Controlling Chemical Reactions

1. Controlling Chemical Reactions Baines Middle School Instructor: G. Nicole Magee

2. Warm Up Which of the following actions would probably speed up the reaction between zinc and hydrochloric acid? Making sure the masses of the reactants and the products are the same Cooling down the reactants before combining them Adding an inhibitor Breaking up the zinc into smaller pieces

3. Energy and Reactions A chemical reaction will not begin until the reactants have enough energy to break the chemical bonds of the reactants. Once the bonds of the reactants are broken, the atoms then begin to form new chemical bonds as the products are formed. Activation energy is the minimum amount of energy needed to start a chemical reaction.

4. Energy and Reactions When hydrogen and oxygen gas react to form water, the reaction gives off a large amount of energy. If you just mix the gases (hydrogen and oxygen) together, you may not get a reaction for years. In order to start the reaction, a tiny amount of activation energy is needed � just a spark. Once a few atoms of hydrogen and oxygen react, the reaction will continue, fueled by the large amount of energy being released as a result of the reaction.

5. Energy and Reactions Overall, the reaction of hydrogen and oxygen gases to form water release more energy than it uses, making this an exothermic reaction. Every chemical reaction needs activation energy to get started. The exothermic or endothermic nature of a chemical reaction determines whether or not a reaction needs more energy from the environment to keep going.

6. Energy and Reactions In an exothermic reaction, the energy of the reactants before the reaction begins is at a certain level. When the activation energy is added, the energy level is at a peak. At the end of the reaction, the products have less energy than the reactants. The drop in the energy level from reactants to products is the result of the release of heat. The burning of fuel is an example of exothermic reactions.

7. Energy and Reactions In an endothermic reaction, the energy of the reactants before the reaction begins is at a certain level. When the activation energy is added, the energy level is at a peak. At the end of the reaction, the products have more energy than the reactants. The increase in the energy level from reactants to products is the result of the addition of energy from the environment. Most endothermic reactions require a continuous source of heat to occur.

10. Rates of Chemical Reactions

11. Rates of Chemical Reactions The rate at which chemical reactions occur can vary a great deal. Some chemical reactions are almost instantaneous, like an explosion. Other chemical reactions take place over a very long period of time, like the rusting of metal. A number of factors can affect the rate of chemical reactions.

12. Rates of Chemical Reactions The key to speeding up a reaction is getting more reactant particles together more often and with more energy. Surface area, concentration of reactants, and temperature are factors that can be manipulated to control the rate of reaction. Catalysts and inhibitors can also be used to control reaction rates. It is possible to speed up reactions and to slow down reactions.

13. Rates of Chemical Reactions: Surface Area The greater the surface area to volume ratio, the greater the exposure of the reactants to one another. For example, which would react more rapidly with acid � a small pebble of calcite, or the same volume crushed to a fine powder?

14. Rates of Chemical Reactions: Temperature When heat is added to a substance, the particles move faster and the temperature increases. Faster moving particles increase the reaction rate in two ways. Chances for a reaction to happen are increased by the particles coming into contact more often Faster moving particles have more energy, which moves particles closer to the activation energy needed for a reaction to occur. Reducing temperature slows down reaction rates.

15. Rates of Chemical Reactions: Concentration Concentration is the amount of substance in a given volume. By increasing concentration of the reactants, the rate of reaction is increased. Increasing concentration of reactants makes more particles available to react.

16. True or False Chemical reactions will speed up if the concentration of the reactants decreases. TRUE FALSE

17. Rates of Chemical Reactions A catalyst is a substance that increases the rate of reaction by lowering the activation energy needed for the reaction to occur. Catalysts are not changed in a reaction, thus they are not considered to be reactants. Enzymes are specialized proteins that act as biological catalysts.

18. Rates of Chemical Reactions An inhibitor is a substance used to slow down the rate of a chemical reaction. Most inhibitors combine either temporarily or permanently to one of the reactants and prevents the reactants from coming into contact with one another. Preservatives added to food products to prevent the food from spoiling or becoming stale are inhibitors Read about the discovery of nitroglycerin on page 59.

19. Checkpoint What would happen if the activation energy for a particular chemical reaction was not available? Describe ways to increase the rate of a chemical reaction. Which has greater surface area: a sugar cube or an equal mass of sugar crystals? Explain

20. Answers The reaction would not occur Increase temperature by adding heat, increase the surface area of the reactants, increase the concentration of the reactants, or add a catalyst The sugar crystals because the smaller the particles for a given mass, the greater the total surface area. Read pgs 54-59 in your science textbook.