Section 4 Rates of Change

Objectives Describe the factors affecting reaction rates. Describe the factors affecting reaction rates. Explain the effect a catalyst has on a chemical reaction. Explain the effect a catalyst has on a chemical reaction. Explain chemical equilibrium in terms of equal forward and reverse reaction rates. Explain chemical equilibrium in terms of equal forward and reverse reaction rates. Apply Le Chatelier’s principle to predict the effect of changes in concentration, temperature, and pressure in an equilibrium process. Apply Le Chatelier’s principle to predict the effect of changes in concentration, temperature, and pressure in an equilibrium process.

Factors Affecting Reaction Rates Temperature Temperature Surface Area Surface Area Concentration Concentration Pressure Pressure Molecular Size Molecular Size Catalysts Catalysts

Temperature An increase in temperature will speed up reaction rates. An increase in temperature will speed up reaction rates. A decrease in temperature will slow down reaction rates. A decrease in temperature will slow down reaction rates.

Surface Area An increase in surface area speeds up reaction rates; in other words, if you break it into tiny pieces, it will react faster. An increase in surface area speeds up reaction rates; in other words, if you break it into tiny pieces, it will react faster.

Concentration The more concentrated the solution, the faster the reaction rates. The more concentrated the solution, the faster the reaction rates. A more concentrated solution has more particles to interact. A more concentrated solution has more particles to interact. Obviously, the less concentrated the solution, the slower the reaction rates. Obviously, the less concentrated the solution, the slower the reaction rates.

Pressure Reactions are faster at higher pressure. Reactions are faster at higher pressure. Reactions are slower at lower pressure. Reactions are slower at lower pressure.

Molecular Size Massive, bulky molecules react slower. Massive, bulky molecules react slower. Smaller, simpler molecules react faster. Smaller, simpler molecules react faster.

Catalysts Catalysts speed up the reaction rate without becoming part of the reaction. Catalysts speed up the reaction rate without becoming part of the reaction. Catalysts are not reactants or products. Catalysts are not reactants or products. Inhibitors slow down the reaction rate. Inhibitors slow down the reaction rate.

Equilibrium Not all chemical reactions are permanent; meaning some of them are reversible. Not all chemical reactions are permanent; meaning some of them are reversible. When a chemical reaction can both ways, you will see  and  between the reactant and product. When a chemical reaction can both ways, you will see  and  between the reactant and product. Equilibrium results when reaction rates balance. Equilibrium results when reaction rates balance. At chemical equilibrium, no changes are apparent even though individual particles are reacting. At chemical equilibrium, no changes are apparent even though individual particles are reacting.

Equilibrium Temperature, pressure, and concentration can affect reactions in equilibrium. Temperature, pressure, and concentration can affect reactions in equilibrium. Temperature – Increasing temperature favors the reaction that absorbs energy. Temperature – Increasing temperature favors the reaction that absorbs energy. Pressure – Increasing pressure favors the reaction that produces fewer molecules of gas. Pressure – Increasing pressure favors the reaction that produces fewer molecules of gas. Concentration – Increasing the concentration of one substance favors the reaction that produces less of that substance. Concentration – Increasing the concentration of one substance favors the reaction that produces less of that substance.

Le Chatelier’s Principle If a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached. If a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached. This principle can be used to control reactions. This principle can be used to control reactions.