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Rates of Reaction

Rates of Reaction. A chemical reaction occurs when: Collisions between molecules have sufficient energy to break the bonds in the reactants. Molecules collide with the proper orientation. Bonds between atoms of the reactants are broken, and new bonds form. Rates of Reaction.

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Rates of Reaction

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  1. Rates of Reaction A chemical reaction occurs when: • Collisions between molecules have sufficient energy to break the bonds in the reactants. • Molecules collide with the proper orientation. • Bonds between atoms of the reactants are broken, and new bonds form.

  2. Rates of Reaction

  3. Activation Energy • the minimum energy needed for a reaction to take place is called the Activation Energy

  4. Reaction Rates • Different reactions take different amounts of time to proceed • Reaction rates are measured by the time needed: • to make a certain amount of product • to use up a certain amount of reactant

  5. What affects reaction rates Temperature • kinetic energy • number of collisions • energy of colliding molecules Concentrations of Reactants • number of reacting molecules • number of collisions

  6. Catalysts Catalysts • Lower activation energy • Provide alternate pathway • NOT used up in reaction • Ex: Enzymes

  7. Catalysts

  8. Chemical Equilibrium Many reactions can go forward AND backwards If the opposite reaction can occur, then this reaction is a reversible reaction At chemical equilibrium, the rate of the forward reaction equals the rate of the reverse reaction

  9. Chemical Equilibrium • Reactions continue at equal (but opposite rates) • No further changes in concentrations of reactants or products occurs

  10. Equilibrium Constants coefficients reactants products

  11. Heterogeneous Equilibrium • Homogeneous equilibrium: all reactants and products in same state (e.g. gas) • Heterogeneous equilibrium: • two or more states are present • concentrations of liquids and solids are constant • Liquids and solid not included in equilibrium expressions

  12. Calculating Equilibrium Constants Write the equilibrium equation for the reaction: Must be determined experimentally 2 N2O4 (g) NO2 (g) At 100 °C, once this reaction reaches equilibrium, the concentration of N2O4 is 0.45 M and NO2 is 0.31 M. What is the value of Kc?

  13. Equilibrium Constants: Large Kc Large Kc value: • Reaction favors products

  14. Equilibrium Constants: Small Kc Small Kc value: • Reaction favors reactants

  15. Equilibrium Constants Will each of the following favor reactants or products? • Reaction with Kc = 2.9 x 10-12 • Reaction with Kc = 0.001 x 105

  16. Calculating Concentrations at Equilibrium • Balance Equation • Write out Equilibrium Expression • Rearrange expression so unknown is alone on one side • Plug in concentrations and/or Kc What is the concentration of methane gas (CH4), a product (along with water vapor) in the equilibrium reaction between carbon monoxide and hydrogen gas? At equilibrium at 1000 °C, the concentrations of hydrogen, carbon monoxide, and water are 0.30 M, 0.51 M, and 2.0 M, respectively. (Kc at 1000 °C = 260)

  17. Le Châtelier’s Principle • At equilibrium, concentrations of substances do not change. • If a stress is put on the reaction at equilibrium, the equilibrium will shift to relieve the stress. What changes count as stress? • Concentration • Adding or removing reactant or product • Volume (Pressure) • Temperature

  18. Effect of Concentration Changes 2 2NO2 (g) NO (g) + O2 (g) • What is the effect on the concentration of each substance? • Add NO2? • Add NO? • Add O2? • Remove NO2? • Remove NO? • Remove O2?

  19. Effect of Volume Changes • Each mole of gas exerts a certain pressure • Decrease the volume… • Increase the pressure… • Shifts to side of reaction with FEWER moles of gas • 2NO2 (g) 2NO (g) + O2 (g) • What happens if we increase the volume? • Shifts to side with MORE moles • What happens if we increase the pressure? • Shifts to side with FEWER moles

  20. Effect of Volume Changes With volume changes and concentration changes, Equilibrium shifts to relieve new stress …in order to return to equilibrium (same Kc)

  21. Effect of Temperature Changes • If T is increased, equilibrium shifts to remove the heat • If T is decreases, equilibrium shifts to create heat • Kc changes! • Unlike for concentration and volume changes • Endothermic: • Heat + • A (g) B (g) + C (g) • Increase the temperature? • Shifts to the products (to use up heat) • Kcincreases

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