Equilibrium: a rate of reaction perspective

1. Equilibrium: a rate of reaction perspective • Forward reaction A + B → C + D • Backward reaction A + B ← C + D • Equilibrium results A + B ↔ C + D Ca2+(aq) + 2HCO3-(aq) ↔ CaCO3(s) + CO2(aq) + H2O (l)

2. Equilibrium is dynamic • All systems at equilibrium involve opposing processes in balance: Chemical reactions, dissolving, phases, diffusion through membranes Rate forward = rate reverse

3. Equilibrium constant • Consider the reaction Ax ↔ Ay • At equilibrium, • Ratef = Rater • Ratef = kf[Ax]; Rater = kr[Ay] • kf[Ax] = kr[Ay] • kf//kr = [Ay]/[Ax] • kf//kr = Keq

4. General expression for reactions aA + bB ↔ cC + dD Products Reactants

5. Not all products and reactants are included • Ignore all pure solids and liquids – they do not have concentrations in the sense of mol/L • Consider MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)

6. Not all products and reactants are included • Ignore all pure solids and liquids – they do not have concentrations in the sense of mol/L • Consider MnO2(s) + 4HCl(aq) = MnCl2(aq) + Cl2(g) + 2H2O(l)

7. Significance of Keq • Keq > 100: products much greater than reactants – almost complete reaction • Keq < .01: products much less than reactants – very little reaction • .01< Keq <100 products and reactants in similar concentrations

8. Calculations – putting numbers in • Consider the reaction 2HI(g) ↔ H2(g) + I2(g) What is the value of Keq if [HI] = 0.54 M, [H2] = [I2] = 1.72 M?

9. Units of Keq… • The units of Keq depend on the ratios of products and reactants in the expression M-2 Unitless

10. Upsetting the applecart • What happens to the equilibrium when changes are made? • Le Chatelier’s Principle If a stress is placed on a system at equilibrium, the system will respond by changing its position to minimize the stress

11. Changes in composition • Consider the reaction at equilibrium 2HI(g) ↔ H2(g) + I2(g) • What happens if additional H2(g) is added? • The system responds by trying to reduce the amount of added material; H2 is converted into HI – the equilibrium shifts away from the point of change 2HI(g) ↔ H2(g) + I2(g)

12. In general: • Add products: products → reactants aA + bB ↔ cC + dD • Add reactants: reactants → products aA + bB ↔ cC + dD • Other effects; • Temperature • Pressure

13. Temperature and equilibrium N2(g) + 3H2(g) = 2NH3(g) + heat • Reaction is exothermic • Supply heat: equilibrium adjusts to disperse heat • Less NH3 is made • Endothermic reactions will show opposite response (why we heat endothermic reactions)

14. Pressure and equilibrium 2HI(g) ↔ H2(g) + I2(g) • 2 moles reactants → 2 moles products • No overall pressure change N2(g) + 3H2(g) = 2NH3(g) • 4 moles reactants → 2 moles products • Increase pressure drives reactants → products