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Reaction Energetics

Reaction Energetics. Factors Affecting Reaction Rates and Equilibrium. Reaction Energetics. Most reactions are accompanied by energy changes, as well as changes of reactants into products. Reactions which give off or produce heat energy are called exo thermic .

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Reaction Energetics

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  1. Reaction Energetics Factors Affecting Reaction Rates and Equilibrium

  2. Reaction Energetics • Most reactions are accompanied by energy changes, as well as changes of reactants into products. • Reactions which give off or produce heat energy are called exothermic. • Reactions which taken in or absorb heat energy are endothermic.

  3. Reaction Energetics Consider the Reaction of methane and oxygen: CH4 + 2 O2 CO2 + 2 H2O + heat This is an energy-releasing, or exothermic, reaction.

  4. Transition state Reaction Energetics CH4 + 2 O2 CO2 + 2 H2O + heat Eact Energy  CH4, O2 H (exothermic) CO2, H2O Reaction Progress 

  5. Transition state Eact CH4, O2 H (exothermic) Energy  CO2, H2O Reaction Progress  Reaction Energetics For most reactions, even exothermic ones like this, energy starts out uphill; and there is an activation energy. Why???

  6. Reaction Energetics • For reactants to start changing, some bonding must begin to break up. • Otherwise, the starting material could never exist. • Bond breaking is always endothermic • For the CH4 + O2 reaction, the original molecules must begin to break up before permanent changes can occur. • Reactant molecules usually acquire their needed energy through forceful collisions.

  7. Reaction Energetics H These bonds must begin to break before reaction can occur O H H C H H O=C=O H This phase of the reaction has endothermic character. O O=O H O=O H Products Reactants

  8. Reaction Energetics H O H As soon as reactant bonds are sufficiently broken, product bonds can begin to form. H C H H O=C=O H O O=O H O=O H Products Reactants

  9. Reaction Energetics H O H As soon as reactant bonds are sufficiently broken, product bonds can begin to form. Initial endothermicity is replaced by exothermicity. H C H H O=C=O H O O=O H O=O H Products Reactants

  10. Transition state Reaction Energetics CH4 + 2 O2 CO2 + 2 H2O + heat Eact Energy  CH4, O2 H (exothermic) CO2, H2O Reaction Progress 

  11. Transition state Transition state Eact Energy  X H Y Reaction Progress  (fast) (slow) XY Reaction Energetics, Eact Eact Energy  A H B Reaction Progress  AB Higher Eact normally means slower reaction.

  12. Transition state, pathway 2 Transition state, pathway 1 Reaction Energetics, Catalysis Pathway 2 occurs with a catalyst and is much faster. Eact Energy  R H (exothermic) P Reaction Progress 

  13. Factors Affecting Reaction Rates • Energy of Activation, Eact • Large Eact  slowreaction Most molecules don’t collide hard enough to react. • Small Eact  fast reaction More molecules will collide hard enough to react. With smaller Eact, more molecules will have enough kinetic energy at given temperature for reaction.

  14. Transition state Transition state Eact Energy  X H Y Reaction Progress  Reaction Energetics Eact Energy  A H B Reaction Progress  Higher Eact normally means slower reaction.

  15. Factors Affecting Reaction Rates • Temperature • Low temperature slowreaction Most molecules don’t collide hard enough to react. • High temperature fast reaction More molecules will collide hard enough to react (average K.E. of molecules higher at higher temperatures).

  16. Eact KEave, low T KEave, high T High T Low T Eact Reaction Energetics No. Molecules  Kinetic Energy 

  17. Factors Affecting Reaction Rates • Reactant Concentrations • Low concentration of reactants slowreaction Low rate of collisions of reacting molecules. • High concentration of reactants fast reaction Higher rate of collision of reacting molecules. • Catalysts • Accelerate certain reactions • Biological catalysts usually are enzymes.

  18. Factors Affecting Reaction Rates Summary • Energy of Activation, Eact • Temperature • Reactant Concentrations • Catalysts/Enzymes

  19. Chemical Equilibrium • An equality of rates of opposing processes (forward and reverse reactions). • Does not imply equal amounts or concentrations of reactants and products. • Results in static, unchanging amounts and concentrations of materials. • Especially important when only small energy difference between reactants and products.

  20. Position of Equilibrium • Some chemical systems attain equilibrium with mostly reactants; Some others with mostly products. • Relative energy difference determines what substances are favored. • More stable substances will be favored (major). • Less stable substances will be disfavored (minor).

  21. Transition state Eact Energy  A (reactants) H (exothermic) B (products) Reaction Progress  A B Since B is more stable than A, it should predominate at equilibrium.

  22. Disturbing Position of Equilibrium LeChâtelier’s Principle “When a system at equilibrium is disturbed, the system will respond to minimize the disturbance.”

  23. Response to “Disturbances” Disturbance Response • Use up reactant (and form more product). • Add reactant • Use up product (and form more reactant). • Add product • Form more reactant (by using up product). • Remove reactant • Form more product (by using up reactant). • Remove product

  24. In water and acids In water and bases. A Real Reaction H2O H+ + OH-

  25. A Real Reaction H2O H+ + OH- Add H+ (acid)… OH- decreases. Add OH- (base)… H+ decreases. Remove H+… OH- increases. Remove OH-… H+ increases.

  26. It’s Like Pushing and Pulling: A + B C + D Add some A…

  27. It’s Like Pushing and Pulling: A + B C + D A + B C + D Add some A…

  28. It’s Like Pushing and Pulling: A + BC + D

  29. It’s Like Pushing and Pulling: A + BC + D “Pushing” on the left causes the reaction to shift to the right to re-establish equilibrium.

  30. It’s Like Pushing and Pulling: A + B C + D Remove some D...

  31. It’s Like Pushing and Pulling: A + B C + D

  32. It’s Like Pushing and Pulling: A + BC + D “Pulling” on the right causes the reaction to shift to the right to re-establish equilibrium.

  33. N2 + 3 H2 2 NH3 Your turn… How should the equilibrium concentration of ammonia, NH3, be affected by increasing the concentration of H2 in the system?

  34. Your turn… N2 + 3 H2 2 NH3 How should the equilibrium concentration of ammonia, NH3, be affected by increasing the concentration of H2 in the system? Hint: we’re “pushing “from the left.

  35. Your turn… N2 + 3 H22NH3 How should the equilibrium concentration of ammonia, NH3, be affected by increasing the concentration of H2 in the system? Good; there’s a rightward shift to minimize the disturbance.

  36. HHb + O2 H+ + HbO2-1 Hemoglobin Oxygenated hemoglobin Acid Another… How should the equilibrium concentration of oxygenated hemoglobin in the blood be affected by increasing the concentration of acid?

  37. HHb + O2H+ + HbO2-1 Hemoglobin Oxygenated hemoglobin Acid Another… How should the equilibrium concentration of oxygenated hemoglobin in the blood be affected by increasing the concentration of acid? Hint: we’re “pushing “from the right.

  38. HHb + O2H++HbO2-1 Hemoglobin Oxygenated hemoglobin Acid Another… How should the equilibrium concentration of oxygenated hemoglobin in the blood be affected by increasing the concentration of acid? Good; “pushing” from the right causes a shift to the left.

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