For the exothermic reaction PCl3(g) + Cl2(g) PCl 5(g) KP = 0.160 at a certain te

ID: 683638 • Letter: F

Question

For the exothermic reaction PCl3(g) + Cl2(g) PCl 5(g) KP = 0.160 at a certain temperature. A flask is charged with 0.500 atm PCl3, 0.500 atm Cl2, and 0.300 atm PCl5 at this temperature. Based on the initial data, which of the following statements are true and which are false ? Drag the items to the appropriate bin. What are the equilibrium partial pressures of PCl3, Cl2, and PCl5, respectively ? Express your answers numerically in atmospheres separated by commas. How will the following changes affect the mole fraction of chlorine gas, cl2, in the equilibrium mixture. Drag each item to the appropriate bin.

Explanation / Answer

Seems like there are a lot of questions buriedhere. Let’s see if this covers them:

PCl3 + Cl2 <---> PCl5

Kp = [ PCl5] = 0.160

[ PCl3 ] [Cl2 ]

PCl3

Cl2

PCl5

Initial

0.5 amt

0.5 atm

0.3 atm

Change

-x

Equilibrim

0.5 + x

0.3 - x

Q= [ PCl5] [ 0.3] =1.2

[ PCl3 ][Cl2 ] = [ 0.5 ] [ 0.5 ]

K = [ 0.3 -x ] = 0.160

[ 0.5 + x ] [ 0.5 + x ]

[ 0.3 - x ] = 0.160 [ 0.5 +x ] [ 0.5 + x ]

[ 0.3 - x ]= 0.160 (0.25 + 1.0x + x2 )

[ 0.3 - x ] = 0.04 +0.16 x + 0.16 x2

0 = 0.16 x2 + 1.16 x -0.26

using the quadratic formula

we get

x = 0.00557 (the second “mathematical” answeris x = -0.191, which may make mathematical sense but doesn’tmake chemical sense. x = 0.00557 is valid and works)

True statements:

The net reaction proceeds to the left to attainequilibrium

(since Q is “too large”, the system will react so asto make it smaller, so reducing the numerator (product) pressureand increasing the denominator (reactant) pressures).

Q is greater than to K

False statements:

The net reaction proceeds to the right to attainequilibrium

(since Q is “too large”, the system will react so asto make it smaller, so reducing the numerator (product) pressureand increasing the denominator (reactant) pressures).

Q is equal to K

Q (the mass action fraction) equals K only when the system hasattained equilibrium

The reaction is at equilibrium

It is at equilibrium only when Q = K

No net reaction will occur

The system will change until equilibrium is attained.

Q is less than K

Mole fractions:

C Cl2 increases when temperature increases

Reaction is exothermic, so heat is considered aproduct. Increasing temperature adds heat, and equilibriumwill shift so as to use up some of the extra heat (using up otherproducts and creating additional reactants in the process)

C Cl2 decreases when volume is reduced

Equilibrium will shift so as to reduce the stress. Product sideinvolves fewer particles so occupies smaller volume under sameconditions, so is “favored” when volume is reduced

C Cl2 increases when volume is increased

Equilibrium will shift so as to reduce the stress. Reactant sideinvolves greater number of particles so occupies (expands to fill)larger volume under same conditions, so is “favored”when volume is increased

C Cl2 decreases when temperature is reduced

Reaction is exothermic, so heat is considered aproduct. Decreasing temperature removes heat, and equilibriumwill shift so as to restore some of the heat lost (using upreactants and creating additional products in the process)