases. Chemical reactions that absorb energy will not occur without a source of energy
ex. Photosynthesis Plants take in light energy to start photosynthesis
II. Energy in Reactions A. B.
C.
Chemical reactions either absorb or release energy Organisms need an energy source (FOOD!) for some chemical reactions necessary for life Activation Energy –energy needed to start a reaction
Activation Energy (Figure 2-20) Energy-Absorbing Reaction
Energy-Releasing Reaction
Question: How do the energy of the reactants and products differ between an energy-absorbing reaction and an energy-releasing reaction? Answer: In an energy-absorbing reaction, the energy of the reactants is less than the energy of the products. In an energyreleasing reaction, the energy of the reactants is greater than the energy of the products.
III. Enzymes A.
Catalyst – substance that speeds up the rate of a chemical reaction 1. 2.
B.
Lowers reaction’s activation energy Important because some processes necessary for life are too slow
Enzyme – protein catalysts that speed up biological reactions See Figure 2-21 (page 52)
Effect of Enzymes (Figure 2-21) Question: What effect does the enzyme have on the activation energy required by this reaction?
Answer: The addition of an enzyme lowers the activation energy of this reaction. The enzyme speeds up the reaction.
C. D. E.
Enzymes provide a site where reactants can be brought together to react Substrate – reactant of an enzymecatalyzed reaction Active Site – where a substrate binds to an enzyme Question: What do you notice about the shapes of the enzyme and substrate? Answer: They are complementary. Enzymes have specific shapes for specific substrates.
An Enzyme-Catalyzed Reaction Question: What happens to the enzyme (sucrase) after the products are released? Answer: The enzyme (sucrase) is free to catalyze another reaction. It is NOT used up!
F.
Enzymes play a role in… 1. 2. 3. 4.
G.
Controlling chemical pathways Making materials that cells need Releasing energy Transferring information
Enzymes can be affected by many variables… 1.
2. 3.
Temperature pH Regulatory molecules
IV. Review: 2.4 Assessment (page 53) 1. a. What happens to chemical bonds during chemical reactions? The bonds change – often they are formed or broken. b. Why is the melting of ice not a chemical reaction? Because new chemicals are not formed 2. a. What is activation energy? The energy that is needed to get a reaction started
b. Describe the difference between a reaction that occurs spontaneously and one that does not. A reaction that occurs spontaneously releases energy. A reaction that does not occur spontaneously absorbs energy. 3. a. What are enzymes? Proteins that act as biological catalysts