ScholarshipEquilibrium and ratesFree

Brown haze over Auckland: nitrogen oxides

Kc, response to compression with observations, quadratic for new equilibrium, kinetics of NO formation

Brown

\ce{NO2}

contributes to urban haze. It is in equilibrium with colourless

\ce{N2O4}

\ce{N2O4(g) <=> 2NO2(g)}

;

\Delta H = +57

kJ mol

^{-1}

. In engines, NO forms from

\ce{N2}

and

\ce{O2}

. Bond enthalpies (kJ mol

^{-1}

): N≡N 945; O=O 498; N=O (in NO) 630.

Facts you may need

Kc and Q
ICE tables and quadratics
Le Chatelier with observations
activation energy
bond enthalpies

(a)

0.0500 mol

\ce{N2O4}

is placed in a sealed 1.00 L flask at 100 °C. At equilibrium

[\ce{NO2}] = 0.0400

mol L

^{-1}

. Calculate

K_c

(b)

The flask volume is suddenly halved at constant temperature. Describe the colour immediately and as equilibrium re-establishes. Calculate Q immediately after compression and the new equilibrium

[\ce{NO2}]

(c)

Predict and explain the colour change, and the effect on

K_c

, when the flask is cooled in ice.

(d)

Discuss why NO forms inside engines but not in the air at normal temperatures, even though air is 99%

\ce{N2}

and

\ce{O2}

. Estimate

\Delta H

for

\ce{N2 + O2 -> 2NO}

from bond enthalpies and consider both the equilibrium position and the rate.

(e)

Catalytic converters remove NO by reacting it with CO to form nitrogen and carbon dioxide. Write the equation and explain the role of the catalyst, including why the converter works poorly for the first minute after a cold start.