AP Chemistry - Semester 2: Equilibrium: the extent of Chemical Reactions

Sections: Introduction   |  Section 1   |  Section 2   |  Section 3   |  Section 4   |  Section 5   |   Lab

17.3 Expressing Equilibria with Pressure Terms Often it is easier to measure the pressure of a gas than its concentration. We can plug in the ideal gas law to help relate the different variables to each other. Algebraically, we can see that the pressure is directly proportional to the molar concentration. Kp is the equilibrium constant obtained when the concentrations of all species are expressed as their partial pressures in atmospheres. Again, using the ideal gas law we can relate Kp to Kc. Kp = Kc(RT) ngas. Example: Calculate Kp for: PCl3(g) + Cl2 > PCl5(g) Kc = 1.67 at 500K There is one mole of gaseous product and two moles of gaseous reactants so ngas = 1-2=-1. Kp= 1.67[(0.0821)(500)]-1 = 0.0407 17.4 Reaction Direction: Comparing Q and K To determine if a reaction has reached equilibrium or to see in which direction the reaction is moving, we can compare Q with K. If Q < K. If the value of Q is smaller than K, the reactants (denominator) are large relative to the products (numerator). The reaction will progress to the products until equilibrium is reached. If Q > K. If Q is larger than K then the products (numerator) will decrease and the reactants (denominator) increase until equilibrium is reached. If Q = K. This situation exists only if the concentrations of both sides have attained equilibrium. Example: using the reaction: N2(g) +O2(g) >2NO(g) Kc = 0.10 at 2000oC If we add 1 mole nitrogen and 1 mole of oxygen to a 1 L container. Initial concentration of N2 = 1mol/L Initial concentration of O2 = 1mol/L Initial concentration of NO = 0 Qc=0 The reaction will proceed to the right. using the same reaction: N2(g) +O2(g) >2NO(g) If we add 1 mole nitrogen,1 mole of oxygen and 1 mole NO to a 10L container Initial concentration of N2 = 1mol/L Initial concentration of O2 = 1mol/L Initial concentration of NO = 0.1mol/L Qc>Kc so the reverse reaction will take place Next page

© 2007 Aventa Learning. All rights reserved.