1 / 19

Chemical Equilibrium

Chemical Equilibrium. Recognizing Rxn Equilibrium. 1. The system is closed. Opposite rxns occur at the same rate. Equilibrium is reached by either starting w/ reactants or products. Temp is constant.

iwalani-david
Download Presentation

Chemical Equilibrium

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chemical Equilibrium

  2. Recognizing Rxn Equilibrium 1. The system is closed. • Opposite rxns occur at the same rate. • Equilibrium is reached by either starting w/ reactants or products. • Temp is constant. **Radioactive tracers can prove th rxn proceeds even w/ no net observable changes: dynamic equilibrium Equilibrium does NOT mean conc of P+R are equal, but th RATES ARE EQUAL!

  3. Equilibrium Constant, Keq • A <=> B • Once equilibrium is reached, the ratio of partial pressures of A and B is constant. • Law of Mass Action • Shows the relationship between the concs (expressed as partial pressures for gases & as molarities for solutions) of the R + P present at equilibrium in any reaction.

  4. Equilibrium Constant, Keq-cont • aA + bB <=> cC + dD • Equilibrium-constant Expression: • Keq = [C]c[D]d [A]a[B]b • Temp dependent • Dimensionless: no units

  5. Magnitude of Keq • Keq can be very large or very small • CO(g) + Cl2(g) <=> COCl2(g) • Keq = PCOCl2 = 1.49 x 108 PCOPCl2 For Keq to be so large >> 1, the numerator, PCOCl2, must be large: The equilibrium lies to the right (toward the P) Keq <<1: equilibrium lies to the left (reactants dominate)

  6. Direction of Chemical Equilibrium & Keq • Since equilibrium can be approached fr either the P or R, wh way we write the equation is arbitrary N2O4 <=> 2NO2 • Keq = (PNO2)2 = 6.46 P N2O4 2NO2 <=>N2O4 • Keq = P N2O4 = 0.155 (PNO2)2 The Keq expression for a rxn written in one direction is the reciprocal of the one for the rxn written in the reverse direction.

  7. Other Ways to Manipulate Eq & Keq Values • When using Hess’s Law to obtain a net equation by adding equations & canceling like terms: • The Keq of a rxn in the reverse direction is the inverse of Keq of the forward rxn. • The Keq of a rxn th has been multiplied by a number is the Keq raised to a power equal to th number. • The Keq for a net rxn made of 2 or more steps is the product of the Keq for the individual steps.

  8. Heterogeneous Equilibria • Homogeneous equilibria -involve subs all in the same phase • Heterogeneous equilibria - subs are in diff phases • Partial pressures of gases are used in Keq expression. • Molar conc of dissolved species are used. • Pure solids, pure liquids, and solvents are NOT included in Keq expression. * these subs must be present for equilibrium

  9. Calculating Keq • If we know the equil conc of at least one species in a rxn, we can use stoichiometry of the rxn to determine the conc of the other species at equilibrium.

  10. Calculating Keq-cont • Tabulate the known initial & equilibrium concs of all species in the Keq expression. • For those where initial & equil conc are known, calc change in conc. • Use stoich of rxn to calc changes in conc for all other species. • Fr initial & change in conc, calc the equil conc.

  11. Applications of Keq • Allows us to predict: • 1) the direction in wh the rxn will proceed to achieve equilibrium • 2) calculate the conc of R+P when equilibrium has been reached

  12. Applications of Keq-cont. • Predicting the Direction of Rxn Q - rxn quotient- calc by substituting in partial press or conc into Keq expression Q = Keq if sy is at equilibrium Q > Keq- reaction proceeds to the left to reach equil Q< Keq - reaction proceeds to the right

  13. Applications of Keq-cont • Calculating Equilibrium Constants - Again use the known initial & equil conc - Set up a ICE table - Calc unknown equil conc

  14. Le Chatlier’s Principle • If a sy at equilibrium is disturbed by a change in temp, press, or the conc of comps, the sy will shift the eq position to counteract the effect of the disturbance.

  15. Le Chatlier’s Principle-cont • Change in R or P Conc • Adding a R or P disturbs equil: • Equil will re-establish by consuming the added R or P • Removing R or P • Equi will re-establish by forming more sub • N2(g) + 2H2(g) <=> 2NH3(g) • Add N2 or H2: shifts right • Add NH3: shifts left • Remove N2 or H2: shifts left • Remove NH3: shifts right

  16. Le Chatlier’s Principle-cont • Effects of Volume & Press Change • ONLY effect gas phase! • A decrease in vol will ^ press • Eq will shift to decrease the press • Eq will shift to the side with the least number of gas particles N2O4(g) <=> 2NO2(g) ^ press will shift to the left

  17. Le Chatlier’s Principle-cont • Effect of Temp Change • Temp will change the Keq • Endothermic: R + heat <=> P • ^ the temp will favor the endothermic rxn • ^ T = ^ Keq • Exothermic : R <=> P + heat • Decreasing the temp will favor the exo rxn • Increasing T = decrease in Keq

  18. Le Chatlier’s Principle-cont • The Effect of Catalysts • Lowers the Ea for both the forward & reverse rxns • Increases the rate at wh equil is achieved, it will not shift the equilibrium or change the equilibrium mixture.

  19. Haber Process • N2(g) + 2H2(g) <=> 2NH3(g) • High conc of H2(g) + N2(g)maintained. • NH3(g)(g) th forms is removed. • High temp to ^ the forward rxn. • 300-600˚C • Catalyst used to ^ rxn rate (est equilibrium faster) • High press used: 200-600 atm

More Related