2. Chapter 7 Section 2

3. How Fast? Fireworks explode in rapid succession on a summer night. Old copper pennies darken slowly while they lie forgotten in a drawer. Rates of Chemical Reactions 2 2

4. How Fast? Some reactions, such as fireworks or lighting a campfire, need help to get going. Others seem to start on their own. Rates of Chemical Reactions 2 2 Not all chemical reactions take place at the same rate.

5. Activation Energy— Starting a Reaction Before a reaction can start, molecules of the reactants have to bump into each other, or collide. The collision must be strong enough. Rates of Chemical Reactions 2 2 This means the reactants must smash into each other with a certain amount of energy.

6. To start any chemical reaction, a minimum amount of energy is needed. Rates of Chemical Reactions 2 2 This energy is called the activation energy of the reaction. Activation Energy— Starting a Reaction

7. Most fuels need energy to ignite. Rates of Chemical Reactions 2 2 The Olympic Torch provided the activation energy required to light the fuel in the cauldron. Activation Energy— Starting a Reaction

8. Reaction Rate A rate tells you how much something changes over a given period of time. Rates of Chemical Reactions 2 2

9. Reaction Rate Chemical reactions have rates, too. Rates of Chemical Reactions 2 2 The rate of reaction tells how fast a reaction occurs after it has started. To find the rate of a reaction, you can measure either how quickly one of the reactions is consumed or how quickly one of the products is created.

10. Temperature Changes Rate You can keep the food you buy at the store from spoiling so quickly by putting it in refrigerator or freezer. Rates of Chemical Reactions 2 2 Food spoiling is a chemical reaction. Lowering the temperature of the food slows the rate of this reaction.

11. Temperature Changes Rate Meat and fish decompose faster at higher temperatures, producing toxins that can make you sick. Rates of Chemical Reactions 2 2 Keeping these foods chilled slows the decomposition process. Bacteria grow faster at higher temperatures, too, so they reach dangerous levels sooner.

12. Temperature Affects Rate Most chemical reactions speed up when temperature increases. Rates of Chemical Reactions 2 2 Molecules collide more frequently at higher temperatures that at lower temperatures. This means they are more likely to react.

13. Temperature Affects Rate This works the other way too. Rates of Chemical Reactions 2 2 Lowering the temperatures slows down most reactions. If you set the oven temperature too low, your cake will not bake properly.

14. Concentration Affects Rate The closer reactant atoms and molecules are to each other, the greater the chance of collisions between them and the faster the reaction rate. Rates of Chemical Reactions 2 2

15. Concentration Affects Rate The amount of substance present in a certain volume is called the concentration of that substance. Rates of Chemical Reactions 2 2 If you increase the concentration, you increase the number of particles of a substance per unit of volume.

16. Surface Area Affects Rate The exposed surface area of reactant particles also affects how fast the reaction can occur. Rates of Chemical Reactions 2 2 Only the atoms or molecules in the outer layer of the reactant material can touch the other reactants and react.

17. Slowing Down Reaction A substance that slows down a chemical reaction is called an inhibitor. Rates of Chemical Reactions 2 2 And inhibitor makes the formation of a certain amount of product take longer. Some inhibitors completely stop reactions.

18. Speeding Up Reactions A catalyst is a substance that speeds up a chemical reaction. Rates of Chemical Reactions 2 2 Catalysts do not appear in chemical equations, because they are not changed permanently or used up.

19. Speeding Up Reactions Many catalysts speed up reaction rates by providing a surface for the reaction to take place. Rates of Chemical Reactions 2 2 Other catalysts reduce the activation energy needed to start the reaction. When the activation energy is reduced, the reaction rate increases.

20. Catalytic Converters Catalysts are used in the exhaust systems of cars and trucks to aid fuel combustion. Rates of Chemical Reactions 2 2 The exhaust passes through the catalyst, often in the form of beads coated with metals such as platinum or rhodium.

21. Catalytic Converters Catalysts speed the reactions that change incompletely burned substances that are harmful, such as carbon monoxide, into less harmful substances, such as carbon dioxide. Rates of Chemical Reactions 2 2

22. Enzymes Are Specialists Enzymes are large protein molecules that speed up reactions needed for your cells to work properly. Rates of Chemical Reactions 2 2 They help your body convert food to fuel, build bone and muscle tissue, convert extra energy to fat, and even produce other enzymes.

23. Enzymes Are Specialists Like other catalysts, enzymes function by positioning the reacting molecules so that their structures fit together properly. Rates of Chemical Reactions 2 2 Click image to view movie.

24. Other Uses One class of enzymes, called proteases (PROH tee ay ses), specializes in protein reactions. Rates of Chemical Reactions 2 2 They work within cells to break down large, complex molecules called proteins. Contact lens cleaning solutions contain proteases that break down proteins from your eyes that can collect on your lenses and cloud your view.

25. 2 2 Section Check Question 1 The minimum amount of energy required to start a reaction is called the _______. Answer Activation energy is the minimum amount of energy required to start a reaction.

26. 2 2 Section Check Question 2 Most chemical reactions _______ when temperature increases. A. abate B. remain the same C. slow down D. speed up

27. 2 2 Section Check Answer The answer is D. Temperature has a direct effect on the reaction rate.

28. 2 2 Section Check Question 3 Most chemical reactions _______ when concentration increases. A. remain the same B. slow down C. speed up D. stop

29. 2 2 Section Check Answer The answer is C. Higher concentration means a greater likelihood of collisions between the atoms in the reactants.