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Lecture

Chapter 16 Elliott

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School
Boston University
Department
Chemistry
Course
CAS CH 102
Professor
Sean Elliott
Semester
Spring

Description
• 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  Rate = k [N O ] r r  2 4 2 o At equilibrium:  f [NO2]  =r  [2 4 ]  Rearrange: k [N O ] f = 2 4 =constant, K k [NO ]2                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 O (g) 2 4 K = [N2O 4] [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 K = (P C) ( P D ) p a b o (P A ) ( PB ) Where P  = units of partial pressure. X • The Value of K: o Value of K indicates extent of reaction: 81  2 H 2g) + O 2g) ⇌ 2 H O2g)     K = 3 × 10 • Very large K: favors formation of products. o 2 CO (g) ⇌ 2 CO(g) + O (g)    K = 3 × 10 ­92 2 2  Very small K: favors reactants; not much product formed at  equilibrium. o H O2g) + CO(g) ⇌ H (g) +2CO (g)  K2= 24  Intermediate value of K: comparable amounts of products and  reactants at equilibrium. • 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(g) + CO(g) ⇌ H (g) + CO (g) 2 2 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 (g2 ⇌ N O (2) 4 o Changing volumes will change partial pressures, change Q relative to K.
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