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Rates of Change

Rates of Change. Section 5.4. What affects the speed of reactions?. Temperature Surface area Concentration Pressure Molecule size Presence of catalysts. Temperature. Kinetic theory: molecules tend to move at speeds proportional to their temperatures Higher speeds  more interactions

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Rates of Change

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  1. Rates of Change Section 5.4

  2. What affects the speed of reactions? Temperature Surface area Concentration Pressure Molecule size Presence of catalysts

  3. Temperature Kinetic theory: molecules tend to move at speeds proportional to their temperatures Higher speeds  more interactions More interactions  faster reactions

  4. Surface Area Cooking a 20 lb. Thanksgiving turkey takes hours Cooking 20 lbs. of hamburgers on a BBQ grill doesn’t take too long (if the grill is big enough) More surface area means more of the reactant is available for the reaction More surface area  faster reactions

  5. Concentration Weak solutions: reactants are spaced apart Concentrated solutions: reactants are close together Higher concentration  more frequent interactions Higher concentration  faster reactions

  6. Pressure Useful where the reactants are gases Higher pressure means more gases in the same amount of space. More gas in same space leads to reactants being closer together Higher pressure  faster reactions

  7. Molecule size Kinetic theory: massive molecules move slower than light molecules at the same temperature Slower moving molecules do not interact as often with other reactants Larger molecules  slower reactions

  8. Catalysts A catalyst: A substance that changes the rate of chemical reactions without taking part in the reaction Catalysts are not reactants nor products They remain unchanged through the reactions Traffic cop analogy Can make reactions go faster or slower. Catalysts that make reactions go slower are called “inhibitors”

  9. Are reactions like one-way streets? • 2H2 + O2 2H2O (combustion) • 2H2 + O2 2H2O (decomposition, electrolysis) • 2H2 + O2↔ 2H2O (in general) • Concept of Equilibrium • Le Châtelier’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.

  10. Example CH4 (gas) + 2O2 (gas) ↔ 2H2O (liquid) + CO2 (gas) What happens if pressure is increased? Le Châtelier’sprinciple states equilibrium will shift to minimize the effects of this change. Higher pressure will cause equilibrium to shift to LOWER pressure More liquid will form Reaction is favored to the right.

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