Chapter 16

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Northeastern University
Chemistry & Chemical Biology
CHEM 1214
Thomas Gilbert

Lecture 10 (1/30/13) • Chemical Equilibrium: o A dynamic process in which concentrations of reactants and products  remain constant over time. o Rate of a reaction in the forward direction matches its rate in the reverse  direction. • When→and ←  Reaction Rates are Equal: o Reaction:  2 NO(g) ⇌ N 2 4g) o From experimental data:  Ratef= kf[NO 22  Rater= kr N2O4]2 o At equilibrium:  f [NO2]  =r  [2 4 ]  Rearrange: k f = [N 2 4 ]=constant, K k 2               [NO 2] • Equilibrium Constant: o Equilibrium Constant Expression:  Ratio of equilibrium concentrations or partial pressures of products  to reactants, each term raised to a power equal to the coefficient of  that substance in the balanced chemical equation.  2 NO(g) ⇌ N 2 4g) [N2O 4] K = [NO ]2    2 o Equilibrium Constant:  The value of K at a specific temperature. • Equilibrium Constant Expressions: o Generic Reaction:  aA + bB ⇌  cC + dD c d [ ] [ ] D K c = a b [ ] [ ] B o Where [X] = concentration units of moles/liter. c d (P C) ( P D ) K p = a b o (P A ) ( PB ) Where P X= units of partial pressure. • The Value of K: o Value of K indicates extent of reaction:  2 H 2g) + O 2g) ⇌ 2 H O(2)     K = 3 × 10 81 • Very large K: favors formation of products. ­92 o 2 CO (g)2⇌ 2 CO(g) + O (g)  2 K = 3 × 10  Very small K: favors reactants; not much product formed at  equilibrium. o H O(2) + CO(g) ⇌ H (g) + 2O (g)  K 2 24  Intermediate value of K: comparable amounts of products and  reactants at equilibrium. Lecture 12 (2/4/13) • Types of Equilibria: o Homogeneous equilibria:  Equilibria involving reactants and products in the same phase.  Mostly gas phase in Ch. 16; aqueous solutions in Ch. 17 o Heterogeneous equilibria:  Equilibria involving reactants and products in more than one  phase, e.g., decomposition of a solid that produces a gas. • Heterogeneous Equilibria: o CaCO (s) 3 CaO(s) + CO (g) 2 o Concentrations of solids are constant at a given  temperature:       Density (g/L)         = mol = (M) Molar mass (g/mol)       L • Le Châtelier’s Principle: o “A system at equilibrium responds to a stress in such a way that it relieves  that stress.” o Factors that will change the relative rates of forward/reverse reactions, or  change the value of Q compared to K, will cause a shift in the position of  equilibrium. • Concentration Stress: o H O(2) + CO(g) ⇌ H (g) + 2O (g) 2 o Remove CO (g): 2  Rate of reverse reaction decreases; reaction proceeds in forward  direction to establish new equilibrium. o Q compared to K:  Removing CO , K2> Q; reaction shifts right (toward products). • Effects of Pressure/Volume: o 2 NO (g)2⇌ N O (g2 4 o Changing volumes will change partial pressures, change Q relative to K. (PN O ) Y K p 242
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