Section 3 – pg 234 Controlling Chemical Reactions Chapter 6 Section 3 – pg 234 Controlling Chemical Reactions

Activation Energy Pg 235 Chemical reactions require energy to start The energy is used to break the chemical bonds of the reactants The atoms then begin to form the new chemical bonds of the products Activation Energy: the minimum amount of energy needed to start a chemical reaction

Exothermic and Endothermic Reactions Every chemical reaction needs activation energy to get started Whether or not a reaction needs still more energy from the environment is dependant on if the reaction is exothermic or endothermic Exothermic Reaction Endothermic Reaction Heat released Heat Absorbed Pg 236

The products have less energy than the reactants Pg 236 The products have less energy than the reactants Due to release of energy (heat) to surroundings IE: burning of fuel Activation Energy Heat Released

Pg 236 Endothermic reactions need activation energy to get started and needs energy to keep going The energy of the products is higher than that of the reactants due to heat absorption Activation Energy Heat Absorbed

Rates of Chemical Reactions Pg 237 Rates of Chemical Reactions Chemical reactions do not occur at the same rates Some are fast – like in explosions Some are slow – like the formation of rust Factors that affect rates of reaction include surface area, temperature, concentration, and the presence of catalysts or inhibitors

Pg 237 Surface Area During a reaction between a solid and a liquid or gas, only the particles on the surface of the solid come in contact with the other reactants If the solid is broken down into smaller pieces, there is more surface area (more particles are exposed), and the reaction will happen faster

Pg 238 Temperature Increased temperature increases the rate of reaction Particles move faster causing them to interact more The faster moving particles have more energy providing the reaction with its activation energy Reducing Temperature slows down rates This is why we refrigerate food (slow down the multiplication of bacteria)

Pg 238 Concentration Concentration: the amount of a substance in a given volume Increasing the concentration of reactants supplies more particles to react

Pg 239 Catalysts Another way to control rate of a reaction is to change the activation energy needed Catalyst: a material that increases the rate of a reaction by lowering the activation energy The catalyst is not changed by the reaction so it is not considered a reactant

Pg 239 Most chemical reactions happens at a high enough temperature to kill living things Our bodies contain biological catalysts that lower this temperature so we don’t die (yay!) Enzyme: biological catalyst; each one is specific to a single type of reaction

Pg 239 Inhibitors Inhibitor: a material used to decrease the rate of a reaction Prevent reactants from coming together by combining with one of the reactants either permanently or temporarily

Chapter 6 Section 3 Homework – pg 239

1A. What is activation energy?

1B. What role does activation energy play in chemical reactions?

1C. Look at the diagram in figure 13, and make a generalization about activation energy in exothermic and endothermic reactions

2A. What are four ways that chemists can control the rates of chemical reaction?

2B. Which would react more quickly in a chemical reaction: a single sugar cube or an equal mass of graduated sugar crystals? Explain.