Equilbrium Constant and EXTERNAL EFFECTS

Equilbrium Constant and EXTERNAL EFFECTS Formation of stalactites and stalagmites CaCO3(s) + H2O(liq) + CO2(g)fgCa2+(aq) + 2 HCO3-(aq)

EQUILIBRIUM • Temperature, catalysts, and changes in concentration/ pressure affect equilibria. • The outcome is governed by LE CHATELIER’S PRINCIPLE • “...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”

Equilibrium constant and Concentration Concentration changes • no change in K • only the position of equilibrium changes.

Butane-Isobutane Equilibrium butane isobutane

butane ButaneIsobutane • At equilibrium with [iso] = 1.25 M and [butane] = 0.50 M. K = 2.5. • Add 1.50 M butane. • When the system comes to equilibrium again, what are [iso] and [butane]? isobutane

ButaneeIsobutane Solution Calculate Q immediately after adding more butane and compare with K. Q is LESS THAN K. Therefore, the reaction will shift to the ____________.

ButaneeIsobutane Q is less than K, shifts right toward isobutane. Set up ICE table [butane] [isobutane] Initial Change Equilibrium 0.50 + 1.50 1.25 - X + X 2.00 – x 1.25 + x

ButaneeIsobutane x = 1.07 M At the new equilibrium position, [butane] = 0.93 M and [isobutane] = 2.32 M. Equilibrium has shifted toward isobutane.

Equilibrium Constant and Catalyst • Add catalyst: NO change in K • A catalyst only affects the RATE it approach equilibrium. Catalytic exhaust system

Pressure and EquilibriumN2O4(g) e 2 NO2(g) Increase P in the system by reducing the volume (at constant Temp). e

N2O4(g) e 2 NO2(g) Increase P in the system by reducing the volume. In gaseous system the equilibrium will shift to the side with fewer molecules (in order to reduce the P). Therefore, reaction shifts LEFTand P of NO2 decreases and P of N2O4 increases.

Temperature Effects on Equilibrium Figure 16.6

Temperature Effects on Equilibrium N2O4 (colorless) + heat e 2 NO2 (brown) ∆Ho = + 57.2 kJ (endo) Kc (273 K) = 0.00077 Kc (298 K) = 0.0059

Every T has a unique K Temperature change = change in K Consider the fizz in a soft drink CO2(aq) + HEATe CO2(g) + H2O(l) K = P (CO2) / [CO2] • Increase T. What happens to equilibrium position? To value of K? • K increases as T goes up because P(CO2) increases and [CO2] decreases. • Decrease T. Now what? • Equilibrium shifts left and K decreases.

NH3 Production N2(g) + 3 H2(g) e 2 NH3(g) + heat K = 3.5 x 108 at 298 K

Le Chatelier’s Principle • Change T -changes K • causes change in P or concentrations at equilibrium • Use a catalyst: K not changed. Reaction comes more quickly to equilibrium. • Add or take away reactant or product:K does not change Reaction adjusts to new equilibrium “position”

Examples of Chemical Equilibria Phase changes such as H2O(s) H2O(liq) e

Equilbrium Constant and EXTERNAL EFFECTS