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)

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

Equilibrium constant kf//kr = Keq kf//kr = [Ay]/[Ax] 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

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

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)

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)

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

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?

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

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

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

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

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)

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